The earliest land plants, similar to present-day liverworts, arose
at least 475 million years ago, suggest tiny fossilized fragments from
Oman. Up to 420,000 different plant species may be growing on Earth, of
which perhaps 80% have been found and named. The Index Kewensis, a species
record held at Britain's Royal Botanic Gardens in Kew, near London, lists
more than a million Linnaean titles : plants are going extinct before botanists
can record their existence
Web resources :
Until now, researchers believed that pitcher plants—studied since the 17th
century, Federle said—captured most insects using waxy crystals making
slippery inner walls. However, when Federle's team studied Nepenthes bicalcarata—one
of the few species of pitcher plants that have no slippery waxy inner walls
but are nevertheless able to capture insects—after a rainfall
cellulose : a b(1-4)
D-glucose
homopolysaccharide, is the most abundant carbohydrate polymer in nature.
Although abundant, it is extremely difficult to degrade, as it is insoluble
and is present as hydrogen-bonded crystalline fibres. Anaerobic microorganisms
(symbiont in ruminant guts) have evolved a system to break down plant cell
walls that involves the formation of a large extracellular enzyme complex
called the cellulosome
(also in some Protozoa (symbiont in termite gut) and some Fungi
?)
hemicellulose
xilans : b(1->4) heteropolysaccharides
of D-Xil
glucomannans : b(1->4) Glc- and Man-
containing heteropolysaccharides)
It arises from a cellular plate during mitosis.
phragmoplast = the developing cell wall between the nuclei of a
vegetal cell in telophase
protoplast = a vegetal cell deprived (by lisozime) of its cell wall
in an isotonic environment
median lamella = a pectin structure between adjacent cell walls
plasmodesm = cytoplasmic channel between cells (20-30 nm diameter)
whose lumen is occupied by a central desmotubule arose from smooth
endoplasmic reticulum
glyoxisome = the vegetal equivalent of animal peroxisome
tonoplast = the membrane of the vacuole
plastids = vegetal organelles
chloroplasts
leukoplasts
modification : a mantainance methylase creates N5-methylC
in CpNpGp box in both strands during chromosome replication using SAM as
donor
plant cuticular lipid export : up until now, we knew that plants
produce this waxy coating on their cuticle, which is essential for water
conservation, and for their ecology in general, but no one knew how these
highly hydrophobic molecules that are made in the cells get out of the
cells. Plants export wax from epidermal cells to the surface of their aerial
parts through CER5, a lipid transporter similar to ABC transporters present
in mammalian cells. This is the first component of the plant lipid export
system to be characterized functionally. Traditionally, wax precursors
were thought to be exported by a vesicular pathway from their site of synthesis
to their destination at the plant surface. The identification of the CER5
transporter does not absolutely rule out the vesicular hypothesis. Because
of the limited resolution of fluorescence imaging, it remains possible
that CER5 is localized in a compartment involved in secretion. The simplest
hypothesis, advanced by the authors, is that the transporter located in
the plasma membrane has a pore that goes from the inside to the outside
through which lipids go out. It could also have a side pore, similar to
the bacterial MsbA ABC transporter, through which the lipids enter or exit
the transporter. A third possibility is that the transporter is not a pore
but a flippase that flip-flops fatty acids from the inner to the outer
leaflet of the plant cell plasma membrane
a cellular-automaton model (simple, discrete 'particles' programmed to
switch between different states depending on the states of their neighbours)
can explain the way that plants regulate their uptake of CO2,
which they use for photosynthesis, and their loss of water vapour. Leaves
have openings called stomata that open wide to let CO2 in, but
close up to prevent precious water vapour from escaping. Plants attempt
to regulate their stomata to take in as much CO2 as possible
while losing the least amount of water. But they are limited in how well
they can do this: leaves are often divided into patches where the stomata
are either open or closed, which reduces the efficiency of CO2
uptake. Patches of open or closed stomata in leaves of the cocklebur plan
sometimes move around a leaf at constant speed, for example
stoma : any minute pore, orifice, or opening on a free surface
pistil /gynecium : the female part of a flower
stigma : the uppermost part of a pistil, which secretes a moist,
sticky substance to trap and hold the pollen that reaches it
leaf : a flattened structure of vascular plants, attached to the
plant by a stem and usually green in color, that is the primary site of
photosynthesis and transpiration
root : the lowermost part of a plant or other structure
invading plant species, such as the Centaurea, establish monocultures
in novel habitats by displacing the indigenous plant communities. It is
thought that the absence of "natural enemies" and/or the release of phytotoxins
from the invading plants by allelopathy promote this process. The European
spotted knapweed (Centaurea
maculosa) releases racemic catechin; the antimicrobial (+)-catechin
and the phytotoxin (-)-catechin that inhibits germination and growth of
a number of nature plants (including Arabidopsis
thaliana and Centaurea
diffusa) by altering gene expression, resulting in rapid reactive
oxygen species wave, similar to that observed for root cell death, but
proceeding cell death by 5-10'. This increase in ROSs induced Ca2+-dependent
triggering of cell death. Gene expression analysis of A. thaliana
showed that 10 genes were upregulated 10' after treatment, with 956 genes
being upregulated 50' later. Many of the 956 gene products are linked to
oxidative stress and the phenylpropanoid and terpenoid pathways. At 12
hours, many of these genes were repressed, possibly as a result of the
onset of cell death. Allelopathy challenges the conventional ecological
perspective that a species' invasiveness is mainly due to enhanced resource
competition after escape from natural enemies and highlights the role for
the biochemical potential of the plant as an important determinant of invasive
success
the world's biggest-ever bloom, at more than 2.7 metres tall, the Titan
Arum flowered in the University of Bonn Botanical Garden on May 23, 2003.
It beats the previous record - set 70 years ago - by 7 centimetres. More
than 2,000 people have so far rushed to witness the event, as the spadix,
the blossom's fleshy central column, is expected to collapse on Saturday
afternoon. The growth, smell and warm temperature of the spadix were closely
studied in 4 flowers between 1987 and 1989. Amorphophallus
titanum, the blue whale of botany comes from the rainforests of
western Sumatra, Indonesia. It was discovered in 1878 by the Florentine
botanist Odoardo Beccari. He sent the seeds to Kew Royal Botanical Gardens
in London, where the first cultivated specimen flowered in 1889. Famously,
3 blooms appeared at Kew last year. Hothouse Titan Arums rarely flower,
but when they do, they are hard to ignore. The colossal lily-shaped blooms
smell of rotting flesh - hence the plant's nickname 'the corpse flower'.
By mimicking a carcass in decay, the brownish flowers attract insects that
deposit their eggs inside the plant, spreading its pollen in the process.
most flowering plants arrange their leaves and petals in spirals around
their stems. And in most species, the angle between successive elements
is close to the so-called golden angle, 137.5°, a number derived from
mathematic theory that correlates to artistic aesthetics and biological
patterning. This convergence has puzzled researchers for 250 years : the
arrangement minimizes the shadow each leaf casts on those below it, maximizing
the plant's light-gathering ability. Once built an optimality model of
light capture, the angle between leaves tends towards 137.5° as leaves
became thinner. The model's predictions matched results from an earlier
study on geometry and light-capture efficiency of the forest-dwelling daisy
Adenocaulon
bicolor. But there are many examples where the leaf spiral doesn't
match the golden angle, and some cases where it does but the plant is not
oriented to gather maximum light
trees grow tall where resources are abundant, stresses are minor, and competition
for light places a premium on height growth. The height to which trees
can grow and the biophysical determinants of maximum height are poorly
understood. Some models predict heights of up to 120 m in the absence of
mechanical damage, but there are historical accounts of taller trees. Current
hypotheses of height limitation focus on increasing water transport constraints
in taller trees and the resulting reductions in leaf photosynthesis. The
tallest known tree on Earth (112.7 m) is a redwood (Sequoia
sempervirens) in wet temperate forests of northern California.
Regression analyses of height gradients in leaf functional characteristics
estimate a maximum tree height of 122–130 m barring mechanical damage,
similar to the tallest recorded trees of the past. As trees grow taller,
increasing leaf water stress due to gravity and path length resistance
may ultimately limit leaf expansion and photosynthesis for further height
growth, even with ample soil moistureref.
plant mitochondrial genes are transmitted horizontally across mating barriers
with surprising frequency, but the mechanism of transfer is unclear. Horizontal
gene transfer from parasitic flowering plants to their host flowering plants
occurs as a result of direct physical contact between the 2. Genes can
also be transferred in the opposite direction, from host to parasite plantref.
The unicellular conifer tracheid should have greater flow resistance per
length (resistivity) than the multicellular angiosperm vessel, because
its high-resistance end-walls are closer together. However, tracheids and
vessels had comparable resistivities for the same diameter, despite tracheids
being over 10 times shorter. End-wall pits of tracheids averaged 59 times
lower flow resistance on an area basis than vessel pits, owing to the unique
torus-margo structure of the conifer pit membrane. The evolution of this
membrane was as hydraulically important as that of vessels. Without their
specialized pits, conifers would have 38 times the flow resistance, making
conifer-dominated ecosystems improbable in an angiosperm worldref
When plants experience drought or cold, they cannot get themselves a glass
of water or move to a warmer place. Instead, their ability to survive lack
of water, extreme temperatures and such stresses as high salt levels relies
heavily on a plant hormone called abscisic acid (ABA). Binding of 2 proteins,
FCA and FY, to one another results in a decrease in expression levels of
Flowering Locus C (FLC), causing a transition from vegetative growth to
flowering. The FCA–FY complex also causes synthesis of a truncated, non-functional
FCA messenger RNA in a negative feedback loop that results in fewer full-length
FCA mRNA transcripts and less FCA protein. Binding of abscisic acid
(ABA) to FCA abolishes the interaction of FCA with FY, leading to an
increase in full-length FCA transcripts and — through increased FLC activity
— a delay in flowering. Red lines depict negative regulationref
schematic
of the structures of jasmonate and its precursors
The Arabidopsis Information Resource (TAIR)
by the Carnegie Institution of Washington Department of Plant Biology and
the National Center for Genome Resources (NCGR)
canola (oil) - there are 2 types
of canola, the short-growing season Polish type (Brassica
rapa and Brassica
campestris) and the longer-season Argentine type (Brassica
napus). Canola is produced extensively in Europe, Canada, Asia,
Australia, and to a limited extent in the United States.
maize - Zea
mays (721 379 361 metric tonnes, 2004). The people of Mesoamerica
are largely responsible for the golden corn we grow today, having domesticated
tough teosinte grass thousands of years ago and bred it into modern maize.
A mutant maize that was found in South America in the 1920s is unable to
grow branches or flowers, and happens to resemble a particular rice mutant
in this respect. Because the sequence of the gene that causes the effect
is known for rice, the sequence in maize was pinned down and called barren
stalk1 : the normal version of barren stalk1 regulates how the maize plants
branch and is also located within one of five regions that maize researchers
have identified as targets of domestication. So, was it one of the genes
that the Mesoamericans unknowingly selected for as they tamed teosinte
(Zea
mexicana)? To investigate further, the researchers compared the
number of variants of the barren stalk1 gene in teosinte, which still grows
wild in Mexico's Sierra Madre, with the number in modern maize. In teosinte,
there are about 12 common variants of the gene, all of which probably produced
subtly different branching patterns in the plants. It is common for this
number of variants to be present in a particular species of plant. But
in modern maize, only one variant exists, suggesting that the others must
have been eliminated by rigorous selective breeding. Why did the Mesoamericans
plump so strongly for one branching pattern rather than another? Presumably
there was something about the branching of maize with that particular variant
that was useful. In combination with other genes it probably had some impact
on the architecture that was important to the Mesoamericans, perhaps bigger
seeds. The next step will be to paste all the variants of the barren stalk
that exist in teosinte into modern maize : once you see what the differences
are in maize, it will be easier to guess why a particular variant was chosen.
Teosinte grass (left) compared to "reconstructed" primitive maize, created
by crossing teosinte with Argentine pop corn :
The most critical step in maize (Zea mays ssp. mays)
domestication was the liberation of the kernel from the hardened, protective
casing that envelops the kernel in the maize progenitor, teosinte. This
evolutionary step exposed the kernel on the surface of the ear, such that
it could readily be used by humans as a food source. This key event in
maize domestication is controlled by a single gene (teosinte glume architecture
or tga1), belonging to the SBP-domain family of transcriptional regulators.
The factor controlling the phenotypic difference between maize and teosinte
maps to a 1-kilobase region, within which maize and teosinte show only
seven fixed differences in their DNA sequences. One of these differences
encodes a non-conservative amino acid substitution and may affect protein
function, and the other 6 differences potentially affect gene regulation.
Molecular evolution analyses show that this region was the target of selection
during maize domestication. Modest genetic changes in single genes can
induce dramatic changes in phenotype during domestication and evolutionref.
Aunt
Millie's has selected Hi- maize(R) resistant
starch as its fiber of choice in its new "Fiber and Flavor" potato
bread, which has lower calories and higher fiber per serving than traditional
white bread. The family-owned company - which has produced bread, buns,
rolls, English muffins, bagels and donuts for more than 100 years - introduced
the new bread as part of its "Healthy Goodness" line designed to help consumers
watch their health while still enjoying great flavor. Hi-maize resistant
starch has been utilized in a remarkably broad range of foods from white
bread and cereal to cookies and pizza dough. Along with the great taste
and sensory experience comes fiber-fortification with a broad range of
unique health benefits. Over 200 human clinical studies have shown that
Hi-maize can address important nutritional and health issues like invisible
fiber fortification, calorie reduction and weight management, glycemic
response/management, energy management, and digestive health. Hi-maize
resistant starch is the only commercially available all-natural resistant
starch made from corn available on the market
rice - Oryza
sativa (605 758 530 metric tonnes, 2004). Biologists have found
a gene that enables rice to live longer underwater. The new breed can survive
underwater for up to 2 weeks — most rice plants die within days of being
submerged. The discovery could pave the way for flood-resistant rice, helping
millions of the world's poorest farmers who lose their crops each year
to flooding. We have known for a long time that there are varieties of
rice that have this tolerance and we have tried to breed strains, but we
didn't understand the genetics. Researchers knew the rough location of
the flood-resistant genes on the plant's chromosomes, but not the exact
gene. This meant that every time they tried to breed flood-resistant strains,
unwanted DNA was passed on along with the key genes, potentially affecting
the rice's taste, colour or appearance. I a flood-resistant plant, a cluster
of genes produces ethylene, a plant hormone that responds to environmental
stress. One variant of one of these genes, called Sub1A-1, is found only
in plants that can live underwater. The variant differs by one DNA letter
from its flood-sensitive version, Sub1A-2. We knew the position on the
chromosome that was causing the effect. 10 years later we found that exact
gene. The single DNA change in Sub1A-1 replaces a proline amino acid with
serine. This triggers activity of other genes that help the plant to respire
without oxygen. The discovery is a step forward for African rice farmers.
Knowing the function and identity of the gene that confers water tolerance
means that varieties already adapted to African conditions could be made
flood-tolerant. Xu and his team have already bred Sub1A-1 into a common
Indian rice variety, using traditional breeding techniques rather than
genetic manipulation. The results so far show that the plants have retained
quality and high yields, as well as picking up the ability to survive drowning.
Over the past decade scientists have identified hundreds of useful genes
in rice, helping to boost disease resistance, yield, drought tolerance,
and other useful qualities. Thanks in part to these advances, rice production
has doubled over the past 40 years. But the demand for rice still exceeds
production, making this type of work invaluable. The team is now trying
to identify genes in other varieties of flood-tolerant plants, such as
maize and soy bean, to improve other cropsref.
Common bread wheat/spring wheat is typically planted in Florida in late
November to early December and harvested in late April to May, predominantly
in the panhandle counties. Wheat occupied ca. 10 000 acres in the 2003
growing season. The annual growth rate in wheat production in India dropped
from a healthy 3.57% in the 1980s to 2.11% in the 1990s and below 1% in
the current decade. After hitting a peak of 76 million tons in 2000, wheat
harvest has been hovering around 72 to 74 million tons, according to the
Food Corporation of India (FCI). In fact, the erosion of FCI's stock need
not be viewed with concern, as it has been the result of a deliberate move
to shed burdensome inventories through subsidized exports and liberal use
of grains in welfare and food-for-work programs. The relatively lower wheat
procurement also should not be a cause for much concern. It only reflects
higher purchases by private trade and withholding of some stocks by big
farmers in anticipation of better prices in lean seasons. These stocks
have to, sooner or later, come out in the market. However, the below-anticipation
wheat harvest for the 5th successive year should be a matter of real discomfiture
and should make wheat scientists and policy makers sit up. The agriculture
ministry, which had earlier reckoned the wheat harvest to be around 74.05
million tons, has scaled it down to 73.5 million tons. Wheat experts, who
were hoping to bag a harvest of around 75 million tons, now feel that the
production would be just around 73 million tons. The wheat trade, on the
other hand, is projecting a still lower output of around 71 to 72 million
tons. Some causes for a low wheat yield have, of course, been identified.
Noted wheat expert and Indian Agriculture Research Institute (IARI) director
S. Nagarajan believes that the unexpected emergence of 2 diseases so close
to ripening of the crop is responsible for yield drop. Black rust, a dreaded
plant disease, resurfaced after over 10 years, affecting the predominant
cultivar PBW-343 in Punjab and adjoining areas when the crop was heading
towards maturity. Another relatively lesser-known disease, called head
scab, struck the durum wheat (hard wheat used for making noodles and pasta
products) around the same time to cause yield loss. This disease is endemic
to North America and has not been reported in India in the recent past.
Poor wheat yields in earlier years of this decade had been caused largely
by unfavorable climatic factors, such as untimely rains or temperature
rise. Moreover, 2 dreaded weeds, Phalaris minor and wild oats, had
posed a formidable danger to wheat cultivation. But fortunately, their
control was discovered through pesticides and agronomic management. More
recently, rust diseases -- yellow, brown and black rusts -- spread like
an epidemic throughout the northwestern wheat belt. Fortunately, they were
tamed by breeding rust-resistant varieties and breaking the disease cycle
by saturating the wheat-growing southern hilly region with rust-immune
varieties. That was the region where the rust pathogen used to survive
in summer when the crop was not being grown in the plains. Wheat, being
a non-monsoon-dependent winter season cereal, is critical for the country's
food security, and any deceleration in its production growth is unwarranted.
Wheat output needs to grow annually by at least 2.5% to keep pace with
the rise in demand. The major wheat-producing countries of northern Asia
are India, Pakistan, Nepal, Bangladesh and Myanmar in order of importance.
Most of the wheat is produced in the Ganges and Nurmada basins of India
and the Indus River Valley of Pakistan. Much of the wheat in India and
Pakistan is irrigated, while in Nepal and Bangladesh it is mostly rain
fed. White grain cultivars are preferred and are primarily of spring habit,
but are usually sown in November and December and harvested in April and
May. Consumption is highest in Pakistan at 141 kg/caput and lowest in Myanmar
at 3 kg/caput. World production as of 2004 is 624 093 306 metric tons.
India is one of the largest wheat producers in the world, with about 25
million a [1 hectare= 10,000 square meters] under production and averaging
almost 60 million tons in recent years. > 90% of the area is sown to bread
wheat, which is grown throughout the country. Durum or macaroni wheat accounts
for around 8% of the area. The crop is grown in most parts of the country,
but nearly 70% lies in the northern plains and 20% in central India. A
rice-wheat rotation is the dominant cropping sequence. Crops other than
rice that precede wheat are also used, particularly in the central and
southern regions. In the large wheat research and development program in
India, much germplasm is screened for important biotic and abiotic stresses.
Important biotic pests include the rusts, Karnal bunt, foliar blight, powdery
mildew, common bunt, flag smut and nematode and insect pests. Salt, heat
and drought are the major abiotic stresses. Large amounts of NPK fertilizers
are used to produce the wheat crop in India. Over the past 3 decades, increased
agricultural productivity in Pakistan occurred largely due to the deployment
of high-yielding cultivars, increased fertilizer use and greater availability
of irrigation water. By the mid-1980s, semidwarf wheat cultivars had been
adopted on almost all irrigated land, and over 100 kg/ha on average of
fertilizer was being applied to wheat. Pakistan production averaged 16.1
million tons on 8.2 million ha each year during the period 1993-1995. Rice-wheat,
berseem-wheat and cotton-wheat are major systems of intense cropping in
Pakistan. Wheat scab (also known as Fusarium head blight) is caused
by the fungus Fusarium graminearum. It is a common problem in Europe,
Asia, South Africa, and the Midwestern and Eastern U.S.A. Scab severity
is very erratic and depends heavily on wet weather conditions. Scab often
causes reduction in test weight, sometimes down to near 50 lb/bu. In addition,
scabby kernels count as "damaged" in the grading process. Although scabby
wheat is often very good, it may contain the mycotoxins DON (vomitoxin)
and zearalenone (an estrogen analog). Swine are most sensitive to scabby
wheat mycotoxins, and as little as one ppm of DON can significantly reduce
daily weight gains in pigs. Higher concentrations result in feed refusal
and vomiting. Swine reproduction is also sensitive to disruption by the
zearalenone mycotoxin. Non-breeding cattle and poultry seem to tolerate
both toxins better than swineref1,
ref2,
ref3,
ref4 There are 3 main rust diseases of the cereal crop plant wheat. These
are wheat stem rust (Puccinia
graminis f.sp. tritici), wheat leaf rust (P.
triticina aka P. recondita f. sp. tritici) and wheat
stripe rust (P.
striiformis f. sp. tritici), all caused by species of the
fungus Puccinia. Severe losses that can occur due to wheat stem
rust have been abated in the USA since the 1960's by effective resistance
breeding, though the knowledge of resistance breaking strains of the pathogen,
such as Ug99 in Uganda, is a concern. Severe losses are still a possibility
with leaf rust. As recently as 1993, leaf rust destroyed over 40 million
bushels of wheat in Kansas and Nebraska. In 1985, Texas and Oklahoma lost
95 million bushels of wheat to leaf rust. The USDA Cereal Disease Laboratory
St. Paul, MN publishes regular reports on cereal rusts in the USA during
the crop season. 10 were published between March and September 2005.
Each report gives detailed state-by-state information, including a summary
map, all in pdf file format. Wheat is not doing well in early 2006 in southern
states because of drought, e.g. in Texas and Oklahoma. This condition
is also not conducive for leaf rust, which partly explains the low levels
of incidence reported so far March in 2006.
In Central Asia, yellow rust (Puccinia
triticina)ref,
tan
spot (Pyrenophora triticirepentis),
common
bunt (Tilletia caries), and
Tilletia
laevis are major foliar diseases of spring wheat, although Septoria
leaf blotch (Mycosphaerella graminicola
and Phaeosphaeria nodorum),
leaf
rust (Puccinia recondita f. sp.
tritici), and cereal
cyst nematode (Heterodera latipons) also occur in some areas.
Yellow rust and leaf rust are also very important in the Caucasian countries.
As researchers develop new varieties resistant to these diseases, progress
is tested through a step-by-step evaluation at different locations in the
region. In the first step, resistance to yellow rust, leaf rust, stem rust,
Septoria leaf blotch, cereal cyst nematode, and common bunt is evaluated
at Tel Hadya, ICARDA's headquarters, where wheat nurseries are artificially
inoculated with these diseases. In the 2nd phase, varieties identified
as resistant are further screened for resistance to different diseases
in heavily infected areas in different countries in Central and West Asia
and North Africa (CWANA) in collaboration with local scientists at 3 to
5 sites per country. It is difficult to assess disease loss given the information
available. The fact that about 350 000 ha. were treated with fungicides
indicates that farmers needed to try to control the diseases affecting
their cropsref.
Web resources : Wheat
Food Council
citrus (108 535 488 metric tonnes, 2004) : on a worldwide
basis, there are at least 6 citrus viroids: Citrus
exocortis viroid (CEVd), Citrus
bent leaf viroid (CBLVd), Hop stunt
viroid (HSVd), Citrus viroid III (CVd-III),
Citrus
viroid IV and Citrus viroid OS (CVd-OS).
CEVd causes the bark-scaling citrus exocortis disease on trifoliate orange
(Poncirus trifoliata) [Pt] root stocks, and some HSVd variants cause
citrus cachexia disease in sensitive hosts such as mandarins (Citrus
reticulata). CEVd, HSVd, and CVd -III are predominately found
in introduced cultivars such as lemons and oranges, while CVd-OS, HSVd,
and CVd-III are frequently found in the domestic cultivar 'Shiranui'. Other
viroids induce different degrees of stunting. Since commercial citrus trees
are commonly infected with viroid mixtures, only limited information is
available on their effects in species other than Etrog citron. In general,
yield reduction was associated mainly with loss of production of large
fruits. Bark-cracking of Pt may be caused by CEVd, CVd-IV, and HSVd, but
not by CBLV d or CVd-III. The International Committee on Taxonomy of Viruses
(ICTV) has proposed the name Citrus dwarfing viroid (CDVd) as a replacement
for CVd-III. Disease management depends upon sterilizing pruning
shears with bleach before moving to the next tree. Modern techniques for
detection of viroids in citrus are required as well, because viroid-infected
plants are often symptomlessref1,
ref2.
A multiplex RT-PCR has been developed to detect 6 citrus viroids (Citrus
exocortis viroid (CEVd), Citrus
bent leaf viroid (CBLVd), Hop stunt
viroid (HSVd), Citrus viroid III (CVd-III),
Citrus
viroid IV and Citrus viroid OS (CVd-OS))
and Apple stem grooving virus (ASGV,
synonym: Citrus tatter leaf
virus (CTLV)) from citrus plants. The multiplex RT-PCR was also designed
to distinguish CVd-I-LSS (a distinct variant of CBLVd) from CBLVd. By the
multiplex RT-PCR, 1-8 fragments specific to the pathogens were simultaneously
amplified from 1 sample and identified by their specific molecular sizes
in 6% PAGE. The results of the multiplex RT-PCR were consistent with those
of other diagnoses, such as uniplex RT-PCR, to detect each of the pathogens.
The multiplex RT-PCR provides a simple and rapid method for detecting various
viroids and ASGV in citrus plants, which will help diagnose many citrus
plants at a timeref1,
ref2
potato - Solanum
tuberosum (327 624 417 metric tonnes, 2004). Sprouting is one of
the biggest problems when storing potatoes. Untreated potatoes start to
sprout after 60 days. Lower temperatures reduce sprouting, but they also
increase the sugar content of the potato. When deep-fried, this sugar caramelizes,
giving fries a nasty brown colour that turns consumers off. A quick dose
of low-energy electrons stops potatoes sprouting for up to four months,
even if they are stored at room temperature. The researchers used a Van
der Graaf generator to deliver a beam of electrons to potatoes moving along
a conveyor belt. The electron beam works by preventing cell division within
the sprout bud tissue, similar to the way that radiotherapy stops cancer
cells multiplying. The method could replace the potentially harmful chemical
sprays such as chloropropham (also known as CIPC, whose residues can linger
on a potato's skin, worrying consumers and regulatory bodies alike) and
expensive g-radiation treatments that are currently
used to stem the surging sprouts (a special facility in Hikkedo, Japan,
treats > 100,000 tons of potatoes annually. But the process is expensive
and requires hefty shielding to protect workers). Camote is a type of sweet
potato having somewhat dry, bland, yellowish to white flesh, used as a
staple food in many tropical countries, and is also called boniato or batata
tomato - Lycopersicon
esculentum originated in South America, were introduced into Europe
in the 16th century, and are now a popular food worldwide. The Roma tomato
was developed in the mid-1950s as a firmer and more disease-resistant varietyref.
Uncooked tomatoes have become an integral and nutritious component of the
daily diet. Approximately 5 billion pounds of fresh market tomatoes are
eaten annually in the USA (120 384 017 metric tonnes, 2004)
banana -
Musa
x paridasiaca L., Musa
acuminata is the 4th most important global food crop after rice,
wheat and maize in terms of gross value of production. Total world Musa
production is currently about 97 million tones annually (FAOSTAT, 2003),
of which bananas cultivated for export account for only 10%. Hence, they
are important for food security in the humid tropics and provide income
to farmers. It is a major staple food, supplying up to 25% of the carbohydrates
for approximately 70 million people in Africa's humid forest and mid-altitude
regions. The East and Central Africa sub-region alone produces nearly 20
million tons of bananas annually (71 343 413 metric tonnes, 2004)ref
cassava - Manihot
esculenta - is a hardy, drought-resistant tuber with an edible
root that grows in tropical and sub-tropical areas of the world. It provides
an important source of energy for millions of people in Africa. (202 648
218 metric tonnes, 2004). Cassava is among the most common crops in sub-Saharan
Africa, accounting for > 50% of world production with over 90 million tons
of fresh product, more than any other crop in Africa. Cassava is vital
to the livelihood of over 200 million people and plays a key food security
role for rapidly expanding rural and urban populations and has huge potential
for commercialization, income generation, and poverty reduction
coffee, green - Coffea
arabica (7 761 397 metric tonnes, 2004). 2 species of coffee are
being grown commercially: Coffea arabica (Arabica or highland coffee)
and Coffea canephora (Robusta or lowland coffee). Arabica is usually
preferred for its taste, but Robusta is also grown widely. Over 30 pathogens,
many of them fungal, have been reported to
cause diseases in coffea.
grapevine - Vitis
labrusca, Vitis
vinifera (66 569 761 metric tonnes, 2004). Wine enchants because
of its complexity, but that very trait makes it difficult to regulate.
That bottle full of aromatic red liquid with hints of cherry may be genuine
Pinot Noir from the California coast, or it may be a New Jersey Merlot,
diluted with water and tarted up with sugar or sophisticated synthetic
flavourings. In the arms race between the adulterators and the regulators,
detection systems have become ever more sophisticated, as have the cheaters.
But at least one common ruse - claiming that the wine is one variety, when
it is actually entirely or partly another - may come to a sudden stop if
DNA can be successfully extracted from wine on the shelf.
stable isotope analysis. Carbon, hydrogen and oxygen atoms all occasionally
show up in versions with slightly different mass. The amounts of such isotopes
vary from region to region, so when they are incorporated into grapes,
they tell a tale about where the wine was made. The snag is that they also
vary with local weather conditions, so samples of wine from each region
have to be taken each year for comparison and entered in the European Wine
Data Bank. For example, 2003 wines were collected before the late rains,
so there were values for oxygen that were more like Southern Italy.
chromatography creates a graph showing which wavelengths of light are absorbed,
and software types wines according to absorption pattern with decent accuracy.
Things fall apart a bit, however, with blends. In Germany, the world's
largest wine importer, regulators often compare the ratios of two forms
of anthocyanin, the molecules that make wine red. This ratio is determined
by the genes of the vine, and was thought to be an unalterable marker that
could tell you what variety of wine you were drinking. But some processing
techniques can change this ratio quite significantly. Techniques can involve
long fermentation, high temperatures and adding enzymes. Many producers
of Cabernet Sauvignon have had their wine unfairly rejected by the German
government and been forced to sell it at half price. More kinds of anthocyanins
should be used to type wine.
extraction and purification of grape-skin DNA from bottles of wine, confident
that if forensic scientists can get DNA from ancient skeletons, he can
do the same for a bottle of Pinot Gris. Researchers are also working to
identify a set of genetic markers to differentiate the 2,500 or so varieties
of grape in existence. Short sections that repeat a different numbers of
times in different varieties seem like a good bet.
All this fuss stems not just from a passion for authentic tastes, but from
economic motives. What other food has such a big price range? It goes from
$2 to $200 for the same size bottle. > 50 different viruses and viroids
are known to infect grapevines worldwide, and undoubtedly many unknown
ones that have yet to be detected. Evolution of new viruses is a constant
threat to grapevine producers and their spread by manref1,
ref2,
ref3,
ref4,
ref5.
lettuce - Lactuca
spp.. Ruccola is the Italian word for arugula, a popular leafy lettuce
in the USA. In other parts of the world, it is referred to as raketsla,
roquette, notensla, or zwaardherik.
sugar beet - Beta
vulgaris (249 208 061 metric tonnes, 2004) is a globally important
crop producing 27% of world sucrose supplies. It is grown in Europe, North
America, Chile, Uruguay, China, the Middle East, North Africa, and countries
of the former Soviet Union.
sweet potato - Ipomoea
batatas (127 139 553 metric tonnes, 2004), ranked 7th in worldwide
food production (5th in developing countries). China is the biggest producer
in the world. The most harmful diseases in sweet potatoes are caused by
a complex of viruses of which the most common is the aphid-transmitted
potyvirus sweet potato
feathery mottle virus (SPFMV). When the whitefly-transmitted sweet
potato chlorotic stunt virus (SPCSV) is also present, it breaks down
resistance to other viruses in sweet potato cultivars, which results in
the severe sweet potato virus disease (SPVD) and high or complete
yield loss. SPCSV was tested for but not found in this Italian study, but
it is known in Spain. SPFMV alone can cause severe cracking and corking
of sweet potatoes before and after harvest. When infected roots are used
to produce slips for new plantings, then plants derived from the infected
root will also be diseased. Therefore, it is extremely important to use
virus-free seed piece stockref1,
ref2,
ref3,
ref4,
ref5.
yam - Dioscorea
batatas, Dioscorea
alata, Dioscorea
rotundata (40 048 149 metric tonnes, 2004). Multiple viral infections
are common in field plants and may lead to severe disease due to viral
synergism (see ProMED-mail 20070624.2042). The predominantly vegetative
propagation of yam increases the chance of a build-up of viruses. Viruses
so far found in yam include Dioscorea latent virus (genus Potexvirus),
Cucumber mosaic virus (genus Cucumovirus), Dioscorea bacilliform virus
(genus Badnavirus), Dioscorea dumetorum virus, Dioscorea trifida virus,
YMV, YMMV, and JYMV (all genus Potyvirus). ZYMV and PVY mentioned in the
note above are potyviruses, which are widely distributed but have not been
reported from yam. YMV is the most important and most prevalent virus infecting
Dioscorea spp. in the tropics. It has been reported from several African
countries and the Caribbean. Symptoms include mild mosaic and stunting
of plants. Resistant yam cultivars are being developed for different regions.
JYMV causes similar symptoms on yam species in Asia. YMMV is spreading
in West Africa, Guadeloupe, and Guyana. It causes mild mosaic and vein-banding
symptoms, which are often transient.
grapevine
yellow speckle viroid (GYSVd) is an elusive disease whose outward expression
is conditioned by climatic and possibly varietal factors. Symptoms, when
shown, consist of a few to many minute chrome yellow spots or flecks scattered
over part or all of the leaf surface or gathering along the main veins
to give a vein banding pattern. GYSVd-induced vein banding is very similar,
if not identical, to the symptoms of a disease also known by the name of
vein banding, which has long been regarded as part of the fanleaf degeneration
complex. Although vein banding may show in GFLV-free vines, it is more
often associated with GFLV infections. In fact, it has been suggested that
the presence of GFLV enhances the expression of GYSVd in the form of vein
banding patterns. Similar enhancement may occur in vines concurrently infected
by grapevine chrome mosaic virus. Unlike GFLV-induced yellow discolorations,
the symptoms of yellow fleck appear in the height of summer on a limited
number of mature leaves, and they persist for the rest of the vegetating
season. Yellow speckle symptoms may also show concurrently with symptoms
of other diseases such as, for instance, leafroll. No vector for GYSVd
is known. Natural dissemination occurs by mechanical inoculation through
surface-contaminated cutting tools during management operations (pruning
and propagation); by graft transmission: and by distribution of infected
propagating material. The absence of symptoms in most European scion varieties
and all American rootstocks greatly facilitates inadvertent viroid dissemination,
making viroid dispersal virtually impossible to prevent. None of the grapevine
viroids is known to be seed-transmitted
coconut
cadang-cadang viroid (CCCVd) => cadang-cadang, which comes from
a Bicol term "gadan-gadan" meaning dead or dying, is a premature decline
and death of coconut and palm trees restricted to the Philippines, first
reported in 1937. CCCVd is the most serious of all known viroids because
of its lethality. It has killed over 40 million coconut palms and still
kills between 200 000 - 400 000 palms annually. It is spread by unknown
means, is seed-transmitted (1/320 seedlings), and also transmitted by pollen.
CCCVd contains 246 or 247 nucleotides and is the smallest known nucleic
acid-containing pathogen. Coconut palms less than 10 years old are rarely
affected, but disease incidence increases rapidly to about 40 years, remaining
constant thereafter. Disease management basically involves eradication
of infected palms when they become symptomatic. No resistance to CCCVd
has been identified. In the Philippines, the disease occurs in the central
region (southern Luzon, Samar, Masbate, and smaller islands within a zone
about 600 km x 300 km). Disease management requires use of sterilized pruning
tools to avoid spread of the viroid
coconut
tinangaja viroid (CTiVd) => Tinangaja,
first reported in 1917, shares about 65% overall sequence with CCCVd and
is confined to Guam, where it affected 30% of coconut plants and led to
the end of commercial production on the island following the death of 30
million trees in the period from 1950 to 1980. Incidence of the disease
is about 30%. There is some evidence that CTiVd is seed-transmitted at
very low levels.
hop latent viroid
CTiVd and CCCVd are the only viroids that infect monocotyledons. Both cause
lethal infections in coconut, causing death within 10 years of diagnosis.
Inasmuch as CCCVd and CTiVd occur separately in the Philippines and Guam,
respectively, it would be interesting to monitor differences in genomic
sequences between the 2 viroids over time.
hop
stunt viroid (HSVd) causes a severe disease in hop (Humulus
lupulus). The fruit crop plant plum, Prunus domestica, develops
dapple or no symptoms when infected by HSVd. HSVd consists of a 295-303
nucleotide circular single-stranded RNA. It has been found in a wide range
of herbaceous and woody hosts, where the infection seems to be latent (such
as grapevine, apricot, pear) or may induce specific disease symptoms such
as hop stunt, citrus cachexia, and dapple fruit of plum and peach. It has
also been reported in cucumber. HSVd is not difficult to find in plum in
China, despite this being the 1st report in that host. No symptoms have
been associated with these findings. The detection of HSVd in plum in China
is significant in part because this is the geographical origin of the species
and because of the threat this pathogen could have to the other crops it
infects. Like other viroids, HSVd is transmitted mechanically, by budding
and grafting, and by vegetative propagation. Note that HSVd has been reported
in grape and apricot in Chinaref1,
ref2.
Isolates of HSVd have been classified into 5 groups; 3 major types (plum,
hop and citrus) each containing isolates from only a limited number of
isolation hosts, and 2 minor ones (plum-citrus and plum-hop-citrus) presumably
derived from recombination between members of the main groups. HSVd apparently
has a wide host range, infecting hop, cucumber, grapevine, citrus, plum,
peach, pear, apricot, pomegranate and almond. It has been found latent
in a wide range of herbaceous and woody hosts (grapevine, apricot, pear)
and may induce peculiar symptoms such as hop stunt, dapple fruit of plum
and peach, and citrus cachexia. HSVd is very contagious and is found worldwide.
In the Czech Republic, HSVd incidence in grapevine planted in the vicinity
of hop gardens was found to be about 70%. Previous work has characterized
HSVd isolates from France, Italy, Japan, Spain and USA. Current studies
have revealed 16 new sequence variants from Cyprus, Greece, Morocco, and
Turkey, where HSVd had not been described previously. The origin and route
of the intrusion of HSVd into Canada remain unknown. Both PLMVd and HSVd
are also easily transmitted by grafting or by use of contaminated pruning
tools, but evidence of their transmission by vectors is apparently lacking
for HSVd. Viroid disease management depends upon sanitation; plant viroid-free
trees, sterilize shears after pruning each tree, and eradicate infected
treesref1,
ref2,
ref3,
ref4,
ref5,
ref6,
ref7,
ref8.
Transmission of grapevine yellow speckle viroid 1 and hop stunt viroid
via seeds has been confirmed in 11 seedlings of 8 grapevine Vitis vinifera
varieties using a combination of RT-PCR, dot-blot hybridization and northern
hybridization assays. This indicates that budwood stocks must be checked
for the presence of HSVd. At least 16 new sequence variants of HSVd have
been obtained from 4 Mediterranean countries (Cyprus, Greece, Morocco and
Turkey) where this viroid had not previously been described. At present
it appears that HSVd is limited to Japan. In some hosts, such as grapevine
and apricot, the infection appears to be latent but in other cases, specific
disorders such as hop stunt, dapple fruit of plum and peach and citrus
cachexia have been associated with HSVd infection. Phylogenetic analyses
revealed that sequence variants belonging to the 2 minor recombinant subgroups
are more frequent than previously though
citrus
cachexia viroid is a graft-transmissible viroid that causes phloem
deterioration and blockage in many mandarin, mandarin hybrids, Citrus
macrophylla Wester, Rangpur lime, and sweet lime. This disease causes
decline, stunting, and crop reduction. Sour orange is tolerant to the viroid.
There appears to be consensus among viroid specialists, based on molecular
genetic studies, that the viroid moved from grapevine to hop.
Mexican
papita viroid (MPVd), found in 1982 in symptomlessly infected Solanum
cardiophyllum in Aguascalientes state, suggest that MPVd may be
the putative ancestor of crop viroids. MPVd, present in symptomlessly infected
solanaceous plants, may have been transferred by chewing insects (aphids,
grasshoppers, flea beetles, tarnished plant bug, leaf beetle, and Colorado
potato beetle) to breeding plots and commercial potato crops.
potato
spindle tuber viroid (PSTVd) has been reported from Solanum
tuberosum (France) and tomato
(Lycopersicon esculentum)(Netherlands and UK). In tomatoes,
PSTVd symptoms develop slowly, often not becoming apparent until 4 or 5
weeks after infection. Infected plants become stunted and show "bunchy
top" symptoms (crowded foliage, due to shortening of internodes, and occasional
formation of spindly shoots). Leaf symptoms include yellowing and purpling
as well as considerable leaf distortion including downward curling of the
leaflets (epinasty), curling, and twisting (rugosity). Severe necrosis
along the veins develops later in the lower and middle leaves, which eventually
die. Younger leaves at the top of the plant remain but are reduced in size.
Flowers are often aborted, and fruit ripening is erratic. Fruits becomes
small and hard and can turn dark green. Overall yields can be significantly
reduced. Disease management basically depends on planting viroid-free transplants,
adherence to a strict phytosanitary regimen to prevent contamination and
subsequent spread of the viroid. Benches, tools, storage bins, and sacks
can be disinfected with 3% hypochloriteref.
Once PSTVd has been introduced onto a farm or nursery, it can be rapidly
spread from plant to plant through the use of contaminated cutting tools
and/or machinery, by handling, or simply by direct plant to plant contact.
PSTVd can also be transmitted via infected pollen. To prevent the spread
within growers' fields, good hygienic practices are necessary to prevent
contact with potentially infected plants, and, to avoid subsequent spread
of the viroid. Contaminated benches, tools, etc. should be disinfected.
2-3% hypochlorite has been shown to be effective. PSTVd can reduce yields
by as much as 65 and 50% in potato and tomato, respectively. Other natural
hosts include pepino (Solanum
muricatum), avocado (Persea
americana), and a range of solanaceous crops. PSTVd was reported
in the 1930's to be transmitted by chewing insects (aphids, grasshoppers,
flea beetles, tarnished plant bug, leaf beetle, and Colorado potato beetle)
and is a nasty pathogen
tomato
apical stunt viroid (TASVd)ref1,
ref2
was first found and characterized in Ivory Coast in 1980's, but no data
was given on its epidemiology or economic impact. Another strain was found
in Indonesia in 1980's, but again without data on potential economic impact.
Both cause significant crop loss. The genomes of the 2 viroids are very
similar (99.7% identity) despite their distant geographic origins. A third
strain, isolated from Solanum
pseudocapsicum [Jerusalem cherry], does not occur naturally in
tomato but can infect tomato by mechanical inoculation. First found in
Israel (on tomato (Lycopersicon
esculentum) grown under plastic houses in the coastal region) in
2003 for samples collected in 1999/2000; first found in Tunisia in 2006ref.
The RNA sequence of the Tunisian, Israeli and Indonesian strains are very
similar. Affected tomato plants in Israel showed shortened internodes (bushy
appearance), leaf deformation and yellowing, reduced fruit size, and pale
red discoloration of fruit. Up to 100% disease incidence could be observed
at single sites with heavy yield losses. The present article also noted
100% disease incidence once temperatures were high, which is a characteristic
of several diseases caused by viroids. TASVd can be transmitted from infected
to healthy tomato plants by grafting or mechanical inoculation (in experimental
conditions). No data is available on pollen or seed transmission, though
some viroids are known to be transmitted by these means. Introduction on
transplants is a strong possibility. Control of viroids is difficult in
practice, so it would be desirable to avoid any further spread of a potentially
serious disease of tomatoes. This new report from Tunisia acts as a warning
to other countries. Other viroids that have been detected in Tunisia are
noted in the archive belowref
These pathogens are ubiquitous, and unfortunately some induce ephemeral
symptoms that complicate their detection. PCR-based diagnostic detection
systems are now available for some of them. AGVd appears to be the result
of recombination between nucleotide sequences in PSTVd, CEVd, Apple
scar skin viroid (ASSVd), and GYySVD-1. Interestingly, Vein banding
disease in grapevine results from the synergistic interaction between grapevine
viroids and Grapevine
fanleaf nepovirus (GFLV). There is evidence that some viroids are seed-and
pollen-transmitted in grapevine. Disease management requires the planting
of viroid-free planting stock, sterilization of pruning tools, and eradication
of infected vines.
unclassified viroids
apple
fruit crinkle viroid (AFCVd), a graft-transmissible fruit and/or bark
disorder of apple, was first reported in Japan. I was unable to obtain
information about the viroid outside of Japan. Apple scar skin viroid (ASSVd)
has 37% sequence homology with GYYSVd. Both viroids share a common
sequence in the central region of the molecule, but lack the central conserved
sequence of viroids in the potato spindle tuber viroid group. Other homologous
residues also occur as blocks of base-paired residues in the secondary
structures of ASSVd and GYSVd. ASSVd viroid also has some homology with
members of the Potato spindle tuber viroid group but not with Avocado sunblotch
viroidref1,
ref2,
ref3,
ref4,
ref5,
ref6,
ref7,
ref8
Disease management is based on prevention of viroid transmission, especially
in nursery production of viroid-free planting material. Strict measures
must be in place to reduce infestation of cutting tools and implements
by using chemical compounds to inactivate viroids. Only viroid-free stock
should be planted. However, cold treatment can be effective, e.g. storage
at 4°C for > 6 months, followed by apical shoot-tip-culture grafting,
can be used to eliminate HLVd. Pre-inoculation with protective mild strains
of viroids has proved effective to control PSTVdref1,
ref2.
Viruses
ssDNA viruses
Geminiviridae
=> geminivirus-like symptoms (stunting, reduced leaf size, and leaf
curling "chino")
Web resources : Gemini
Detective : Geminiviridae:Begomoviruses whitefly-transmitted geminiviruses
Begomovirus : plant virologists continue to report new begomoviruses, and
new ones will undoubtedly emerge. The combination of high temperatures,
presence of high populations of whiteflies, and suitable natural host plants
apparently results in a mix that is conducive to generating new begomovirus
strains in the region. The begomovirus-satellite disease complexes are
associated with economically important diseases and have been isolated
from vegetable and fiber crops, ornamental plants, and weeds throughout
Africa and Asia. Their widespread distribution and diversity, coupled to
the global movement of plant material and the dissemination of the whitefly
vector, suggests that these disease complexes pose a serious threat to
tropical and sub-tropical agro-ecosystems worldwide.
=> Ageratum yellow vein disease is caused by the whitefly-transmitted
monopartite begomovirus Ageratum yellow vein virus (AYVV) and a DNA beta
satellite component. Naturally occurring symptomatic plants also contain
an autonomously replicating nanovirus-like DNA 1 component that relies
on the begomovirus and DNA beta for systemic spread and whitefly transmission
but is not required for maintenance of the disease. Systemic movement of
DNA 1 occurs in Nicotiana benthamiana when co-inoculated with the
bipartite begomovirus Tomato golden mosaic virus and the curtovirus Beet
curly top virus (BCTV), but not with the mastrevirus Bean yellow dwarf
virus. BCTV also mediates systemic movement of DNA 1 in sugar beet, and
the nanovirus-like component is transmitted between plants by the BCTV
leafhopper vector Circulifer tenellus. A 2nd nanovirus-like component,
referred to as DNA 2, has only 47% nucleotide sequence identity with DNA
1. AYVV disease occurs throughout the Indian subcontinent (Nepal, India,
Pakistan), China and parts of Africa. The disease may be associated with
distinct virus species at these locations. Ageratum conyzoides,
a weed species widely distributed throughout southeast Asia, frequently
exhibits striking yellow vein symptoms associated with infection by AYVV
Geminiviridae (genus Begomovirus). The widespread presence of AYVV-infected
Ageratum
conyzoides constitutes a major inoculum source. Ageratum ranges from
Southeastern North America to Central America, but the center of origin
is in Central America and the Caribbean. Most taxa are found in Mexico,
Central America, the Caribbean, and Florida. Ageratum conyzoides
now is found in several countries in tropical and sub-tropical regions,
including Brazil. It is widely utilized in traditional medicine by various
cultures worldwideref1,
ref2,
ref3
There are at least 5 strains/isolates of CLCuV known, most of which
are reported from Pakistan and western India. Cotton leaf curl Gezira virus
is the more cosmopolitan strain, occurring in Egypt and Sudan, and is known
to be present in Central Africa, Chad, Nigeria, Togo and West Africa. Cotton
leaf curl disease (CLuCD) is a serious disease of cotton and several
other malvaceous plant species that is transmitted by the whitefly Bemisia
tabaci. Control of CLCuD is mainly based on insecticide treatments
against Bemisia
tabaci. Roguing, particularly of ratoon cotton from the previous
seasons crop, is recommended but appears to have little affect in reducing
disease incidence. Resistant cotton cultivars have been introduced that
were developed by conventional breeding and selection. However, recent
reports have suggested that the virus complex has overcome the resistanceref1,
ref2,
ref3,
ref4,ref5,
ref6,
ref7,
ref8,
ref9,
ref10,
ref11
cotton
leaf curl Multan virus (CLCuMV) causes cotton leaf curl disease
(CLCuD), a major constraint to cotton production on the Indian subcontinent.
It has been shown to be caused by a monopartite begomovirus and a novel
ssDNA satellite molecule termed CLCuD DNA-beta. The satellite molecule
is trans-replicated by CLCuMV but does not possess the iteron sequences
of this virus. Field surveys across all the cotton-growing regions of Pakistan
indicate that dual and multiple infections are the norm for CLCuD with
no evidence of synergism. Despite the diversity of begomoviruses associated
with CLCuD, only a single class of DNA-beta has been detected, suggesting
that this satellite has the capacity to be recruited by unrelated begomoviruses.
As of 2003, an 26 additional DNA beta molecules, associated with diverse
plant species obtained from different geographical locations, have been
cloned and sequenced. They were shown to be widespread in the Old World,
where monopartite begomoviruses are known to occur. Analysis of the sequences
revealed a highly conserved organization for DNA-beta molecules consisting
of a single conserved open reading frame, an adenine-rich region, and the
satellite conserved region (SCR). The SCR contains a potential hairpin
structure with the loop sequence TAA/GTATTAC, similar to the origins of
replication of geminiviruses and nanoviruses. 2 major groups of DNA-beta
satellites were resolved by phylogenetic analyses. One group originated
from hosts within the Malvaceae and the 2nd from a more diverse
group of plants within the Solanaceae and Compositae. Within
the 2 clusters, DNA-beta molecules showed relatedness based both on host
and geographic origin. These findings strongly support coadaptation of
DNA-beta molecules with their respective helper begomoviruses. The begomovirus-satellite
disease complexes are associated with economically important diseases and
have been isolated from vegetable and fiber crops, ornamental plants, and
weeds throughout Africa and Asia. Their widespread distribution and diversity,
coupled to the global movement of plant material and the dissemination
of the whitefly vector, suggests that these disease complexes pose a serious
threat to tropical and sub-tropical agro-ecosystems worldwide.
East
African cassava mosaic virus (EACMV) : cassava mosaic disease (CMD)
is the most important constraint to cassava
(Manihot esculenta) production in Africa. Since the 1990s, the
importance of the disease has been greatly increased by the spread through
East and Central Africa. The most likely means of spread of EACMV-UG into
Burundi would be via viruliferous whiteflies (Bemisia
tabaci) and movement of infected cassava cuttings. The fact that
EACMCV has reached epidemic status in Nigeria threatens the food security
of that country and is a setback to plant pathologists who have been contending
with control of CMD. The virus is present in many parts of Africa (Congo
Republic, Democratic Republic of Congo, Kenya, Malawi, Rwanda, Tanzania,
Uganda and possibly in Madagascar, Zimbabwe, Zambia and Mozambique). Disease
management strategies include cultural control (planting of virus-free
stocks and use of stem branches of moderately resistant and resistant genotypes
as sources of cuttings), development of resistant lines through crosses
between cassava and wild species (Manihot
glaziovii) to restore root quality and resistance to CMD, as well
as lines derived from Nigeria and local Nigerian genotypes. Moreover, breeding
for the specific needs of farmers in different regions of Nigeria should
be consideredref1,
ref2,
ref3.
Cassava production in Africa faces new challenges from CMD. Disease has
spread in recent years, bringing increased risk of food insecurity to millions
of rural and urban households, particularly in eastern Africa. Research
and extension programs have helped limit the geographic spread of CMD,
but the potential magnitude of the problem threatens to overwhelm these
efforts. CMD continues to be prevalent in all the main cassava-growing
areas in the ECA (Economic Commission for Africa) sub-region and is regarded
as the most important disease, causing 20-90% crop losses based on the
cultivar, viral strain and environmental factors. Deterioration in the
status of CMD is a fact in East Africa, Uganda, DR Congo, and Kenya. Plants
infected via white fly-transmission dominate affected areas. Lower leaves
of infected plants look apparently healthy, while leaves above the point
of 1st infection show severe symptom expression, drastic reduction in leaf
size with marked distortion. Plants harbor numerous adult white fly (Bemisia
tabaci (Bt)) populations on young shoots and large nymph populations
on the lower surface of the apparently healthy lower leaves. Lack of alternative
propagation stock in disease-infected areas leaves farmers no choice but
to use material from the previous harvest of infected plants as planting
stock for the next generation. Environmental factors favoring the development
and fecundity of Bt enhance disease spread, and spread of CMD is therefore
highly linked to the vector. To alleviate the situation, a number of African
countries (Kenya, Burundi, and Madagascar) have made significant progress
in selecting resistant/tolerant clones, which are being evaluated within
their different ecological zones. The research discussed in this piece
was completed in 2002, and normally I would not post a piece that took
almost 3 years to be published in "New Disease Reports, but I believe that
it is important to recognize new virus threats to the food security of
Sudan and other African countriesref1,
ref2,
ref3
mungbean
yellow mosaic India virus (MYIMV) : the interaction between MYIMV and
satellite DNA-beta results in more severe symptoms compared to plants inoculated
with MYINV alone. Similar results have been reported for other DNA-beta
molecules and viruses. DNA-beta molecules are symptom-modulating, ss-DNA
satellites associated with monopartite begomoviruses belonging to the Geminiviridaeref1,
ref2,
ref3,
ref4
pepper
golden mosaic virus (PepGMV), formerly designated as Serrano golden
mosaic virus (SGMV) and Texas pepper virus (TPV), is present
in Costa Rica, Guatemala, Honduras, Mexico (7 states), and USA (Texas),
and causes significant crop losses. Scientists in Mexico are attempting
to select tomato plants that appear to be asymptomatic following inoculation
with PepGMV, suggesting that they may be useful in developing resistant
cultivars. Part of the resistance mechanism appears to be the lack of virus
movement in infected plants. At least 2 closely related strains of PepGMV
(Tamaulipas and La Paz) are present in Mexico.
It is transmitted by 2 whitefly species -- the sweet potato whitefly Bemisia
tabaci and the silverleaf whitefly, Bemisia
argentifolii -- although some taxonomists suggest that these species
are really biotypes. Squash and watermelon
(Citrullus lanatus) are preferred hosts. Disease management
utilizes cultural control (eradication of infected plants, use of UV-absorbing
greenhouse plastic films, and aluminum plastic mulches) and biological
control (use of parasitoids such as Encarsia
spp. and Eretmocerus
spp.). Use of insecticides is not very efficacious because whiteflies
tend to congregate on the undersides of leaves. In most instances, the
incidence of SLCV-affected plants in Israel was close to 100% and was always
associated with high populations of the whitefly, Bemisia tabaci.
SCLV has been reported from USA (Arizona, Texas, and California) as well
as from Guatemala, Honduras, Sinaloa and Sonora states in Mexico, Nicaragua,
and Panama. Epidemic caused by a 'New World' geminivirus in the Eastern
Hemisphere has also been reported.
=> leaf curl in the crop plant sweet
potato, Ipomoea batatas. SLCV or sweet potato with leaf curl
symptoms has been detected or reported in Taiwan, Japan, Brazil, China,
Mexico, Puerto Rico, and the USA, and Kenya (2005). The causal agent is
the whitefly-transmitted SPLCV. Viral genomic ssDNA can be detected in
plant samples using PCR with SPLCV-specific primers. SPLCV-like isolates
cluster into 3 groups, and all of them might have evolved from the same
common ancestor possibly from the Old Worldref1,
ref2
=> leaf crumple disease in soybean
(Glycine max) in India. An incidence of > 80% was observed in
Lucknow. Diseased plants exhibited severe yellowing, crumpling and distortion
of leaves. Infected plants were stunted, and had a very low yield.
The virus is not uncommon in solanaceous crops like tomato in India, but
detection in soybean is a 1st report. The complexity of different
begomoviruses in India and neighboring countries that cause similar diseases
can be followed in the posts listed below. Diversity is driven by an abundant
active vector, the whitefly Bemisia
tabaci, many weed and crop reservoirs, and the capacity for this
group of ssDNA viruses to share/swap genetic information thereby creating
new viruses and strains. Their ability to cause severe disease is demonstrated
in this reportref1,
ref2,
ref3.
tomato
leaf curl Mayotte virus. The agricultural sector on Mayotte is divided
into subsistence farming and farming for export. Subsistence farming, which
provides the staples making up 75% of the islanders' diet, consists of
coconuts, cassava, bananas and rice. Small quantities of fish and meat
are also consumed. Mayotte is not self-sufficient and must import a large
portion of its food requirements, mainly from France. Exports are mainly
ylang-ylang (perfume essence), vanilla, copra, coconuts, coffee, and cinnamon
=> leaf curl disease in potato
(Solanum tuberosum), Nicotiana
benthamiana, tomato
(Lycopersicon esculentum), and watermelon
(Citrullus lanatus) crops in the Indian subcontinentref.
Chilli
pepper (Capsicum annuum), an important crop on the Indian subcontinent,
often shows symptoms similar to tomato leaf curl, such as yellowing, leaf
curling, reduction in leaf size and stunting. Since chilli and tomato crops
overlap in the field, chilli peppers may become infected with tomato begomoviruses.
Tomato-infecting begomoviruses are particularly damaging to solanaceous
crops. In the 1970's there were only 3 tomato-infecting begomoviruses in
the Americas, but at present there are at least 14 new ones, of which 7
are distinct ToLCNDV species. Recombination or pseudorecombination are
driving forces in the evolution of new viruses, especially in tropical
regions. Disease management of ToLCV depends in part on preventing movement
of Bt-infested plants (e.g.. tomato transplants) to virus-free areas, where
the virus can become established and implementation of phytosanitary procedures.
Various control options include removal of infected plants (roguing) and
removal or burial of infected crop residues and intercropping in combination
with chemical insecticides and use of available resistant cultivars. Use
of plastic UV-absorbing screening material to exclude Bt is another method.
Genetic resistance to begomoviruses has been reported in some wild Lycopersicon
species such as L. hirsutum and L. peruvianum which might
be transferred to tomato. In Pakistan, resistance to leaf curl virus has
been incorporated into tomato and chili cultivarsref1,
ref2,
ref3,
ref4.
A whitefly-transmitted begomovirus originating in sponge
gourd (Luffa cylindrica) in Northern India has been shown to
be closely related to ToLCV-NDe. What is interesting is that the new virus
is a relatively rare example of a sap-transmissible begomovirus. Based
on genome sequence analysis, it is closely related to ToLCV-NDe but less
closely related to other ToLCV isolates/strains. At least 3 other begomovirus
strains/isolates have been reported from Asia. The one associated with
cucumber yellow leaf curl disease in Thailand shares 95% nucleotide sequence
similarity with the DNA-A of ToLCV-NDe, and pumpkin yellow vein mosaic
virus is most closely related to the same virus.
In June 2003, symptoms of stunting and leaf curling resembling symptoms
of tomato leaf curl disease, as well as reductions in yields, were observed
on tomato plants in the western (Combani and Kahani) and eastern (Dembeni,
Kaoueni, and Tsararano) regions of Mayotte, a French island in the Comoros
Archipelago located in the northern part of the Mozambique Channel. The
whitefly, Bemisia
tabaci, was observed colonizing tomato plants and other vegetable
crops at low levels
there are 7 ToLCNDV isolates that have been reported from India and 2 from
Thailand. Recombination or pseudo-recombination are driving forces in the
evolution of new begomoviruses, especially in tropical regions. Disease
management of ToLCNDV depends in part on preventing movement of Bt-infested
plants (e.g.. tomato transplants) to virus-free areas. Various control
options include removal of infected plants (roguing), removal or burial
of infected crop residues and intercropping in combination with chemical
insecticides and the use of available resistant cultivars. Use of plastic
UV-absorbing screening material to exclude Bt is another method. Genetic
resistance to begomoviruses has been reported in some wild Lycopersicon
species such as L. hirsutum and L. peruvianum, which might
be
transferred to tomato. In Pakistan, resistance to leaf curl virus has
been incorporated into tomato and chili cultivarsref1,
ref2,
ref3,
ref4,
ref5,
ref6
tomato
severe leaf curl virus (ToSLCV) : strains have been reported from Mexico,
Guatemala, Honduras, Nicaragua, and Cuba. Cucumber (Cucumis
sativus) and tomato
(Lycopersicon esculentum) are natural hosts. These strains likely
originated in Central America and the Caribbean region. The genomes of
these viruses are either monopartite or bipartite, the viruses are transmitted
by whiteflies (Bemisia
tabaci), and they infect a range of dicotyledonous plants. Disease
management involves procedures to delay infection, regulated applications
of insecticides, use of biopesticides (parasitoids and predators), and
planting resistant cultivars if they are available.
=> tomato yellow leaf curl (TYLC)ref1,
ref2
is one of the most devastating viral diseases of cultivated tomato in tropical
and subtropical regions. It is spread efficiently by an insect vector (Bemisia
tabaci [Bt]), and the difficulty of its management is compounded
by the existence of distinct biotypes, of which the 'B' and 'Q' biotypes
are of key interest in southern Europe. The disease is difficult to identify
because of great variation in symptom expression. Infected seedling transplants
may serve as the route for long-distance spread of the virus and, when
introduced into areas of high Bt populations, results in extensive and
rapid spread of the virus. High levels of resistance to TYLCV were detected
in 7 of 9 accessions of Lycopersicon
peruvianum and in all 5 accessions of Lycopersicon
chilense tested. In contrast, plants of 7 accessions of Lycoperison
hirsutum and 3 of 4 accessions of Lycopersicon
pimpinellifolium were highly susceptible. Plants of accession CIAS
27 (L. pimpinellifolium) showed moderate resistance to TYLCVref1,
ref2,
ref3,
ref4,
ref5,
ref6,
ref7.
Its introduction into the region is an excellent example of the interaction
between the whitefly vectors (Bemisia
tabaci [Bt] and Bemisia
argentifolii) and tomato that results in rapid spread of a geminivirus.
Bt is an efficient vector of TYLCV; susceptible host plants, such as squash,
can serve as virus reservoirs. TYLCV acquired during a short period of
time by 1-2 day-old Bt adults remain associated with the insect for several
weeks and were detectable for the life of the insect. Although infectivity
decreased with age, Bt was able to infect test plants for > 4 weeks. It
is present in most Mediterranean countries and parts of sub-Saharan Africa,
Middle East, South East Asia, Japan, Australia, Central America, Mexico,
the Caribbean Islands (including Guadeloupe), and locally in the U.S. states
of Florida, Georgia, and Louisiana. Tomato yellow leaf curl disease is
well-established in Reunion Island after 7 years of cropping : Bt predominates
on the island, but some specimens belonged to another biotype (also present
in Madagascar, Mauritius, Seychelles and designated biotype Mascareignes).
Genetic resistance to TYLCV in tomato cultivars appears to controlled by
at least 5 genes, and crossing experiments yielded only tolerant hybrids.
Disease losses can be catastrophic, leading to complete loss. Disease management
includes the use of available resistant cultivars, use of virus-free transplants,
control of the whitefly vector with chemical insecticides, application
of strict phytosanitary measures, avoidance of peak times of vector activity,
changes in cultural practices such as the control of between-season, alternate
hosts, as well as the identification of resistance sources and the production
of transgenic plants containing resistant genes from wild speciesref1,
ref2,
ref3,
ref4,
ref5,
ref6.
tomato yellow leaf curl Morondava virus (TYLCMV) isolates from Madagascar
tomato leaf curl virus (TLCV-Aus) is a begomovirus that has caused
disease in tomato in Australia since 1971 and is described and mapped in
the A2 list of EPPO for TYLCV and similar viruses. TLCV-Aus is known to
be very similar to a strain of TYLCV from Thailand (TYLCV-Thai). It remains
to be seen which of the multiple strains of TYLCV this new Australian isolate
resembles. A summary of the Australia begomoviruses and a set of diseased
tomato photographsref.
Recent outbreaks of begomoviruses causing severe disease in tomato in Indonesia,
Uganda and South Carolina USA
Tomato plants are severely stunted with shoots becoming erect. Leaflets
become reduced in size and pucker. Leaves curl upwards, become distorted,
and have prominent yellow margins. Flowers wither or appear normal, and
the fruits that set either show no symptoms from the viral infection or
they may be small, dry and unsaleable when infections come early in the
season. While it is impractical to completely eradicate a vector-borne
viral disease, a combination of production practices may minimize the impact
of the disease. These include using disease-free transplants, roguing
the infected plants (early in the season), and managing whitefly populations
using various insecticides or reflective mulches. The Ugandan virus is
similar to TYLCV from Dembeni, Mayotte, Comoros Islands but differs from
other African strains. The Indonesian virus is similar to pepper yellow
leaf curl Indonesia virus and an eggplant isolate of TYLCV from Thailand.
The USA - South Carolina virus is similar to TYLCV isolates from Florida,
the Dominican Republic, Cuba, Guadeloupe, and Puerto Rico. The combination
of reports points out the genetic diversity of this virus family. The USA
find in South Carolina seems to be an example of the introduction of a
virus from another state on transplanted seedlings, in this case tomato
and pepperref1,
ref2,
ref3,
ref4
and plant immunology
