Chicago, Illinois
July 8, 2007
In research that will be presented
at the annual meeting of the
American Society of Plant Biologists (ASPB) in Chicago (July
7-11, 2007), South African scientists at the
University of Cape Town
developed a genetically engineered (GE) maize variety that
resists infection by the highly destructive maize streak virus
(MSV), which can destroy maize crops over a large area. Maize is
an important African food crop that can supply 50% of daily
caloric intake. MSV is transmitted by a leafhopper, and
infection of susceptible plants produces dwarfed plants and
deformed cobs. The transgenic maize has proved consistently
resistant to MSV, and the trait is reliably passed on to the
next generation and in crosses with other varieties. Field
trials will begin soon to test the effectiveness and safety of
this new GE maize. The technology can potentially be adapted to
other crops that are also infected by geminiviruses like MSV.
Maize streak viruses (MSV),
geminiviruses that can destroy most of a maize crop, are endemic
to sub-Saharan Africa and adjacent Indian Ocean islands where
they are transmitted by leafhoppers in the genus Cicadulina.
Maize can supply 50% of the caloric intake in sub-Saharan Africa
but, in certain years, a farmer’s entire crop can be wiped out.
Now, scientists at the University of Cape Town, South Africa,
along with colleagues at the South African seed company,
PANNAR Pty Ltd, have
developed a resistant variety of maize that they hope will help
alleviate food shortages as well as promote the reputation of GE
technology in Africa. Dr. Dionne Shepherd of the University of
Cape Town will be presenting the results of her recent work and
that of coauthors B. Owor, R. Edema, A. Varsani, D.P. Martin,
J.A. Thomson and E.P. Rybicki, at the annual meeting of the
American Society of Plant Biologists in Chicago (July 8, 2007,
11:20 AM) in a major symposium on Plant Biology in Sub-Saharan
Africa organized by Debby Delmer of UC Davis.
Maize, which originated in Mexico, was carried to Africa in the
1500s and eventually displaced native food crops such as sorghum
and millet. Maize streak virus, an endemic pathogen of native
African grasses, was then carried to maize plants by
viruliferous leafhoppers. African scientists have been working
for more than a quarter century on developing resistant
varieties of maize by selecting and crossing varieties with
various degrees of resistance to the virus. However, resistance
requires multiple genes located on different chromosomes, so the
process is not straightforward. The group at the University of
Cape Town took the opposite approach. They mutated a viral gene
that encodes a protein that the virus needs to replicate itself
and inserted it into maize plants. When the virus infects one of
these transgenic maize plants, the mutated protein, which is
expressed at a high level, prevents the virus from replicating
and killing the plant. The transgenic maize variety has proven
consistently resistant to MSV and the trait can be reliably
passed on to the next generation and in crosses to other
varieties. Field trials are scheduled to begin soon, not only to
test the effectiveness of the technology in the field but also
to ensure that the GE maize variety has no unintended effects on
beneficial organisms that may feed on it. The resistant maize
will also be tested to ensure that the viral protein is
digestible and non-allergenic. The MSV-resistant maize is the
first GE crop developed and tested solely by Africans.
This group of scientists also surveyed 389 Ugandan MSV isolates
to assess the diversity and genetic characteristics of this
destructive pathogen. They found that the most prevalent strain
of this virus is a product of recombination of different viral
genotypes, thus identifying an important source of new
pathogenic variants and illustrating the constantly changing
evolutionary battle between plants and pathogens. MSV was first
sequenced in 1984 and found to contain a genome of only 2700 DNA
bases in a circle of single-stranded DNA. When it infects
susceptible plants, they produce deformed cobs and are often
severely dwarfed. As the name of the virus suggests, the leaves
are marked with parallel, yellow-white streaks.
The timing of infection, the maize genotype, and prevailing
climatic conditions can all influence the extent of damage
wreaked by this viral pathogen. Young plants cannot survive the
infection but older plants are better able to contain the
infection, resulting in smaller losses of grain. However,
drought can have a devastating effect on maize fields over a
wide geographical area. Under warm and wet conditions, a
long-bodied morph of the leafhopper C. mbila emerges, but this
form only travels short distances of 10 meters or less, thus
limiting its damage to crops. Under drought conditions, a
stronger, short-bodied morph that can fly great distances
spreads the disease over large areas, thus exacerbating the
effects of the drought itself.
Disease caused by similar geminiviruses, Wheat dwarf virus (WDV)
and various sugarcane streak viruses, also affect other crops,
including barley, wheat, oats, sugarcane, and millet. Thus, the
technology developed for MSV could potentially be adapted to
develop resistance in these other crops. Virologist Edward
Rybicki and microbiologist Jennifer Thomson are hopeful that
this year’s field trials will demonstrate not only the
effectiveness of this technology in producing resistance to a
destructive pathogen but also the safety of GE technology. Part
of the objective is to provide seed that will be sold at a
minimal profit to subsistence farmers.
ASPB, headquartered in Rockville, Maryland, was founded in
1924. This professional society has a membership of nearly 5,000
plant scientists from the United States and more than 50 other
nations. ASPB publishes two of the most widely cited plant
science journals in the world: The Plant Cell and Plant
Physiology.
RELATED RELEASE:
South African crop scientists develop maize
streak virus resistant maize variety
Other news
from Pannar |
|
