Scientists have developed a transgenic maize variety resistant to
maize streak virus. The transmission of MSV by a leafhopper is exacerbated
during drought conditions, resulting in devastated crops over large areas.
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 genetically engineered
(GE) foods 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 on
July 8.

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 foods. Part of the objective is to provide seed that will
be sold at a minimal profit to subsistence farmers.



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