Mir1 Gene Stalls Corn Pests
Fall armyworms may grow to only half their normal inch-and-a-half size if the
Mir1 gene from corn germplasm lines Mp704 and 708 can be bred into commercial
To pupate from caterpillars into adult moths, fall armyworms and
southwestern corn borers must go on a 10- to 20-day feeding binge. Their fare
is the stalks, leaves, husks, and ears of corn plants. Unchecked, they can
cause losses of 30-plus percent.
But now scientists may have a way to turn the tables on the pests:
genetically engineer the plants so they churn out a rare natural enzyme that
curbs the caterpillars' destructive appetiteso much so, they'd grow to
less than half their normal size and weight.
"The idea is to reduce their ability to chew up the plant and to slow
their reproduction," explains Dawn S. Luthe, a Mississippi State
University (MSU) molecular plant biologist.
Except for so-called tropical corn, domestic hybrids generally lack the
genetic muscle to withstand the pests on their own. Thus the need for
insecticide, especially for sweet corn production, notes Luthes
colleague, W. Paul Williams. He is a plant geneticist in the ARS Corn Host
Plant Resistance Research Unit at MSU, near Starkville.
Weekly insecticide applications can help ensure worm-free ears, he says. But
timing when to spray can be a costly, hit-or-miss proposition.
Intrigued by the prospect of a natural alternative, the researchers pushed
their early investigations of the enzyme furtherwith help from other
researchers and graduate students.
In recent lab studies they used molecular techniques such as PCR (polymerase
chain reaction) and RFLP (restriction fragment length polymorphism) to trace
the enzyme to an elusive gene, Mir1. Until then, this gene had only been
found in callus tissue cultures from two corn germplasm lines, Mp 704 and Mp
708, that were developed at Mississippi State.
In spring of 1996, ARS and MSU applied for a patent on the Mir1 gene
and its enzyme, 33 kd cysteine proteinase. Now Luthe says, Were in
the process of negotiating an option agreement with DeKalb Genetics to explore
commercial potential. The agreement would allow the company to insert the
gene into greenhouse plants and regenerate them to study Mir1s
The scientists envision their new found enzyme would serve as a built-in
caterpillar deterrent in corn and maybe even cotton. "If this is something
that proves to be worthwhile, it could be possible to increase the plant's
concentration of the enzyme," says Williams.
That would ensure a caterpillar ingests a high enough dose that it either
stops feeding or doesnt cause further serious harm. Lab and field studies
suggest this could mean fewer than 50 percent of the pests survive to
On another front, the Mir1 genes enzyme could replace or
prolong the effectiveness of toxin-making genes from the bacterium Bacillus
Bts genes now protect commercial lines of bioengineered corn and
cotton from boll worms, tobacco budworms, and other caterpillar pests. But
concern that pests are genetically adapting to tolerate Bts toxins has
fueled a search for alternatives and counter strategies. Among these is
gene-stacking, or putting more than one resistance gene into a plant.
Its possible that if you could combine these different
genes, says Williams, you could slow development of resistance in
So how long until new, worm-resistant corn hits the farm field? Possibly 2
or 3 yearsif ongoing research and commercial interest hold course.
"It's only going to come if we can provide the gene and associated
technology to the seed companies," Williams says. They would "move
this from research into reality." -- By Jan Suszkiw, ARS.
Williams is in the USDA-ARS Corn Host Plant Resistance Research Unit, Room
340, Dorman Hall - Stone Blvd., Mississippi State, MS, 39762; phone (662)