THE SPREAD of the soybean cyst nematode has, since at least the middle of the 1950’s, been a significant drag on what has otherwise been a magnificently productive period for international soybean cultivation.
These tiny, parasitic pests — a type of roundworm whose destructive life cycle was detailed in the Fall/Winter 2010 issue of Illumination — make their homes in the roots of soybean plants, creating breeding and feeding sites that eventually cause the plants above to wither. According to the USDA, cysts typically cause close to $1 billion in American soybean crop losses each year.
For now, efforts to combat infestations consist chiefly of crop-management strategies intended to limit roundworm reproduction. One option involves cultivating varieties of soybeans that naturally resist cyst nematodes. But even this is tricky. Not all resistant soybeans are unappetizing to all types of nematodes, so “race matching” is essential. And exclusive planting of soybean cyst nematode-resistant varieties, particularly varieties deriving their resistance from the same genetic source, has been shown to “select” for virulent nematodes; that is, the pests quickly adapt to feeding and breeding on previously resistant varieties.
So what to do? Melissa Mitchum, a molecular nematologist and associate professor of plant sciences at MU’s Bond Life Sciences Center, has spent the better part of the past decade working to answer that question. The way forward, she says, seemed relatively simple: discover the genes that confer the highest form of resistance, then engineer varieties that couple resistance with high yields. But making it happen proved difficult. “Nine years ago, when I began investigating the molecular basis of soybean resistance to nematodes in an effort to identify the genes involved, I never imagined it would be this complex,” Mitchum says.
In October, Mitchum and research collaborator Khalid Meksem, an associate professor at Southern Illinois University, announced in the journal Nature that they had at last accomplished their goal: they had finally isolated the gene. Oddly enough, it turned out to be one already familiar to geneticists everywhere.
“The gene responsible for nematode resistance was completely unexpected. The gene, called serine hydroxymethyltransferase (SHMT), is common in nature and found in different kingdoms, including both animals and plants. In humans, mutations in the SHMT gene can lead to a deficiency of folate, a B vitamin that is essential to the production and maintenance of cells, and this has been linked to a variety of diseases.”
Demonstrating that SHMT was indeed responsible for resistance involved a great deal of high-tech heavy lifting. First, Mitchum, Meksem and their respective research teams pitched in to discover the location of the gene in the soybean genome. Next they identified resistant soybean plants with a mutated form of the SHMT gene. Those plants, they found, were no longer resistant to nematodes. In another experiment, they shut down the SHMT gene using gene-silencing techniques. These soybeans, too, became susceptible. A third test put the resistant form of the SHMT gene into susceptible soybeans. These became resistant.
“Plant breeders can put our results to use immediately,” Mitchum says. “For farmers, developing new forms of resistance to SCN in soybeans can’t come soon enough. Nematodes are developing their own ways around natural defenses. Hopefully, our discovery has paved the way to enhance the durability of resistant varieties of soybean.”