Soybean roots sit under fluorescent light in rows of petri dishes, which may seem unremarkable to anyone except those who work in this laboratory. The simplicity of the setup belies the complexity of the research that went into creating the roots, according to an announcement from The Ohio State University.
For decades, the genes of the seeds that produced these roots have been tinkered with to create a plant that resists a common and highly destructive soybean disease: Phytophthora root and stem rot. Across Ohio, the disease accounts for $50 million in losses every year, ranking it as one of the top three most vexing soybean diseases for Ohio farmers. Phytophthora sojae thrives in wet, warm soil, particularly poorly draining soil, which is common in northwest Ohio.
The soybean roots growing in Leah McHale’s lab at Ohio State might have the genes to fend off the disease, but the answer is at least two years away from being determined.
In a month or so, the soybean roots will be exposed to phytophthora and then watched to see whether they acquire the disease and die. If their resistance changes — getting better or worse — that means McHale and her colleagues, including soybean pathologist Anne Dorrance, have identified another gene that affects resistance to phytophthora. That gene then becomes a building block in creating a new variety of soybean seed that can thwart the disease, the announcement said.
A soybean plant carrying that newly identified gene improving resistance will be crossed with another soybean plant with that same gene. The resulting seeds next will be grown into plants and tested for their disease resistance.
That process will be repeated for a few more generations of plants grown throughout Ohio and at universities nationwide until McHale and other researchers can definitively say that the plant variety they created will not succumb to phytophthora. The seed from those plants can then go to market for soybean growers.
This journey, which begins in a petri dish and ends with a new variety of seed that resists phytophthora, can stretch five years or more. That’s partly because a soybean has a surprisingly large number of genes — about 66,000, roughly three times higher than the number of genes people have. Soybeans are fairly complex.
“You think plants like soybeans just sit there and grow, but they have to respond to sunlight, lack of rain, pathogens, insects and other circumstances,” said McHale, an associate professor of soybean breeding and genetics in the Ohio State College of Food, Agricultural & Environmental Sciences (CFAES).
Anywhere from 10 to hundreds of different genes are involved in resisting phytophthora. Some genes have more of an influence than others in whether the plant succumbs to the disease. McHale and Dorrance help identify which combination of genes offers resistance to phytophthora and create that combination in a single plant by crossing varieties of soybeans.
In the simplest of breeding schemes, a resistant variety may be crossed with, for example, a high-yielding variety, and the emergent plant is tested to see if it indeed offers desired disease resistance as well as high yields. The DNA of the resulting seeds are then analyzed to determine if they have the set of genes that can fight off the disease. Once that’s done, the seeds with the desired genes are allowed to self-pollinate for several generations, and each resulting plant is tested to see if it can, in fact, fend off phytophthora.
As director of the Center for Soybean Research in CFAES and an international authority on soybean diseases, Dorrance tests the varieties of soybeans McHale creates by exposing them to phytophthora and seeing how they fare. If a plant survives the exposure, it’s closer to going to market for farmers to purchase.
While CFAES researchers have long been studying phytophthora and creating soybean seeds that resist it, the disease continues to evolve. A soybean variety that once resisted phytophthora can succumb to a newly evolved version of the pathogen, Ohio State said, so new varieties of soybean seeds are always needed.
“We always have to try to keep one step ahead of the pathogen,” said Dorrance, who has worked for more than a decade to identify the genes that contribute to phytophthora disease resistance.
After years of research, McHale and Dorrance and their team are in the final stages of locating which of the tens of thousands of genes in the soybean plant are responsible for resistance to phytophthora. “We’ve done a lot to get to this point,” McHale said.
As a result, McHale, her colleagues and their graduate student assistants can express and suppress genes that make up a soybean to see how that affects the plant’s ability to resist the phytophthora pathogen in its roots. The disease attacks the plant's roots first and also can attack the leaves when infected soil particles are splashed or blown onto the leaves in a storm.
Even though researchers know the location of the soybean genes responsible for resistance, the trait for resistance has low heritability, meaning that the environment plays a big part in whether the genes will be expressed or not, even if they are present in the plant.