WOOSTER, Ohio – A new way of characterizing partial resistance to one of the most devastating soybean diseases may enable germplasm companies to incorporate effective genes more quickly into plant lines that are the most beneficial to growers.
Ohio State University plant pathologists and soybean breeders are teaming up with researchers from the Virginia Bioinformatics Institute and the Department of Crop and Soil Environmental Sciences at Virginia Polytechnic Institute to study the mechanisms of partial resistance to Phytophthora sojae – the pathogen that causes Phytophthora root and stem rot. With a five-year, $6.74 million National Science Foundation grant, the team is evaluating 289 genetic lines from a Virginia plant population to identify those with partial resistance. The researchers are using microarray chips – technology that produces an instant readout of which genes might be most useful in producing germplasm with high levels of partial resistance.
Anne Dorrance, an Ohio State plant pathologist with the Ohio Agricultural Research and Development Center, said that using microarray chips produces a more informative result than the standard marker technology.
"A standard marker amplifies a region in a genome where resistance is expressed, and then maps that area. At most, we know where the region is but not what the mechanisms are that control this trait," said Dorrance. "With microarray chips, we can map over 30,000 genes at the same time and know instantly which genes are involved in partial resistance and which are not. It allows us to more quickly target which genes are important for companies to incorporate in their germplasm and get it into the hands of growers faster."
The technology also enables researchers to better understand the mechanisms of gene expression and how partial resistance to Phytophthora works. Dorrance, who also holds a partial Ohio State University Extension appointment, said that partial resistance is more durable, more consistent and more effective in controlling Phytophthora than single resistance genes alone. A combination of the two will give growers the best protection.
"The 'R' genes only have a certain life span. Using these R-genes wisely will get the longest length of time out of genes, but eventually plants with just R-gene resistance will no longer be effective against the disease," said Dorrance. "High levels of partial resistance helps maintain yields across disease pressures and disease locations. Of course we continue to identify R-genes because if you have both partial and R-gene resistance in a plant, you aren't going to see losses to Phytophthora."
Single-resistance genes, like the newly discovered Rps8, work by killing the pathogen before it ever has a chance to establish in the plant. However, if the pathogen is not detected by the resistant gene, then that gene becomes ineffective and the plant succumbs to disease.
"It's the reason why so many single-resistant gene packages, specifically Rps1a, Rps1b, Rps1c, Rps1k, Rps3a and Rps6, are no longer able to control Phytophthora in many Ohio fields," said Dorrance.
Partial resistance genes allow Phytophthora to colonize a soybean plant, but only to a certain extent, keeping the disease at bay and preventing it from killing the plant as long as resistance is high enough.
"Partial resistance basically means that the pathogen has little effect on the plant once it has grown up and out of the ground," said Dorrance. "Partial resistance varieties can be very effective, sometimes having a 30 percent difference in yields compared to soybean plants that have no resistance to Phytophthora at all, depending on the disease pressure."
One advantage of partial resistance genes is that, unlike single-resistance genes, they are not race specific, meaning that partial resistance works against any Phytophthora isolate that exists. The result is partially resistant soybean cultivars that yield consistently, no matter what race of Phytophthora may be present in a particular field.
Phytophthora is a major problem in Midwest states that have heavy clay soils, such as Ohio. Heavy rains saturate the soil producing areas with standing water, which provides an outlet for the pathogen to infect plant roots. This water mold grows in the roots and into the plant stem, eventually killing the plant. Economic losses to Phytophthora can be as high as $120 million in any given year, with yield reductions ranging from five to 30 bushels per acre depending on variety.