The discovery of two genes in the soybean genome that are highly resistant to a soilborne pathogen could save U.S. soybean growers millions of dollars in yield lost annually, according to new research published by Theoretical and Applied Genetics.
Purdue University researchers identified the genes—which resist root and stem rot from a disease called Phytophthora—through a study led by Jianxin Ma and Teresa Hughes. The new discovery could lead to the development of soybean cultivars that better withstand Phytophthora sojae, which costs growers more than $250 million each year.
Naturally occurring Phytophthora resistance exists in soybean germplasm, but most previously resistant genes have lost their ability to fight off the pathogen because of Phytophthora's developed immunity. The two newly identified genes appear stronger than most earlier genes, and they could remain viable for many more years, said Ma, a soybean geneticist in Purdue's Department of Agronomy.
"These two genes demonstrate resistance to all the predominant isolates of this pathogen found in Indiana, and many other isolates that are virulent to previously identified resistance genes," he said. "If these two genes are effectively used in Indiana and other Midwest soybean crops, an annual net increase in soybean production would be anticipated."
Phytophthora has been a problem for Indiana soybean farmers since it was first found in the state in 1948. The pathogen thrives in wet, cool conditions and produces spores that move in water and onto soybean roots. Diseased roots form lesions that can move up the stem and kill the entire soybean plant. In addition, the pathogen produces spores that can remain dormant in soil through the winter and become active when warm weather returns.
Even in normal crop years, the disease causes 8% to 15% of the crop loss nationwide.
Because the soybean plant's own genetic resistance to Phytophthora has proven to be the best way to control the pathogen, the mapping of the soybean genome in recent years has improved the odds of finding other resistant genes. However, the Purdue team made its discovery looking for a genetic answer to another soybean problem, said Hughes, a U.S. Department of Agriculture (USDA) plant pathologist and adjunct professor in Purdue's Department of Botany and Plant Pathology.
"We were originally looking for possible resistance to Asian soybean rust," she said. "Our experimental locations had high Phytophthora pressure, and we found that these genes did very well against that disease. That was our first clue that they might have good resistance to Phytophthora sojae."
During the three-year study, researchers developed molecular "markers," or identifying tags, to expedite the transfer of the resistant genes to soybean cultivars in a process known as marker-assisted selection.
"There are about 46,000 predicted gene models in what we call the reference soybean genome," Ma said. "These markers allow rapid pyramiding of multiple resistant genes into a single cultivar in order to boost the effectiveness of resistance."
Although Phytophthora eventually could render the two resistant genes ineffective, the pathogen itself would likely become much weaker, Hughes said.
"Every time a pathogen overcomes resistance in its plant host, it has to give up something itself," she added. "So if it turns out that in order for the pathogen to overcome this new resistance, it ends up having a fitness penalty—for instance, it can't compete as well or it doesn't survive as long in the soil— then these genes will last longer."
Hughes also said the team believes the genes are durable, but they do not have enough research to predict how effective they will be, or for how long.
The agricultural industry may also benefit from a newly discovered gene that gives wheat plants resistance to a deadly wheat stem rust pathogen.
