Researchers discover genes resistant to soybean pathogen

July 18, 2013  


WEST LAFAYETTE, Ind. - Purdue University researchers have identified two genes within the soybean genome that are highly resistant to a soilborne pathogen that causes Phytophthora root and stem rot, a disease that costs U.S. soybean growers more than $250 million annually in lost yield.

The discovery, made by a team of scientists led by Jianxin Ma and Teresa Hughes, could lead to the development of soybean cultivars better able to withstand the pathogen Phytophthora sojae. The Purdue research was published online by Theoretical and Applied Genetics and is to appear in the journal's November print edition.

Naturally occurring Phytophthora sojae resistance exists in soybean germplasm. Most previous resistant genes, however, have lost their ability to fight off the pathogen, which has developed immunity to them. Together, the two newly identified genes appear stronger than most earlier genes and 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 sojae 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. The pathogen also produces spores that can remain dormant in soil through the winter and become active when warm weather returns.

Even in normal crop years Phytophthora sojae is responsible for 8-15 percent crop loss nationwide.

Because the soybean plant's own genetic resistance to Phytophthora sojae 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. But the Purdue team made its discovery looking for a genetic answer to another soybean problem, said Hughes, a U.S. Department of Agriculture 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 its three years of study the Purdue researchers have developed molecular "markers" - identifying tags - that can be used to expedite the transfer of the resistant genes to soybean cultivars. That process is 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 sojae eventually could render the two resistant genes ineffective, the pathogen itself likely would become much weaker, Hughes said.

"Every time a pathogen overcomes resistance in its plant host it has to give up something itself," she said. "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.

"We believe these genes are durable, but we don't know enough about them yet to predict how effective they could be, and for how long."

Ma, Hughes and collaborating Purdue researchers Scott Abney, Feng Lin, Meixia Zhao, Jieqing Ping, Austin Johnson and Biao Zhang plan to continue their research. They next hope to move their work from greenhouses and into field trials. After that the resistant lines could make their way into commercial cultivars.

"This has the potential to provide a higher profit margin for soybean farmers, as well as reducing the use of harmful chemicals and promoting a cleaner environment," Ma said.

The Theoretical and Applied Genetics paper, "Molecular mapping of two genes conferring resistance to Phytophthora sojae in a soybean landrace PI 567139B," can be viewed at http://link.springer.com/content/pdf/10.1007%2Fs00122-013-2127-4.pdf

The Purdue research was supported by checkoff funds from the Indiana Soybean Alliance.

Writer: Steve Leer, 765-494-8415, sleer@purdue.edu

Sources: Jianxin Ma, 765-496-3662, maj@purdue.edu

Teresa Hughes, 765-496-1843, hughestj@purdue.edu


ABSTRACT

Molecular mapping of two genes conferring resistance to Phytophthora sojae in a soybean landrace PI 567139B

Feng Lin, Meixia Zhao, Austin Johnson, T. Scott Abney, Teresa J. Hughes, Jianxin Ma

Phytophthora root and stem rot (PRR), caused by the soil-borne oomycete pathogen Phytophthora sojae, is one of the most destructive diseases of soybean. PRR can be effectively controlled by race-specific genes conferring resistance to P. sojae (Rps). However, the Rps genes are usually non-durable, as populations of P. sojae are highly diverse and quick to adapt, and can be overcome 8-15 years after deployment. Thus, it is important to identify novel Rps genes for development of resistant soybean cultivars. PI 567139B is a soybean landrace carrying excellent resistance to nearly all predominant P. sojae races in Indiana. A mapping population consisting of 245 F2 individuals and 403 F2:3 families was developed from a cross between PI 567139B and the susceptible cultivar 'Williams,' and used to dissect the resistance carried by PI 567139B. We found that the resistance in PI 567139B was conferred by two independent Rps genes, designated RpsUN1 and RpsUN2. The former was mapped to a 6.5 cM region between SSR markers Satt159 and BARCSOYSSR_03_0250 that spans the Rps1 locus on chromosome 3, while the latter was mapped to a 3.0 cM region between BARCSOYSSR_16_1275 and Sat_144, approximately 3.0-3.4 cM upstream of Rps2 on chromosome 16. According to the 'Williams 82' reference genome sequence, both regions are highly enriched with NBS-LRR genes. Marker assisted resistance spectrum analyses of these genes with 16 isolates of P. sojae, in combination with the mapping results, suggested that RpsUN1 was likely to be a novel allele at the Rps1 locus, while RpsUN2 was more likely to be a novel Rps gene.


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