Traits Get Tricky

October 27, 2010 09:27 AM

Planting soybean cyst nematode (SCN) varieties is a no-brainer for Howard Haas. The Algona, Iowa, grower says that modern varieties containing SCN resistance yield better today than those without built-in protective traits.

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University of Missouri soybean breeder Grover Shannon has bred some new varieties with sources of resistance that are helping Southern growers.

Still, Haas and almost every other soybean grower face a challenge in their dependence on resistant varieties. SCNs are getting wily and have begun to adapt to common sources of resistance. The dilemma has researchers and seed companies racing to pursue native and transgenic approaches to multi-race SCN resistance.

Part of the problem is that the same resistant soybean cultivars have been used for breeding since the first detection of SCN in 1954. Although more than 100 sources of SCN-resistant germplasm are known, the primary source today is PI-88788. Another trait source, Peking, runs a distant second. Because of this lack of diversity, the suggestion that growers rotate to varieties with unique sources of resistance is difficult to carry out.

State of the industry. Ask Bill Rhodes, vice president of research for eMerge, why the seed industry depends on limited cyst-resistant germplasm and you’ll get a look of exasperation.

“It boils down to one word—yield. Varieties with PI-88788 yield more than varieties with other sources of SCN resistance, even when they are growing in a susceptible environment. Yield is still the bottom line for the grower,” says the plant breeder.

Aaron Robinson, Monsanto soybean product development manager, says the reason most commercial varieties have SCN resistance derived from PI-88788 is that it doesn’t bring the negative agronomics that other sources of resistance come with.

“Marker-assisted breeding and selection are helping us get past some of those negative aspects with alternative sources of resistance,” Robinson says. Monsanto intends to introduce a few varieties with new sources of SCN resistance in the near future.

“If varieties with SCN resistance derived from other sources aren’t available, we recommend rotating varieties. There are different levels of resistance expressed within PI-88788,” he says.
John Soper, Pioneer Hi-Bred crop genetics research and development vice president, estimates that 90% or more of the SCN varieties on the market today contain PI-88788 resistance. “We’ve been doing work in recent years, particularly in the early Group I, Group II and Group III maturity ranges, with Peking,” he says.

“Going forward, it’s pretty obvious that SCN will continue to be dynamic in terms of its race structure, and we’re looking at additional sources of resistance for the future,” Soper says.

New hope. PI-437654, the source of resistance in the public variety Hartwig and the CystX lines, provides another source of resistance. However, plant breeders have struggled to get multiple genes within this soybean line into varieties without sacrificing yield.

University of Missouri soybean breeder Grover Shannon used Hartwig to breed two public, mid–Group V maturity, conventional releases called Jake and Stoddard. “Although all of the resistance was not captured from Hartwig, resistance is good enough that these releases give Southern growers SCN options they’ve not had in the past without yield penalties,” he says.

Jake and Stoddard are popular because they both have broad resistance to SCN and root knot nematode. Jake also carries resistance to reniform nematodes. Shannon is working to
incorporate the broad SCN resistance and resistance to other nematode species into elite Group IV or earlier-maturing soybean lines.

“Further enhancement to reduce losses to SCN could be accomplished by pyramiding genes from other unique resistance sources with Hartwig-type resistance,” he says. “That might lead to the development of soybean cultivars with more durable resistance to the range of genetically variable field populations of SCN.”

Tough row. Growers need to understand that resistance to SCN is a difficult problem for scientists to solve, says Vincent Klink, a Mississippi State University biologist.

“SCN is a species composed of at least 16 different races—also known as populations or virulence types—that infect soybeans,” he says.

“Many fields will have more than one SCN race in them. One race may be dominant with other minor races present in any given field,” Klink explains. “Continued planting of the same variety year after year allows for a proliferation of the minor SCN races.”

What makes this race shift possible is that each individual race of SCN is equipped with a toolbox of substances that it injects into a specific cell type in the root that allows infection. “Different SCN races have unique sets of substances that can presumably be used in various combinations to infect different soybean cultivars,” Klink says.

“In any soybean field that has both major and minor races of SCN, there will always be enough genetic variation in SCN to overcome the resistance that is bred into soybean cultivars. That’s why it’s so critical to manage SCN by using different varieties bred for different SCN races.”

Genetic optimism. The two major tools that are helping molecular biologists combat SCN are the soybean genome and the SCN genome.

“The sequenced soybean genome has aided in the understanding of where some resistance genes are located. It’s important to know that the soybean cultivar used to sequence its genome is fully susceptible to SCN,” Klink says.

He says there are five major regions, known as loci, in the soybean genome that provide most of the resistance bred into commercial cultivars.

“We have learned that certain genes have been duplicated or deleted in regions in and around the resistance loci in soybean cultivars with resistance,” Klink notes. “This is an area of intense research activity.”

Only part of the SCN genome is sequenced. “There is a very specific way SCN gene sequences can be used to engineer resistance by biologically inactivating SCN genes,” Klink says. Known as RNA interference, it may one day be used to genetically alter plants to combat all SCN races.

Using methods adapted from cancer research, Klink and fellow researchers have found a genetic footprint for susceptible and resistant SCN reactions.

“There are soybean genes and gene pathways activated only during the resistant reaction. Genes can be manipulated in specific ways to determine how they function, and it’s likely some are resistance genes,” he says.

Soybean growers like Haas welcome all the help they can get.

“We’d love to see new trait solutions, but they will need to come to the farm without a yield drag,” he says.

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