If you owned one pair of blue jeans and wore them every day for two decades, you'd expect some wear and tear.
In much the same way, the genetic resistance you depend on to protect against soybean cyst nematode (SCN) is fraying. Some 95% of the commercially available SCN-resistant varieties depend on a single source of resistance called PI 88788.
"In the last decade, we've seen a reduction in the effectiveness of PI 88788 in the prime soybean-growing areas," says Kaitlyn Bissonnette, a University of Missouri plant pathologist. "Soybean farmers need to be clamoring for increased access to new sources of varietal resistance."
Soybean breeders have been working for years to insert other types of genetic resistance into elite soybean varieties. PI 548402 (Peking), PI 90763 and PI 437654 (Hartwig) are the most promising, but they are much more difficult to work with than the PI 88788 variety.
Brian Diers, a University of Illinois plant breeder, explains that PI 88788 involves one major gene: Rhg1. Peking, on the other hand, involves two genes: Rhg1 and Rhg4. It's simply harder for breeders to work with two genes, and it takes time to achieve yield parity.
"But, as you continue cycles of breeding, you are able to incorporate these genes more readily into elite, high-yielding lines," Diers says. Better genetic markers also help breeders select the genes needed and speed varietal development.
VARIETY ACCESS GROWS
Soybean growers, particularly in the Midwest, are seeing more soybean varieties enter the market with the Peking source of resistance, offering farmers more opportunities to rotate sources of varietal resistance. "We have a large amount of evidence showing that this reduces selection pressure on SCN populations to continually adapt," Diers says.
More cultivars with PI 437654 (Hartwig) are coming, too. Diers says his Illinois program released two high-yielding lines, which were commercialized by companies through licenses from the university.
Diers' team also recently released a variety with a three-gene stack. "We combined the two resistance genes from wild soybean with Rhg1 from PI 88788 and have shown that this combination gives greater resistance than Rhg1 alone," he says.
He's also developed a four-gene stack -- two new resistance genes from wild soybean stacked with Rhg1 from PI 88788, plus another resistance gene from PI 567516C. "If you look in the literature, there are many SCN-resistance genes that have been mapped," he says. "We worked on the gene from PI 567516C because it can give a greater increase in resistance than most other genes identified."
ANOTHER TYPE OF ROTATION
Farmers battling aggressive nematode populations should note that not all varieties containing PI 88788 are created equal. Simply rotating between soybean varieties may be helpful, Diers notes.
"Varieties derived from PI 88788 resistance do not all have the same level of resistance, and this may be related to the number of copies of the Rhg1 gene. There are normally 10 copies of the Rhg1 gene in varieties with PI 88788 resistance, but some may have fewer copies. With PI 88788, the higher the copy number, the higher the resistance," he explains.
For More Information:
> The SCN Coalition: www.thescncoalition.com
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