Soybeans like most plants pair with soil fungi in a symbiotic mycorrhizal relationship. In exchange for a bit of sugar, the fungus acts as an extension of the root system to pull in more phosphorus, nitrogen, micronutrients and water than the plant could on its own.
Mycorrhizal fungi occur naturally in soil and are commercially available as soil inoculants. But new research from the University of Illinois suggests not all soybean genotypes respond the same way to their mycorrhizal relationships.
In our study, root colonization by one mycorrhizal species differed significantly among genotypes and ranged from 11 percent to 70 percent. To arrive at that finding we grew 350 diverse soybean genotypes in pots filled with spores of a common mycorrhizal fungus. After six weeks we looked at the roots under a microscope to evaluate the level of colonization.
It was a little bit of a gamble because we didn't know much about soybean’s relationship with mycorrhizae. We didn’t know if differences in colonization among the soybean genotypes would occur. So when we screened the soybean genotypes and found differences, it was a big relief. That meant there was also a potential to find genetic differences.
The process of root colonization starts before fungal spores even germinate in the soil. Roots exude chemicals, triggering spores to germinate and grow toward the root. Once the fungus makes contact, there’s a complex cascade of reactions in the plant that prevents the usual defensive attack against invading pathogens. Instead the plant allows the fungus to enter and set up shop inside the root. There it creates tiny tree-like structures known as arbuscules; those are where the fungus and plant trade sugar and nutrients.
The study suggests there is a genetic component to root-colonization rates in soybeans. To find it we compared the genomes of the 350 genotypes. We honed in on six genomic regions associated with differing levels of colonization in soybean. We were able to use all the information we have on the soybean genome and gene expression to find possible causal genes within those six regions.
According to our study the genes control chemical signals and pathways that call fungus toward roots. That allows the plant to recognize mycorrhizal fungus as a “good guy,” help build arbuscules and more. For almost every step in the colonization process we were finding related genes within those regions.
Knowing which genes control root colonization could lead breeders to develop soybean cultivars with a better affinity for mycorrhizal fungus. That could mean improved nutrient uptake, drought tolerance and disease resistance.
“This environmentally friendly approach to improving soybean production may also help reduce the overuse of fertilizers and pesticides, and promote more-holistic crop-production systems,” said Glen Hartman, plant and crop pathologist in the U.S. Department of Agriculture-Agricultural Research Service’s Department of Crop Sciences.
The article, “Whole-genome resequencing identifies quantitative trait loci associated with mycorrhizal colonization of soybean,” is published in” Theoretical and Applied Genetics.” Authors are Michelle Pawlowski, Tri Vuong, Babu Valliyodan, Henry Nguyen and Glen Hartman. Funding was obtained from the United Soybean Board and the USDA Agricultural Research Service.