The relationship between plant roots and soil fungi is a complex symbiotic association in which both organisms can benefit.
Soybeans pair up with mycorrhizal (root) fungi. In exchange for sugar, the fungus acts as an extension of the plant’s root system to draw in phosphorus, nitrogen, micronutrients, and water that the plant cannot reach on its own.
The process of root colonization starts before fungal spores even germinate in the soil. Roots give off chemicals that trigger fungal spores to germinate and then grow toward the root. Once they make contact, a complex cascade of reactions takes place in the plant to prevent what would be a usual defensive attack against an invading pathogen.
The plant, recognizing the fungus, allows it to enter the root where it grows tiny tree-like structures known as arbuscules. It is here that the fungus and plant trade sugar and nutrients.
Scientists at the University of Illinois have shown that some soybean genotypes respond differently to arbuscular mycorrhizal fungi (AMF) as the relationship becomes established.
“If we can understand the relationship between soybean and beneficial microbes better, we may be able to use them more reliably,” said Michelle Pawlowski, postdoctoral fellow in the university’s crop sciences department.
“We wanted to know to what degree does the soybean genotype control its relationship with a beneficial microbe and how can that be applied to increasing efficacy of using beneficial microbes for biological control.”
She said the first step was to screen a diversity panel of soybean genotypes to identify a possible genetic basis to the relationship.
Pawlowski’s team grew 350 soybean genotypes in pots that were filled with spores of a common mycorrhizal fungus. After six weeks, they examined the roots to evaluate the levels of colonization. They found that the root colonization by just one mycorrhizal species ranged among those soybean genotypes from 11 to 70 percent.
Pawlowski said multiple genes are involved in controlling the relationship.
“In our study, six genomic regions were identified based on associations with the levels of colonization. There may also be numerous small-effect genes that may play a role in this interaction. The combination of genes an individual soybean has will all attribute to the level of colonization.”
Discovering those genomic regions was, she said, a great relief.
“It was a little bit of a gamble because we didn’t know much about soybean’s relationship with mycorrhizae and did not know if differences in colonization among the soybean genotypes would occur. When we screened the soybean genotypes and found differences, it was a big relief. That meant there was a potential to find genetic differences, too. For almost every step in the colonization process, we were finding related genes within those regions.”
The study showed a lot of different factors come into play to influence the success of the union. Some genes, she said, have more of an effect than others.
“It is really amazing. There is evidence of mycorrhizal associations that formed over 450 million years ago and may have played a large role in the evolution of land plants,” she said.
“They were essentially the root system before plants developed their own roots. That is the origin of the reliance plants have on the fungi. They needed them for a long time in order to take up water and nutrients.”
Knowing which genes control root colonization could help breeders develop soybean cultivars with a higher affinity for mycorrhizal fungus, which could lead to improved nutrient uptake, drought tolerance, and disease resistance.
The next stage in the research is to focus on the profiles of colonization and the potential genes that control it so that those particular genes can be used or enhanced in breeding programs. They are also looking at how genotypes impact the level of mycorrhizal responsiveness.
The research was published in Theoretical and Applied Genetics.