PS 12-166 - Genetic variation for responsiveness of legume symbiosis traits to nitrogen fertilization

Abstract: Mutualistic relationships with microbes play an important role in plant performance, but the benefits of mutualism can be altered depending on environmental conditions. In the legume-rhizobium symbiosis, rhizobium bacteria fix nitrogen in exchange for carbohydrates and shelter from their plant host. Nitrogen fertilization can reduce the value of symbiotically fixed nitrogen to a legume, and plants can plastically decrease the number and size of root nodules formed in response to fertilization. The degree of plasticity of symbiosis can be influenced by plant genetic variation, making it an attractive target for crop breeding for more sustainable crops. However, little is known about whether legume genotypes vary in the responsiveness of symbiotic traits to nitrogen fertilization. To investigate this, we examined the effects of nitrogen fertilization on symbiosis traits and performance in 141 soybean recombinant inbred lines (RILs) resulting from a cross between wild and domesticated soybean genotypes. Plants were inoculated with Bradyrhizobium diazoefficiens USDA 110 and allowed to fix nitrogen. We measured nodule number, nodule size, SPAD, shoot mass, and root mass for 1200 soybean plants under low and high levels of nitrogen fertilization. We find that wild and domesticated soybean genotypes and genotypes of their hybrid progeny differ in the responsiveness of nodule number and size to nitrogen fertilization. For most genotypes, nitrogen fertilization also caused SPAD in leaf tissue to increase, and shoot mass shoot mass to decrease, though the responsiveness of these performance metrics to nitrogen fertilization showed abundant genetic variation. These findings highlight the abundance of genetic variation in symbiotic plasticity, which could serve as the raw material for evolutionary responses or crop breeding for superior symbiotic outcomes.