Abstract
Sinorhizobium fredii is a fast-growing rhizobial species that can establish a nitrogen-fixing symbiosis with a wide range of legume species including soybeans (Glycine max). In soybeans, this interaction shows a high level of specificity such that particular S. fredii strains nodulate only a limited set of plant genotypes. Here we report the identification of a dominant gene in soybeans that restricts nodulation with S. fredii USDA193. Genetic mapping in an F2 population revealed co-segregation of the underlying locus with the previously cloned Rfg1 gene. The Rfg1 allele encodes a member of the Toll-interleukin receptor/nucleotide-binding site/leucine-rich repeat class of plant resistance proteins that restricts nodulation by S. fredii strains USDA257 and USDA205, and an allelic variant of this gene also restricts nodulation by Bradyrhizobium japonicum USDA122. By means of complementation tests and CRISPR/Cas9-mediated gene knockouts, we demonstrate that the Rfg1 allele also is responsible for resistance to nodulation by S. fredii USDA193. Therefore, the Rfg1 allele likely provides broad-spectrum resistance to nodulation by many S. fredii and B. japonicum strains in soybeans.
Highlights
The leguminous plants are able to establish a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia
Our study suggests that the Rfg1 locus is involved in the determination of nodulation specificity with multiple S. fredii and B. japonicum strains in soybeans
We concluded that the restriction of nodulation by S. fredii USDA193 in Williams 82 was not due to a failure in Nod factor perception but caused by the block of bacterial infection
Summary
The leguminous plants are able to establish a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The symbiosis is featured by the formation of root nodules where the bacteria in nodule cells can convert atmospheric nitrogen into ammonia and make it available to the plant. This symbiotic partnership has important implications in sustainable agriculture because it reduces the need for nitrogen-based fertilizers. Bacteria are first entrapped by curled root hairs and multiply to form micro-colonies referred to as infection foci. From these foci, plant-made tubular-like structures, called infection threads, start to develop through local cell wall hydrolysis and invagination of the plant plasma membrane.
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