Identification of Bradyrhizobium elkanii Genes Involved in Incompatibility with Vigna radiata.

The establishment of a root nodule symbiosis between a leguminous plant and a rhizobium requires complex molecular interactions between the two partners. Compatible interactions lead to the formation of nitrogen-fixing nodules, however, some legumes exhibit incompatibility with specific rhizobial strains and restrict nodulation by the strains. USDA61 is incompatible with mung bean ( cv. KPS1) and soybean cultivars carrying the allele. Here, we explored genetic loci in USDA61 that determine incompatibility with KPS1. We identified five novel genes that contribute to this incompatibility. Four of these genes also control incompatibility with soybean cultivars carrying the allele, suggesting that a common mechanism underlies nodulation restriction in both legumes. The fifth gene encodes a hypothetical protein that contains a box in its promoter region. The box is conserved in genes encoding the type III secretion system (T3SS), which is known for its delivery of virulence effectors by pathogenic bacteria. These findings revealed both common and unique genes that are involved in the incompatibility of with mung bean and soybean. Of particular interest is the novel T3SS-related gene, which causes incompatibility specifically with mung bean cv. KPS1.