The Bradyrhizobium diazoefficiens type III effector NopE modulates the regulation of plant hormones towards nodulation in Vigna radiata

Host-specific legume-rhizobium symbiosis is strictly controlled by rhizobial type III effectors (T3Es) in some cases. Here, we demonstrated that the symbiosis of Vigna radiata (mung bean) with Bradyrhizobium diazoefficiens USDA110 is determined by NopE, and this symbiosis is highly dependent on host genotype. NopE specifically triggered incompatibility with V. radiata cv. KPS2, but it promoted nodulation in other varieties of V. radiata, including KPS1. Interestingly, NopE1 and its paralogue NopE2, which exhibits calcium-dependent autocleavage, yield similar results in modulating KPS1 nodulation. Furthermore, NopE is required for early infection and nodule organogenesis in compatible plants. Evolutionary analysis revealed that NopE is highly conserved among bradyrhizobia and plant-associated endophytic and pathogenic bacteria. Our findings suggest that V. radiata and B. diazoefficiens USDA110 may use NopE to optimize their symbiotic interactions by reducing phytohormone-mediated ETI-type (PmETI) responses via salicylic acid (SA) biosynthesis suppression.

Automated summary

The symbiosis between Vigna radiata and Bradyrhizobium diazoefficiens USDA110 is determined by the type III effector NopE, which triggers incompatibility with certain varieties of V. radiata but promotes nodulation in others. NopE is crucial for early infection and nodule organogenesis in compatible plants, and it is highly conserved among Bradyrhizobia and other plant-associated bacteria. The findings suggest that NopE may be used by V. radiata and B. diazoefficiens USDA110 to optimize their symbiotic interactions by suppressing salicylic acid biosynthesis and reducing phytohormone-mediated responses.