Scrutiny of NolA and NodD1 Regulatory Roles in Symbiotic Compatibility Unveils New Insights into Bradyrhizobium guangxiense CCBAU53363 Interacting with Peanut (Arachis hypogaea) and Mung Bean (Vigna radiata).

Bradyrhizobium guangxiense CCBAU53363 efficiently nodulates peanut but exhibits incompatible interaction with mung bean. By comparing the common region with those of other peanut bradyrhizobia efficiently nodulating these two hosts, distinctive characteristics with a single isoform () and a truncated were identified. However, the regulatory roles of NodD1 and NolA and their coordination in legume-bradyrhizobial interactions remain largely unknown in terms of explaining the contrasting symbiotic compatibility. Here, we report that was important for CCBAU53363 symbiosis with peanut but restricted nodulation on mung bean, while was dispensable for CCBAU53363 symbiosis with peanut but essential for nodulation on mung bean. Moreover, exerted a cumulative contribution with to efficient symbiosis with peanut. Additionally, mutants lacking delayed nodulation on peanut, and both and were required for competitive nodule colonization. It is noteworth that most of the nodulation genes and type III secretion system (T3SS)-related genes were significantly downregulated in a strain 53Δ mutant compared to wild-type strain CCBAU53363, and the downregulated nodulation genes also had a greater impact than T3SS-related genes on the symbiotic defect of 53Δ on peanut, which was supported by a more severe symbiotic defect induced by 53Δ than that with the 53Δ, 53Δ, and 53Δ mutants. NolA did not regulate gene expression but did regulate the T3SS effector gene in an indirect way. Meanwhile, , , and some T3SS-related genes besides were also demonstrated as new "repressors" that seriously impaired CCBAU53363 symbiosis with mung bean. Taken together, the roles and essentiality of and in modulating symbiotic compatibility are sophisticated and host dependent. The main findings of this study were that we clarified that the roles and essentiality of and are host dependent. Importantly, for the first time, NolA was found to positively regulate T3SS effector gene to mediate incompatibility on mung bean. Additionally, NolA does not regulate genes, which are activated by NodD1. exerts a cumulative effect with on CCBAU53363 symbiosis with peanut. These findings shed new light on our understanding of coordinated regulation of NodD1 and NolA in peanut bradyrhizobia with different hosts.