Genetic and transcriptomic analysis of the Bradyrhizobium T3SS-triggered nodulation in the legume Aeschynomene evenia.

Some Bradyrhizobium strains nodulate certain Aeschynomene species independently of Nod factors, but thanks to their type III secretion system (T3SS). While different T3 effectors triggering nodulation (ErnA and Sup3) have been identified, the plant signalling pathways they activate remain unknown. Here, we explored the intraspecies variability in T3SS-triggered nodulation within Aeschynomene evenia and investigated transcriptomic responses that occur during this symbiosis. Furthermore, Bradyrhizobium strains having different effector sets were tested on A. evenia mutants altered in various symbiotic signalling genes. We identified the A. evenia accession N21/PI 225551 as appropriate for deciphering the T3SS-dependent process. Comparative transcriptomic analysis of A. evenia N21 roots inoculated with ORS3257 strain and its ∆ernA mutant revealed genes differentially expressed, including some involved in plant defences and auxin signalling. In the other A. evenia accession N76, all tested strains nodulated the AeCRK mutant but not the AeNIN and AeNSP2 mutants, indicating a differential requirement of these genes for T3SS-dependent nodulation. Furthermore, the effects of AePOLLUX, AeCCaMK and AeCYCLOPS mutations differed between the strains. Notably, ORS86 nodulated these three mutant lines and required for this both ErnA and Sup3. Taken together, these results shed light on how the T3SS-dependent nodulation process is achieved in legumes.