Dual effect of Mesorhizobium loti T3SS functionality on the symbiotic process

Mesorhizobium loti MAFF303099 has a functional type III secretory system (T3SS) involved in the nodulation process on Lotus tenuis and Lotus japonicus. Four putative M.similar to loti T3SS effectors (Mlr6358, Mlr6331, Mlr6361, and Mlr6316) have been previously described, and it has been demonstrated that the N-terminal regions of Mlr6361 and Mlr6358 mediate the secretion via a T3SS. Here, we demonstrate the capacity of Mlr6316 and Mlr6331 N-terminal regions to direct the secretion of a translational fusion to a reporter peptide through T3SS. By using single, double, and triple mutants, we demonstrated the positive and negative participation of some of these proteins in the determination of competitiveness on Lotus spp. Low competitiveness values correlated with low nodulation efficiency for a mutant deficient in three of the putative M.similar to loti effectors. Our data suggest that the net effect of M.similar to loti T3SS function on symbiotic process with Lotus results from a balance between positive and negative effects.