Dual effect of Mesorhizobium loti T3SS functionality on the symbiotic process

Abstract

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. 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. loti effectors. Our data suggest that the net effect of M. loti T3SS function on symbiotic process with Lotus results from a balance between positive and negative effects.

Nodulation outer proteins: double-edged swords of symbiotic rhizobia

“…When single T3 effector genes were mutated in rhizobial strains, positive, negative or no effects on symbiosis were observed ( Figures 2C-2F). Altered symbiotic phenotypes that can be attributed to the lack of a specific T3 effector have been reported for nopL [58,74], nopM [57,75] and nopT [56,57] of S. fredii NGR234, as well as for nopBG (mlr6316) and nopBX (mlr6361) of M. loti MAFF303099 [15][16][17]72]. Symbiotic phenotypes were also observed for nopP mutants of various S. fredii strains [51,[77][78][79] as well as for Rhizobium etli CNPAP512 [53].…”

“…GFP or FLAG tag) can be expressed in wild-type rhizobia and a T3SSdeficient mutant for further analysis. T3SS-dependent secretion of such test proteins can then be analysed by fluorescence microscopy or immunological methods [16,68,72]. By definition, Nops should be secreted by wild-type bacteria, but not by a T3SS-knockout mutant.…”

“…By definition, Nops should be secreted by wild-type bacteria, but not by a T3SS-knockout mutant. Using this approach, four FLAGtagged candidate effectors of Mesorhizobium loti MAFF303099 were found to be secreted via the T3SS [16,72]. Another way to characterize candidates is to test them in a heterologous Pseudomonas-Arabidopsis translocation system.…”

“…In other cases, however, no or only minor effects on symbiosis were observed when strains lacking a single effector gene were tested in nodulation experiments. Examples for such effector genes are nopJ of NGR234 [57], nopBV (mlr6331) and nopBW (mlr6358) of M. loti MAFF303099 [17,72], as well as nopAA (gunA2), nopAC (pgl), nopAD (pme), nopE1 and nopE2 of B. japonicum USDA110 [29,80]. This prompted researchers to mutate further already constructed T3 effector gene mutants.…”

NopD of Bradyrhizobium sp. XS1150 Possesses SUMO Protease Activity

“…Mesorhizobium loti MAFF303099 secretes a related protein (mlr6316) in a T3SS-dependent manner. Mutant analysis and inoculation experiments with host plants suggested a possible symbiotic role of this protein in nodulation or nodulation competitiveness (Hubber et al, 2004;Sánchez et al, 2012). Moreover, the two recently identified bradyrhizobial proteins inducing Rj4mediated nodulation blockage (BEL2_5, Faruque et al, 2015;MA20_12780, Tsurumaru et al, 2015) can be considered as NopD family proteins.…”