The Overexpression of an EnvZ-Like Protein Improves the Symbiotic Performance of Mesorhizobia

Bacteria can sense changes in the environment and adapt to survive under different conditions. The two-component signal transduction system EnvZ/OmpR is described to mediate response to osmotic stress. This system also regulates genes involved in other processes such as virulence, fatty acid uptake, exopolysaccharide production, peptide transportation and flagella production. Considering that some of these processes are known to be important for a successful symbiosis, the present study addresses the role of EnvZ in rhizobia-legume interaction, evaluating the effects of extra envZ-like gene copies in the Mesorhizobium-chickpea symbiosis. Five Mesorhizobium-transformed strains, expressing the envZ-like gene from M. mediterraneum UPM-Ca36T, were evaluated in terms of symbiotic performance. Chickpea plants inoculated with envZ-transformed strains (PMI6envZ+ and EE7envZ+) showed significant higher symbiotic effectiveness as compared to the corresponding empty-vector strains. In plants inoculated with PMI6envZ+, a higher number of infection threads was observed, and nodules were visible 4 days earlier. Overall, our results showed that the expression of extra-copies of the envZ-like gene influenced the symbiotic process at different stages, leading to strain-dependent effects. This study represents the first report on the involvement of an EnvZ-like protein in the rhizobia-legume symbiosis and contributes to elucidate the role of these systems in bacteria-host interaction.