Homospermidine synthase contributes to salt tolerance in free-living Rhizobium tropici and in symbiosis with Phaseolus vulgaris

Background and aims Homospermidine is known to be the most abundant polyamine in root nodules of Phaseolus vulgaris induced by Rhizobium tropici . In addition, homospermidine is involved in the stress tolerance of fast-growing rhizobia and in the bacteroid protection from environmental changes. For that reason, in this work, we have investigated the role of the rhizobial homospermidine synthase in the response of P. vulgaris root nodules to salinity. Materials and methods A mutant strain of R. tropici impaired in the synthesis of homospermidine has been constructed (Rt hss ::Ω, Sp r ) and the response to salinity of the free-living and symbiotic bacteria has been analyzed. Plants of P. vulgaris inoculated with the mutant strain were treated with 100 mM NaCl, and the concentration of polyamines was determined in different nodular fractions together with nitrogen fixation and gene expression of polyamine biosynthetic enzymes. Results Neither homospermidine nor 4-aminobutilcadaverine was detected in the free-living mutant bacteria and in nodules of plants inoculated with Rt hss ::Ω, Sp r . This strain was more sensitive to salinity than the wild type, and plants inoculated with the mutant bacteria had lower nodule fresh weight than with the wild type. Conclusion Homospermidine synthase contributes to salt stress in both free-living and symbiotic bacteria. Despite the synthesis of homospermidine, this enzyme produces also 4-aminobutilcadaverine. Based on the lower nodule fresh weight and plant biomass reduction induced by salinity in plants inoculated with the mutant strain, this enzyme seems to be involved in nodule organogenesis and salt tolerance.