Symbiotic effectiveness, ecological adaptation and phylogenetic diversity of chickpea rhizobia isolated from a large-scale Australian soil collection

Aims To investigate phylogenetic and phenotypic diversity of resident chickpea-nodulating rhizobia from Australian cropping soils. Methods Eighty Mesorhizobium strains collected from across Australian cropping regions were analysed in a pot experiment to evaluate nodulation and symbiotic effectiveness (SE%) in chickpea plants. In vitro testing of these strains tolerant to specific stresses (pH, temperature, antibiotics, heavy metals and NaCl) was performed. In addition, phylogenetic analyses were carried out using16S-23S rRNA, atpD , recA, nodC and nifH. Results All strains were members of the genus Mesorhizobium based on phylogenies of 16-23S rRNA IGS and core genes. Strains with diverse 16S-23S rRNA IGS carried symbiosis genes that had high similarity to those in chickpea symbionts, suggesting the resident rhizobia in Australian soils may have acquired symbiotic traits from inoculant strains through horizontal gene transfer (HGT). Inoculation of chickpea revealed that variation in SE% among isolated strains was correlated with phylogenetic relatedness to the commercial inoculant strain Mesorhizobium ciceri CC1192. Strain A47 collected from Queensland gave the highest shoot biomass and two strains (A78 and A79) from Western Australia grew under acidic conditions (pH 4.4). In a field experiment, inoculation with four selected strains or strain CC1192 all increased shoot biomass compared with an uninoculated control; chickpea inoculated with strain A47 had the highest nodulation. Conclusion Australian cropping soils contain resident chickpea rhizobial genotypes that are genetically and phenotypically diverse, with some genotypes having equal or superior SE compared with the inoculant strain CC1192. This study provides evidence to support HGT of symbiosis genes in Australian soils, and has identified strains adapted to different environmental conditions.