Plant secondary metabolite profiling evidences strain-dependent effect in the Azospirillum-Oryza sativa association

Azospirillum is a plant growth-promoting rhizobacterium (PGPR) able to enhance growth and yield of cereals such as rice, maize and wheat. The growth-promoting ability of some Azospirillum strains appears to be highly specific to certain plant species and cultivars. In order to ascertain the specificity of the associative symbiosis between rice and Azospirillum, the physiological response of two rice cultivars, Nipponbare and Cigalon, inoculated with two rice-associated Azospirillum was analyzed at two levels: plant growth response and plant secondary metabolic response. Each strain of Azospirillum (Azospirillum lipoferum 4B isolated from Cigalon and Azospirillum sp. B510 isolated from Nipponbare) preferentially increased growth of the cultivar from which it was isolated. This specific effect is not related to a defect in colonization of host cultivar as each strain colonizes effectively both rice cultivars, either at the rhizoplane (for 4B and B510) and inside the roots (for B510). The metabolic profiling approach showed that, in response to PGPR inoculation, profiles of rice secondary metabolites were modified, with phenolic compounds such as flavonoids and hydroxycinnamic derivatives being the main metabolites affected. Moreover, plant metabolic changes differed according to Azospirillum strain×cultivar combinations; indeed, 4B induced major secondary metabolic profile modifications only on Cigalon roots, while B510, probably due to its endophytic feature, induced metabolic variations on shoots and roots of both cultivars, triggering a systemic response. Plant secondary metabolite profiling thereby evidences the specific interaction between an Azospirillum strain and its original host cultivar.