Root exudate supplemented inoculant of Azospirillum brasilense Ab-V5 is more effective in enhancing rhizosphere colonization and growth of maize

Abstract

This study was aimed to evaluate inoculants of Azospirillum brasilense strain Ab-V5 supplemented with root exudates (collected from maize seedlings) for improving the inoculation response of maize, by stimulation of root colonization by the bacterium. Root exudates collected from seedlings of two commercial maize genotypes were evaluated, and their effects on seeds, seedling colonization and plant development under greenhouse conditions were addressed. Compared with seeds treated with water, seeds soaked with maize root exudate extract (MREE) increased mean bacterial count by three log-unity. Plantlets inoculated with MREE-supplemented inoculant suspensions showed 0.4 log-unity higher mean counts than those inoculated with non-supplemented bacterial suspension. The plant biomass for inoculants supplemented with MREE increased by 50% and the root area and number per plant increased by 19% when compared to non-supplemented bacterial suspensions. Photosynthetic physiological responses were also observed when the MREE inoculants were used, although these effects varied according to the plant genotype and MREE source. The results demonstrate that soluble compounds exuded by maize seedlings can increase the colonization of maize plants by A. brasilense Ab-V5 and suggest a strategy to improve the effectiveness of plant–PGPB associative systems by introducing chemoattractant molecules into inoculant formulations.