Inoculation with Azospirillum brasilense (Ab-V4, Ab-V5) increases Zea mays root carboxylate-exudation rates, dependent on soil phosphorus supply

Abstract: Background and aims: We aimed to investigate the effects of root carboxylate exudation in the interaction between Azospirillum brasilense and Zea mays. We hypothesized that root carboxylate exudation is a mechanism that increases colonization of the maize rhizosphere by A. brasilense and that carboxylate exudation would increase at a low soil phosphorus (P) availability. Methods: We conducted a greenhouse experiment, using maize seeds inoculated and uninoculated with A. brasiliense. Seeds were planted in pots, supplied with nutrient solution, varying in P concentration. After 45 days we measured total plant biomass, root length and area, plant nutrient status, and the root carboxylate-exudation rate. Results: Inoculation increased the root length and area, and this effect increased with increasing P supply. Inoculated plants also showed an increased root carboxylate-exudation rate. For inoculated treatments, the exudation rate was positively correlated with root architecture parameters; however, it was negatively correlated with leaf manganese concentration, a proxy for the amount of carboxylates in the rhizosphere. Conclusion: Inoculation of A. brasilense stimulated root carboxylate exudation, which was positively correlated with root length and area. These positive correlations are probably mediated by the effect of carboxylates on the rhizosphere microbial community. This indicates a positive feedback in which A. brasilense inoculation stimulates root carboxylate exudation, influencing the rhizosphere microbial community. It results in positive effects on maize root architecture. The root length of inoculated plants was positively correlated with P supply, indicating that P supply positively affects the microbial community, modulating the interaction between A. brasilense and Z. mays.