Growth and aluminum tolerance of maize roots mediated by auxin- and cytokinin-producing Bacillus toyonensis requires polar auxin transport

Plant Growth Promotion Rhizobacteria (PGPR) control plant development by modulation of their phytohormone activities. To probe how PGPR trigger plant growth and mitigate stress induced by aluminum (Al), the beneficial effect of Bacillus toyonensis strain Bt04 (Bt04), isolated from the Algerian Sahara rhizosphere, on maize roots was assessed. Our results revealed that the strain Bt04 is producing indole-3-acetic acid (IAA) both endogenously and from tryptophan (Trp). Surprisingly, the production of another endogenous auxin, a non-indole phenylacetic acid (PAA), was found at higher concentrations than IAA. Additionally, the production of cytokinins (CKs) by Bt04 has been demonstrated. Inoculation with Bt04 resulted in a promotion of maize growth and an enhancement of root development under aluminium (Al) toxicity condition. Importantly, Bt04 reduced Al accumulation in the young maize roots. Moreover, we could demonstrate that Bt04 itself exhibits tolerance to high concentrations of AlCl3. Bt04 might exert its growth promoting effect partially by increasing protection against oxidative stress since bacterial treatment reduced lipid peroxidation in maize seedlings under Al toxicity. To investigate a possible mechanism of this growth induction of maize seedlings we treated them under control and stress conditions with auxin transport inhibitors and demonstrated that the auxin transport pathway is needed for the Al induced stress response after inoculation with the PGPR.