Plant growth‐promoting abilities of Methylobacterium sp. 2A involve auxin‐mediated regulation of the root architecture.

Methylobacterium sp. 2A, a plant growth‐promoting rhizobacteria (PGPR) able to produce indole‐3‐acetic acid (IAA), significantly promoted the growth of Arabidopsis thaliana plants in vitro. We aimed to understand the determinants of Methylobacterium sp. 2A–A. thaliana interaction, the factors underlying plant growth‐promotion and the host range. Methylobacterium sp. 2A displayed chemotaxis to methanol and formaldehyde and was able to utilise 1‐aminocyclopropane carboxylate as a nitrogen source. Confocal microscopy confirmed that fluorescent protein‐labelled Methylobacterium sp. 2A colonises the apoplast of A. thaliana primary root cells and its inoculation increased jasmonic and salicylic acid in A. thaliana, while IAA levels remained constant. However, inoculation increased DR5 promoter activity in root tips of A. thaliana and tomato plants. Inoculation of this PGPR partially restored the agravitropic response in yucQ mutants and lateral root density was enhanced in iaa19, arf7, and arf19 mutant seedlings. Furthermore, Methylobacterium sp. 2A volatile organic compounds (VOCs) had a dose‐dependent effect on the growth of A. thaliana. This PGPR is also able to interact with monocots eliciting positive responses upon inoculation. Methylobacterium sp. 2A plant growth‐promoting effects can be achieved through the regulation of plant hormone levels and the emission of VOCs that act either locally or at a distance. Summary statement: Arabidopsis thaliana–Methylobacterium sp. 2A interaction and communication involve the modulation of plant hormones, root colonisation, and the production of volatile organic compounds. Methylobacterium sp. 2A promotes plant growth via auxin‐mediated regulation of the root architecture. [ABSTRACT FROM AUTHOR]