Résumé
Using the interaction between Arabidopsis thaliana and the Phyllobacterium brassicacearum STM196 strain that was isolated from the rhizoplane of field‐grown canola, we have previously shown that a plant‐growth‐promoting rhizobacterium can affect the root system architecture by modifying the tissue distribution of indole acetic acid IAA in the root (Contesto et al., 2010). To confirm that auxin is involved in that rhizobacterium‐induced regulation of root development, we inoculated tir1 Arabidopsis mutant plants deficient in the key IAA receptor with STM196. As expected, the increased lateral root growth seen in Arabidopsis wild‐type plants was abolished in the tir1 mutant. Using a strain of Mesorhizobium loti MAFF303099, a close genetic relative of P. brassicacearum, expressing DsRed fluorescent protein, we observed that the bacteria are epiphytic and thrive on the site of emerged lateral roots, suggesting that the lateral root elongation could be dependent on the colonization pattern. Finally, consistent with the effect of rhizobium on the auxin pathway, the root response to gravitropism was enhanced in inoculated plants.