Azospirillum argentinense Modifies Arabidopsis Root Architecture Through Auxin-dependent Pathway and Flagellin

To evaluate if root architecture changes observed in Arabidopsis thaliana inoculated with Azospirillum argentinense Az39 depend exclusively on the bacterial capacity to produce indole-3-acetic acid (IAA) and plant ability to sense IAA levels. Azospirillum argentinense Az39, A. argentinense Az39 ipdC–, flagellin from A. argentinense Az39, and pure IAA were applied to A. thaliana Col-0 (wild-type) and tir1.1 (a lateral root deficient mutant) seedlings. Inoculation with heat-inactivated A. argentinense Az39 cells and a non-PGPR bacterium (Escherichia coli DH5α) was also tested. The primary root (PR) length, lateral roots (LR) number, and root hair (HR) density were assessed, and the root transcriptome was sequenced (Illumina HiSeq), followed by DEGs and GO term enrichment analyses. Inoculation with both A. argentinense strains resulted in a shorter PR and an increased number of LR and RH. IAA application (0.1 µM) led to a similar root phenotype than inoculation with Az39 (10⁸ CFU mL⁻¹). The addition of 1 µM flagellin, as well as plant exposure to non-lysed A. argentinense Az39 or E. coli DH5α cells, enhanced RH formation. Genes related to auxin signaling were highly expressed in the roots of Az39-inoculated seedlings; genes related to jasmonate and salicylic acid metabolism were highly expressed in the roots of plants inoculated with ipdC −. Root architecture changes in A. thaliana inoculated with A. argentinense Az39 do not depend exclusively on root IAA levels/IAA plant perception. This PGPR induces root morphological changes through both IAA-dependent and IAA-independent mechanisms. Flagellin may be a key molecule involved in IAA-independent mechanisms.