Auxin and Root Hair Defective Six‐Like 4 Regulate Azospirillum brasilense‐Induced Root Hair Development in Arabidopsis

ABSTRACT Beneficial microbiomes significantly modulate root hair morphogenesis, yet the underlying signalling pathways remain unclear. Through comprehensive phenotypic analysis, we demonstrated that Azospirillum brasilense , a well‐known plant‐growth‐promoting rhizobacteria (PGPR), promotes root hair development by prolonging the elongation period. The promotion of root hair growth by A. brasilense is attributed to increased auxin accumulation in root apices and enhanced shootward transport from the root tip, which is mediated by the accumulation of the auxin importer auxin transporter protein 1 (AUX1) in the lateral root cap and the overlying epidermis. Chemical or genetic disruption of auxin signalling abolished bacteria‐induced root hair responses. Furthermore, A. brasilense upregulated several root hair‐related basic helix‐loop‐helix family transcription factors, including root hair defective 6 (RHD6) and RHD6‐LIKE 1 (RSL1), which control root hair development through RSL4. Morphological analysis revealed that while the rsl2rsl4 double mutant remained hairless after inoculation, A. brasilense successfully rescued root hair growth in rhd6rsl1 . These findings suggest that A. brasilense can bypass RHD6 to activate RSL4, which then drives root hair development dependent on ROS. Collectively, our study uncovers the core signalling components modulated by A. brasilense to fine‐tune root hair responses.