<i>Pseudomonas chlororaphis</i> subsp. <i>aurantiaca</i> Stimulates Lateral Root Development by Integrating Auxin and Reactive Oxygen Species Signaling in <i>Arabidopsis</i>

Plant growth-promoting rhizobacteria (PGPR) can promote lateral root formation, while the underlying mechanisms are not fully understood. Here, we found that <i>Pseudomonas chlororaphis</i> subsp. <i>aurantiaca</i> inoculation enhanced auxin accumulation in lateral root primordia (LRP). Upon reaching LRPs, auxin activated the AUXIN RESPONSE FACTOR 7 and 19 (ARF7/19) and promoted lateral root formation in <i>Arabidopsis</i>. Moreover, we found that reactive oxygen species (ROS) is required for auxin-dependent lateral root emergence, and <i>P. chlororaphis</i> upregulated the expression of RESPIRATORY BURST OXIDASE <i>D</i> and <i>F</i> (<i>RBOHD</i>/<i>F</i>), leading to the accumulation of ROS in LRP. Although scavenging ROS or <i>rbohd</i>/<i>f</i> mutants exhibited decreased lateral roots after <i>P. chlororaphis</i> inoculation, the bacteria-triggered auxin signals were not altered. Conversely, the application of auxin or mutants defective in auxin signaling disturbed <i>P. chlororaphis</i>-derived ROS accumulation in lateral roots. Collectively, these results suggest that ARF7/19-dependent auxin signaling activates <i>RBOHD</i>/<i>F</i> to produce ROS, coordinately facilitating lateral root development after P. chlororaphis treatment.