SAUR15 Promotes Lateral and Adventitious Root Development via Activating H⁺-ATPases and Auxin Biosynthesis

Abstract

<i>SMALL AUXIN-UP RNA</i>s (<i>SAUR</i>s) comprise the largest family of early auxin response genes. Some SAURs have been reported to play important roles in plant growth and development, but their functional relationships with auxin signaling remain unestablished. Here, we report Arabidopsis (<i>Arabidopsis thaliana</i>) <i>SAUR15</i> acts downstream of the auxin response factors ARF6,8 and ARF7,19 to regulate auxin signaling-mediated lateral root (LR) and adventitious root (AR) formation. The loss-of-function mutant <i>saur15-1</i> exhibits fewer LRs and ARs. By contrast, plants overexpressing <i>SAUR15</i> exhibit more LRs and ARs. We find that the <i>SAUR15</i> promoter contains four tandem auxin-responsive elements, which are directly bound by ARF6 and ARF7 and are essential for <i>SAUR15</i> expression. LR and AR impairment in <i>arf6</i> and <i>arf7</i> mutants is partially reduced by ectopic expression of <i>SAUR15</i> Additionally, we demonstrate that the ARF6,7-upregulated SAUR15 promotes LR and AR development using two mechanisms. On the one hand, SAUR15 interacts with PP2C-D subfamily type 2C protein phosphatases to inhibit their activities, thereby stimulating plasma membrane H⁺-ATPases, which drives cell expansion and facilitates LR and AR formation. On the other hand, SAUR15 promotes auxin accumulation, potentially by inducing the expression of auxin biosynthesis genes. A resulting increase in free auxin concentration likely triggers LR and AR formation, forming a feedback loop. Our study provides insights and a better understanding of how SAURs function at the molecular level in regulating auxin-mediated LR and AR development.