Rice actin-binding protein RMD is a key link in the auxin–actin regulatory loop that controls cell growth

Significance The positive feedback loop between the auxin pathway and actin cytoskeleton is essential for auxin self-organizing responsive signaling during plant development; however, its underlying mechanism remains largely unknown. Here, we showed that an actin-binding protein, rice morphology determinant (RMD), acts as a key component mediating the auxin–actin loop pathway, affecting cell growth and morphogenesis. Auxin directly promotes RMD expression via binding of Oryza sativa auxin response factor 23 (OsARF23) and OsARF24 heterodimers on the RMD promoter, triggering changes in F-actin organization. In turn, RMD-dependent F-actin arrays affect auxin intracellular signaling, including polar auxin transport, localization and recycling of auxin efflux carriers, and auxin distribution in root cells. Our work identifies RMD as a key link in the auxin–actin self-organizing regulatory loop that is required for auxin-mediated cell growth.