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

Abstract

The plant hormone auxin plays a central role in plant growth and development. Auxin transport and signaling depend on actin organization. Despite its functional importance, the mechanistic link between actin filaments (F-actin) and auxin intracellular signaling remains unclear. Here, we report that the actin-organizing protein Rice Morphology Determinant (RMD), a type II formin from rice (Oryza sativa), provides a key link. Mutants lacking RMD display abnormal cell growth and altered configuration of F-actin array direction. The rmd mutants also exhibit an inhibition of auxin-mediated cell elongation, decreased polar auxin transport, altered auxin distribution gradients in root tips, and suppression of plasma membrane localization of auxin transporters O. sativa PIN-FORMED 1b (OsPIN1b) and OsPIN2 in root cells. We demonstrate that RMD is required for endocytosis, exocytosis, and auxin-mediated OsPIN2 recycling to the plasma membrane. Moreover, RMD expression is directly regulated by heterodimerized O. sativa auxin response factor 23 (OsARF23) and OsARF24, providing evidence that auxin modulates the orientation of F-actin arrays through RMD. In support of this regulatory loop, osarf23 and lines with reduced expression of both OsARF23 and OsARF24 display reduced RMD expression, disrupted F-actin organization and cell growth, less sensitivity to auxin response, and altered auxin distribution and OsPIN localization. Our findings establish RMD as a crucial component of the auxin-actin self-organizing regulatory loop from the nucleus to cytoplasm that controls rice cell growth and morphogenesis.auxin signaling | actin cytoskeleton | rice morphogenesis

Rice Morphology Determinant-Mediated Actin Filament Organization Contributes to Pollen Tube Growth

“…In addition, RMD determines the F-actin organization and network in rice root cells Li et al, 2014). To determine the role of RMD in regulating F-actin organization and distribution in pollen tubes, we first observed F-actin arrays in wild-type and rmd-1 mature pollen grains using staining with Alexa Fluor 488-phalloidin (Supplemental Fig.…”

“…To quantify the extent of actin filament density in pollen tubes, fluorescence intensity and skewness were measured as described previously by Li et al (2014). A statistical analysis of fluorescence intensity showed a higher density of F-actin in the shank region of rmd-1 (n = 36) and rmd-2 (n = 31) pollen tubes, while a lower density of F-actin was observed at the tip region of rmd pollen tubes compared with that of the wild type (n = 32; Fig.…”

“…The rmd-1 and rmd-2 mutants, the RMD RNAi line Ri-S, and the proRMD:: GUS transgenic rice (Oryza sativa) lines have been described previously Li et al, 2014). The complementary line was created by expressing RMD-eGFP driven by its own promoter in the rmd-1 background.…”

“…Our previous work demonstrated that a type II formin protein, RICE MORPHOLOGY DETERMINANT (RMD; also called BENT UPPERMOST INTERNODE1) from rice (Oryza sativa), functionally regulates cytoskeleton organization by nucleating, capping, and bundling actin in vitro and also plays a crucial role in regulating F-actin dynamics and arrays in rice cells (Yang et al, 2011;Zhang et al, 2011;Li et al, 2014). rmd mutants exhibit altered microfilament and microtubule arrays, causing defective plant morphology .…”

“…rmd mutants exhibit altered microfilament and microtubule arrays, causing defective plant morphology . In addition, RMD is a key regulator of the auxin-actin self-organizing regulatory loop that controls the diffuse cell growth of rice roots (Li et al, 2014). However, the role of RMD-mediated F-actin organization in polarized cell growth is poorly understood.…”