RICE MORPHOLOGY DETERMINANTEncodes the Type II Formin FH5 and Regulates Rice Morphogenesis

Zheng Zhang,Wanqi Liang,Zheng Yuan,Haiyun Ren,Jianping Hu,Hexin Tan,Gang Li,Ying Wang,Yi Zhang,Dabing Zhang

doi:10.1105/tpc.110.081349

Abstract

Multicellular organisms contain a large number of formins; however, their physiological roles in plants remain poorly understood. Here, we reveal that formin homology 5 (FH5), a type II formin mutated in rice morphology determinant (rmd), plays a crucial role in determining rice (Oryza sativa) morphology. FH5/RMD encodes a formin-like protein consisting of an N-terminal phosphatase tensin (PTEN)-like domain, an FH1 domain, and an FH2 domain. The rmd mutants display a bending growth pattern in seedlings, are stunted as adult plants, and have aberrant inflorescence (panicle) and seed shape. Cytological analysis showed that rmd mutants have severe cell elongation defects and abnormal microtubule and microfilament arrays. FH5/RMD is ubiquitously expressed in rice tissues, and its protein localization to the chloroplast surface is mediated by the PTEN domain. Biochemical assays demonstrated that recombinant FH5 protein can nucleate actin polymerization from monomeric G-actin or actin/profilin complexes, cap the barbed end of actin filaments, and bundle actin filaments in vitro. Moreover, FH5 can directly bind to and bundle microtubules through its FH2 domain in vitro. Our findings suggest that the rice formin protein FH5 plays a critical role in determining plant morphology by regulating actin dynamics and proper spatial organization of microtubules and microfilaments.

Highlights

Variation in organ morphology contributes to the diversified architecture of flowering plants
We show that formin homology 5 (FH5), a type II formin-like protein encoded by Rice Morphology Determinant (RMD), functions as an actin-nucleating protein that regulates rice plant morphology by modulating the microtubule- and microfilament-based cytoskeletal systems
To test whether FH5 binds to microtubules in vivo, we introduced the translationally fused construct of FH5 FH1FH2-red fluorescent protein (RFP) into onion epidermal cells, using green fluorescent protein (GFP)MAP4 (Microtubule-associated protein 4) as the marker for the microtubule cytoskeleton (Marc et al, 1998)

Summary

Introduction

Variation in organ morphology contributes to the diversified architecture of flowering plants. The microfilament cytoskeleton, which is composed of the filamentous actin assembled from G-actin monomers, is a well-organized and dynamic system. It plays crucial roles in diverse cellular processes, including cell shape and polarity establishment and maintenance, cell division, cytoplasmic streaming, organelle movement, and responses to external signals (Wasteneys and Galway, 2003; Smith and Oppenheimer, 2005; Hussey et al, 2006; Staiger and Blanchoin, 2006; Paul and Pollard, 2009). The Arp2/3 complex stimulates branched filament formation from preexisting actin filaments (Mullins et al, 1998; Pollard and Beltzner, 2002), whereas formins regulate the assembly of parallel actin filaments into actin bundles or cables

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