BENT UPPERMOST INTERNODE1Encodes the Class II Formin FH5 Crucial for Actin Organization and Rice Development  

Abstract

The actin cytoskeleton is an important regulator of cell expansion and morphogenesis in plants. However, the molecular mechanisms linking the actin cytoskeleton to these processes remain largely unknown. Here, we report the functional analysis of rice (Oryza sativa) FH5/BENT UPPERMOST INTERNODE1 (BUI1), which encodes a formin-type actin nucleation factor and affects cell expansion and plant morphogenesis in rice. The bui1 mutant displayed pleiotropic phenotypes, including bent uppermost internode, dwarfism, wavy panicle rachis, and enhanced gravitropic response. Cytological observation indicated that the growth defects of bui1 were caused mainly by inhibition of cell expansion. Map-based cloning revealed that BUI1 encodes the class II formin FH5. FH5 contains a phosphatase tensin-like domain at its amino terminus and two highly conserved formin-homology domains, FH1 and FH2. In vitro biochemical analyses indicated that FH5 is capable of nucleating actin assembly from free or profilin-bound monomeric actin. FH5 also interacts with the barbed end of actin filaments and prevents the addition and loss of actin subunits from the same end. Interestingly, the FH2 domain of FH5 could bundle actin filaments directly and stabilize actin filaments in vitro. Consistent with these in vitro biochemical activities of FH5/BUI1, the amount of filamentous actin decreased, and the longitudinal actin cables almost disappeared in bui1 cells. The FH2 or FH1FH2 domains of FH5 could also bind to and bundle microtubules in vitro. Thus, our study identified a rice formin protein that regulates de novo actin nucleation and spatial organization of the actin filaments, which are important for proper cell expansion and rice morphogenesis.

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

“…Further, the osarf23/35S::RMD plants exhibited stronger longitudinal F-actin bundles with higher density compared with those of the WT (Fig. 3 C-G), consistent with a role for RMD in actin bundling organization (22,23). In response to IAA, osarf23 also displayed less sensitivity with respect to auxin-regulated root cell growth and RMD expression and the rearrangement of F-actin arrays (Fig.…”

“…RMD plays a pivotal role in regulating morphogenesis by modulating cytoskeleton organization in rice (22,23). To investigate the role of RMD during rice root growth, we used two null mutant alleles of RMD, rmd-1 and rmd-2 (22).…”

“…The rmd mutants exhibit abnormal microfilament and microtubule organization, causing altered plant morphology, including defective root and shoot growth as well as aberrant inflorescence and seed shape (22,23). Here, we show that auxin modulates actin filament (F-actin) array orientation by directly regulating RMD expression via Oryza sativa auxin response factor 23 (OsARF23) and OsARF24 heterodimers.…”

Rice TUTOU1 Encodes a Suppressor of cAMP Receptor-Like Protein That Is Important for Actin Organization and Panicle Development

“…F-actin was stained using the previously described method (Olyslaegers and Verbelen, 1998;Yang et al, 2011;Zhang et al, 2011). Mature rice leaves were incubated in PEM buffer (100 mM PIPES, 10 mM EGTA, 0.3 M mannitol, and 5 mM MgSO 4 , pH 6.9) and 0.66 mM Alexa Fluor 488-phalloidin (Life Technologies).…”

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

“…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 .…”