Encasing otherwise spherical protoplasts, cell walls define the shape of plant cells. For years microtubules were thought to be the key cytokskeletal element in plant cell morphogenesis by spatially regulating cell wall deposition. Our work on the Arabidopsis ROP2 GTPase provides evidence for the actin cytoskeleton as a new important player in cell morphogenesis. ROP2 is a member of the plant-specific subfamily of Rho-type GTPases that are well known for their roles in actin-based cell behaviors in animals and fungi. ROP2 regulates cell shapes through activating localized accumulation of a novel form of cortical diffuse F-actin. Our ROP2 work also provides new insights into a long-standing question in plant cell biology: whether and how cellular signaling networks control plant cell shapes. The publication of this work coincides with a recent surge of studies on the role of the cytoskeleton in plant cell morphogenesis using both pharmacological and genetic approaches, contributing to the high citation.

Our new findings may aid future studies to understand developmental regulation of cell morphogenesis and intracellular signaling networks controlling cell shapes. Cell shape formation in plants occurs in two sequential developmental stages: the early ROP2 GTPase-dependent and the late ROP2-independent stages. The discovery of ROP2-dependent F-actin will likely lead to new interests in investigating how the actin cytoskeleton coordinates with microtubules to control cell shapes in plants and whether the Rho GTPase-mediated/actin-based mechanism provides a common ground for cell shape formation between plant and animal cells.

Specific cell shapes are critical for the function of cells and development of tissues and organs. Joint cells within a developing tissue/organ must talk to each other to coordinate the timing and the direction of cell growth, resulting in mature cells, tissues and organs with specific shapes and sizes. Therefore signals generated from cell-cell communication are expected to regulate intracellular signaling networks that control cell shapes. Our work on ROP2 is the first step in a long journey to mapping out the intercellular and intracellular signaling networks that define cell shapes.

We have been interested in the roles of ROP GTPase signaling in Arabidopsis for quite a few years. In the course of characterizing the phenotypes of transgenic plants expressing constitutively active and dominant negative mutants of ROP2, we discovered cell shape changes in these plants. We were particularly intrigued by the alteration of jigsaw-shaped pavement cells in leaves. The formation of intercalary lobes and sinuses in these cells was thought to depend on cortical MTs and cellulose microfibrils associated with the sinuses. However, our earlier work shows that a pollen-specific homolog of ROP2, ROP1, controls pollen tube growth (a specialized form of polar growth known as tip growth) through regulating the dynamics of tip-localized F-actin. This led us to the investigation of ROP2 involvement in the regulation of F-actin in pavement cells. The intercalary jigsaw-shaped pavement cells provide a particularly interesting system to study the regulation of cell shapes by cell-cell communication.

Zhenbiao Yang
Associate Professor
Center for Plant Cell Biology
Department of Botany and Plant Science
University of California, Riverside
Riverside, CA, USA

Ying Fu
Postdoctoral Research Associate
Center for Plant Cell Biology
Department of Botany and Plant Science
University of California, Riverside
Riverside, CA, USA