title: The ROP2 GTPase controls the formation of cortical fine F-actin and the early phase of directional cell expansion during Arabidopsis organogenesis.
abstract: Polar cell expansion in differentiating tissues is critical for the development and morphogenesis of plant organs and is modulated by hormonal and developmental signals, yet little is known about signaling in this fundamental process in plants. In contrast to tip-growing cells, such as pollen tubes and root hairs, cells in developing tissues are thought to expand by diffuse growth. In this study, we provide evidence that these cells expand in two phases with distinct mechanisms. In the early phase, cell expansion can occur in both longitudinal and radial or lateral directions and is mediated by Rop GTPase signaling, a mechanism known to control tip growth. The expression of a dominant-negative mutant for ROP2 (DN-rop2) inhibited polar cell expansion, whereas the expression of a constitutively active mutant (CA-rop2) caused isotropic expansion in the early phase. In the late phase, expansion occurs only in the longitudinal direction and is not affected by DN-rop2 or CA-rop2 expression. The transition from the early to the late phase coincides with the reorientation of cortical microtubules from random to transverse arrangements. Thus, cell expansion in the late phase is consistent with polar diffuse growth, in which polarity probably is defined by transverse cortical microtubules. We show that the direction of cell expansion in the early phase is associated with the localization of diffuse fine cortical F-actin in leaf epidermal cells. DN-rop2 expression specifically inhibited the formation of this F-actin, but not actin cables, whereas CA-rop2 expression caused delocalized distribution of this fine F-actin throughout the cell cortex. Furthermore, green fluorescent protein-ROP2 was localized preferentially to the cortical region of the cell, where expansion apparently occurs. These observations suggest that ROP2 control of the polar expansion of cells within tissues is analogous to the Rop control of tip growth and of tip-localized F-actin in pollen tubes and root hairs and that the tip growth mechanism also may modulate polar cell expansion in differentiating tissues.
authors.affiliation: Department of Botany and Plant Sciences, Center for Plant Cell Biology, University of California, Riverside, CA 92521, USA.
authors.forename: Ying
authors.initials: Y
authors.lastname: Fu
authors.forename: Hai
authors.initials: H
authors.lastname: Li
authors.forename: Zhenbiao
authors.initials: Z
authors.lastname: Yang
mesh_terms: D000199:Actins / Q000378:metabolism*; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D002454:Cell Differentiation / Q000235:genetics; D002455:Cell Division / Q000235:genetics; D020690:GTPase-Activating Proteins / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D019441:Plant Epidermis / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D018515:Plant Leaves / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D018514:Plant Structures / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified; D011058:Pollen / Q000166:cytology / Q000235:genetics / Q000254:growth & development; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D013997:Time Factors; D020741:rho GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D000199:Actins; D020690:GTPase-Activating Proteins; D008164:Luminescent Proteins; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D020741:rho GTP-Binding Proteins