Rho GTPases in Plant Development: A Genetic and Cell Biological Analysis

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0111078FowlerBecause plant cells are immobile, and are fixed within tissues by a cell wall, their shapes and sizes (morphology) must be generated by precise regulation of the sites of cell expansion and/or the planes of cell division. Current data indicate that regulation of expansion and division depends on interactions among factors associated with the cell cortex, the plasma membrane and the cell wall. However, a mechanistic understanding of these factors (proteins and signals) and their functions in plant cell morphogenesis is incomplete. Evidence is building that the plant-specific Rop subfamily of the Rho GTPases is involved in regulating these morphogenetic processes at the cell cortex, including pollen tube growth, the distribution of the actin cytoskeleton, and cell wall synthesis. However, the in vivo functions of specific Rops throughout plant growth and development are poorly understood.Using the model crop plant Zea mays, the P.I. has isolated a large collection of mutations in five rop genes, and evaluation of mutant phenotypes indicates that at least one of these rop genes, ropB, functions in the pollen tube. The P.I. will take a combined genetic, molecular, and cell biological approach to understanding the functions of the Rop family in plant development. Specifically, the P.I. will determine the in vivo functions of maize ROPB by a detailed characterization of the ropB mutant phenotype. Growth in the maize style (the silk) of mutant pollen tubes from single and double rop mutant plants will be analyzed. Mutant pollen tubes will be tested in culture with a variety of inhibitors, to dissect the signaling pathways downstream of ROPB. The P.I. will also generate and characterize maize plants expressing fluorescent reporters of the plant cytoskeleton, and use these to more precisely dissect the effects of rop mutations on cytoskeletal distribution in vivo.This work will provide insight into these important signaling molecules, and should help clarify their roles in several processes (e.g., pollination and pollen tube growth, cell wall synthesis). In addition to contributing to basic scientific knowledge, our results may suggest ways to manipulate specific rops that would lead to crop improvement. The close relationship of the monocot Zea mays to other important grass crops (rice, wheat) suggests