Pollen Projects

Profilin and Corn POPs
Supported by the USDA-NRICGP (94-37304-1179, 97-35304-4876 & 99-35304-8640)
Staff: Faisal Chaudhry & Dr. Tracie Matsumoto

A remarkable example of cellular morphogenesis, pollen germination and tube growth, is essential for plant reproduction. In order to deliver non-motile sperm to the embryo sac, the vegetative cell of the pollen grain forms a tip-growing protuberance that extends at rates up to 1 cm/h. A dynamic network of filaments and associated proteins, the actin cytoskeleton, organizes the pollen cytoplasm, provides the molecular railroad tracks for cytoplasmic streaming, and orchestrates the delivery of secretory vesicles to the growing apex. To understand the molecular mechanism of tip growth, we have focussed attention on a gene family for the actin monomer-binding protein, profilin, from the crop plant Zea mays. Our central hypothesis is that profilin is a fundamental regulator of actin polymerization and its cellular function is mediated through interactions with a diverse set of binding partners. We have already characterized the interaction of recombinant and native maize profilins with three major ligands (G-actin, PtdIns(4,5)P2 and polyproline) and discovered several important differences between plant and non-plant profilins and among plant profilin isoforms. Live cell and in vitro mutagenesis studies demonstrate that profilin binding to both actin and proline-rich sequences is important for cellular function. In this proposal we will test two working hypotheses. First, we propose that the major function of pollen profilin is to bind actin monomers and prevent actin polymerization, i.e. profilin is a simple sequestering protein. The second hypothesis is that pollen contains novel and previously identified interacting proteins, or partners of profilin (POPs), which regulate profilin function. Biochemical, cytological and genetic experiments are designed to test these hypotheses. This research is significant because it will provide detailed understanding of the strategies used by plant cells to regulate actin organization and function. Furthermore, it provides the potential to uncover plant-specific mechanisms that underpin cytoplasmic organization, cell polarity, and vesicle trafficking. Our findings will dramatically improve knowledge of pollen biology and provide tools and approaches for manipulation of plant sexual reproduction.

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