Control of polar plant cell growth: functional characterization of regulators and effectors of the tobacco pollen tube Rac/Rop GTPase Nit-Rac5

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The goal of the proposed study is to elucidate molecular mechanisms regulating polar pollen tube growth by controlling localized Rac/Rop activation at the tip of these cells, or cellular processes downstream of this event. Results obtained will enhance current knowledge concerning Rac/Rop signaling, and the molecular control of polar plant cell growth, of plant development and of the direction of pollen tube growth during fertilization. Pollen tubes are large cells that grow rapidly in a strictly polar manner through female flower tissue and transport sperm cells enclosed in their cytoplasm to egg cells during fertilization. Tobacco pollen tubes are an excellent model system to investigate polar plant cell growth, because they elongate normally in culture, are amenable to experimental manipulation, can be grown in vitro in large quantities and are genetically tractable (transient and stable transformation, ongoing genome sequencing project). Our published work has shown that Rac/Rop GTPases, plant relatives of Rho family GTPases with apparently diverse and largely uncharacterized functions, are associated with the plasma membrane specifically at the tip of pollen tubes and are key regulators of the polar growth of these cells. Here, we propose to functionally characterize using a variety of approaches proteins that show yeast two-hybrid interaction with the tobacco pollen tube Rac/Rop GTPase Nt-Rac5. Substantial preliminary data based on yeast two-hybrid assays, transient over-expression and GFP tagging indicate that these proteins act either as regulators or effectors of Nt-Rac5.