"Highly polarized cells (pollen tubes, root hairs) are an ideal system to investigate signal transduction mechanisms in plant cells. Technical advances in genetics, molecular and cell biology made them excellent models for studying signal perception and transduction in effecting morphogenetic changes. This proposal aims to evaluate how signaling pathways previously identified modulate membrane trafficking and microfilament architecture connecting structure and function. The grant focuses on the role of SAC and syntaxin proteins in polar growing cells. Mutation of SAC1, an Arabidopsis SAC Domain Phosphoinositide Phosphatase, causes alterations in cell morphogenesis and actin organization. We have selected SAC genes highly expressed in tip-growing cells to determine how mutations in these genes affect growth and morphogenesis. We will test SAC dependency over phosphoinositide levels and how this affects the actin cytoskeleton (imaged with GFP-fusion proteins). Using imaging and molecular methods we will test SAC localization and interaction with lipid kinases and syntaxins. Should it prove positive, we will characterize it genetically, biochemically and cellularly. With these experiments we will evaluate if such signaling pathways operate independently or in cascade mode. Previous experiments have also shown that Rab GTPases are key regulators of membrane trafficking and the actin cytoskeleton, namely through modulation of lipid kinase and syntaxin activity. This has only been done independently in distinct cell models and we will test if these small GTPases are a central link of these signaling pathways."