AbstractsBiology & Animal Science

Rab GTPase proteins are important regulators of membrane trafficking. Arabidopsis thaliana (A. thaliana) has 57 Rab family members, most of which are currently uncharacterized. We found that one RabA family member, RabA4d, is required for normal pollen tube growth. EYFP-RabA4d-labeled membrane compartments are tiplocalized in growing pollen tubes and loss of RABA4D leads to abnormal pollen tube growth and guidance. As several RabA family members are expressed in pollen this provided an opportunity to study RabA functional redundancy. While loss of RabA4d, the sole RabA4 subfamily member in pollen, led to abnormal pollen tubes, loss of xvii RabA1i, one of several RabA1 subfamily members expressed in pollen, led to formation of normal pollen tubes. Pollen tubes and root hairs are highly similar cells, with many growth requirements in common. Phosphatidylinositol 4-phosphate (PI-4P) is important for root hair tip growth. We investigated the function of PI-4P synthesis in pollen tube growth. We found that phosphatidylinositol 4-OH kinase β1 (PI-4Kβ1), an enzyme predicted to catalyze formation of PI-4P and identified as a RabA4b effector, also interacts with RabA4d in a GTP-dependant manner. Loss of PI-4Kβ1 and PI-4Kβ2 led to short, crooked pollen tubes, indicating importance of PI-4P in pollen tube growth. Additionally, PI-4P is enriched in the membrane at the tip of growing pollen tubes. Rab GTPase proteins regulate membrane trafficking by recruiting various effector proteins to intracellular membranes. We characterized the putative RabA4b effector PLANT U-BOX PROTEIN13 (PUB13). PUB13 belongs to a family of predicted E3 ubiquitin ligase proteins which function in plant responses to extracellular stresses such as pathogens. We found that pub13/pub13 plants showed increased resistance to the plant pathogen Pseudomonas syringiae DC3000. Overall, these data lead to an increased understanding of RabA family function in A. thaliana.