Cellular signaling mechanisms during pollen-pistil interactions in Brassicaceae

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Plants maintain their yield, vigour and genetic diversity through appropriate selection of genetically diverse mates while rejecting related partners. Given the fact that crop-yield is the most important agronomic trait, understanding the molecular mechanism behind plant reproduction is a major research priority. Successful plant reproduction is dependent on a compatible interaction between the pollen/pollen tube and the pistil. In plant species of Brassicaceae, which are characterized by dry stigmas, only compatible pollen can induce the pistil to provide all the necessary resources for its hydration and penetration into the stigmatic tissue. In contrast, during Brassica (canola, broccoli and cabbage) self-incompatibility (SI) (a selection mechanism to prevent inbreeding), recognition of the self-pollen, induces a highly ordered signaling pathway that rejects the self-pollen (blocks allocation of pistil resources for pollen hydration/germination). During SI response in Brassica, interaction between the pollen and the pistil, leads to recruitment of ARC1 E3 ligase to the proteasome (protein degradation machinery). The targets of ARC1 are proposed to be compatibility factors, which when targeted for degradation by ARC1 results in pollen rejection. My long-term goal is to gain a thorough understanding of the signaling mechanisms that regulate compatible and incompatible pollination events during plant reproduction. Using proteomics approach to identify proteins targeted for destruction following SI response in Brassica stigmas, we identified various metabolic, cytoskeletal and signaling proteins. We have pursued a few of these candidates (Glyoxalase 1 (GLO1), tubulin) and have exciting data that suggest that these proteins are compatibility factors required for successful pollination. The first theme of this proposed research program will focus on determining how the compatibility factor GLO1 is targeted to the proteasome for degradation by the SI response pathway. In addition, the role of methylglyoxal, the cytotoxic metabolite detoxified by GLO1, during SI, will also be addressed using biochemical and molecular approaches. The second major theme is to identify the upstream regulators of MAPKs that we have identified to be involved in mediating compatible pollination. Identification of the activator of these MAPKs would allow us to build the compatible signaling pathway. The third theme is to identify and characterize the phosphatase that controls the phospho-status of MAPKs and one of its likely targets, EXO70A1. Identification of the phosphatase could reveal another mechanism through which SI response could be functioning other than the proteasome-mediated degradation pathway. This research will significantly advance our understanding of the early signaling during pollen-pistil interactions, a key event for successful reproduction.