Deciphering Male- and Female-coordinated Gating Mechanisms that Ensure Plant Reproductive Success - Project Summary Reproduction in flowering plants depends on multiple male (pollen)-female (pistil) interactive steps to deliver sperm, which are non-motile and transported as cytoplasmic cargos by the pollen tube through several specialized pistil tissues to the female target for fertilization, leading to seed production. The proposed research addresses key cell-cell communicative events in three distinct prezygotic (i.e. prior to sperm-egg fusion) phases during the pollen/pollen tube journey in the pistil to enable fertilization. Long- term efforts in our lab have set discovery milestones for the field and recently elucidated key molecular players in each of these phases, providing critical advances and unprecedented opportunities for the mechanistic dissection proposed here. The pistil supports pollen germination on its receptive surface, the stigma, to produce a pollen tube that grows inside the transmitting tissue to reach the target egg-bearing chamber, the female gametophyte located inside an ovule. Once arriving at the female gametophyte, the pollen tube burst, releasing sperm for fertilization, producing seed. The pistil also set up barriers to ward off unwanted mates or invasive disease agents, and to effectively prevent multiple pollen tubes from penetrating the same female gametophyte to suppress polyspermy and ensure progeny health. We discovered three related signaling modules, each critical for one the three prezygotic phases. Phase 1 supports pollen germination on the stigma. Phase 2 achieves two goals, one ensuring pollen tube integrity until it reaches its target, the other ensuring single pollen tube entry into an ovule. Phase 3 occurs at the pollen tube/ovule and pollen tube/female gametophyte interfaces. Interactions here induce bursting of the first-arriving pollen tube in the female gametophyte and trigger a mechanism for a local polyspermy block to further ensure against supernumerary pollen tube entry into an already penetrated female gametophyte. These signaling modules are anchored by the pistil-expressed receptor kinase FERONIA or pollen-expressed homologs, each partnering with a GPI-anchored protein (GPI-AP) LORELEI (LRE) or LRE-like GPI-AP1,2,3 (LLG1,2,3) to serve as coreceptors for peptide ligands called RALFs (Rapid Alkalinization Factors). Here we propose experiments to elucidate how these signaling modules and additional regulatory factors impact the molecular interactions, biochemical processes and cellular conditions in pistillate cells to mediate success for these prezygotic phases, enabling fertilization, and to prevent unwanted intrusions. Plants have evolved but hidden in the most protected location these highly sophisticated cell-cell communication strategies to ensure their own proliferation. Our expertise positions us uniquely capable of achieving these goals, which will fill a complete mechanistic void, setting paradigms to guide studies in many ecologically and agriculturally important plants, and inform rational designs to safe-guard reproductive success, ensuring food security to provide for global nutritional needs.
