After pollen binds to the stigma, the extracellular coating diffuses along the stigma surface and facilitates water transfer to the desiccated pollen grains. Mutations in CER genes cause a defect in pollen hydration by decreasing the levels of long-chain lipids on the pollen surface and destabilizing the entire pollen coating. The pleiotropic nature of the cer defects has identification of a mutation in the GRP17 gene has revealed the function of the major pollen coat protein. grp17 mutants have a normal complement of all other coat proteins and lipids, yet display severe delays in pollen hydration. Genes encoding five other coat proteins have been identified in Dr. Preuss' laboratory; identifying mutations in those genes will make it possible to determine whether their products interact with GRP17, contribute to pollen hydration, or play a role in maintaining pollen coat structure. Interestingly, most of the pollen coat proteins are encoded by a tandem array of GRP genes; this arrangement raises the possibility that mating specificity in plants, like that observed in animals or algae, is controlled by concerted evolution of a single locus. There is one specific aim: to investigate the requirements for pollen hydration a) identify mutants defective in individual coat proteins, b) investigate interactions between pollen coat proteins and lipids by combining grip and cer mutations, c) determine the extent of GRP functional redundance by characterizing the effects of a deletion of the entire GRP locus and d) localize GRP proteins, determining their association with lipid droplets and migration onto the stigma surface. Fertilization in flowering plants depends on as elaborate series of cell-cell interactions that enable female pistil cells to discriminate among pollen grains and inhibit, on an individual basis, the invasive growth of foreign pollen and pathogens. These interactions are essential for plant development and species identity, yet in most cases the molecular basis is not understood. The long-term goal of this research is to uncover molecules required for Arabidopsis reproduction, including those that mediate pollen adhesion, recognition, hydration, tube initiation, and growth to the ovules. This project focuses on the initial step in pollination - the interactions between pollen and stigma cells.
Nsf-funded investigations in Dr. Preuss' laboratory have determined that pollen-stigma adhesion relies on species-specific molecules within the extracellular pollen wall [1], and that the subsequent hydration of pollen requires a glycine-rich, lipid binding protein within the hydrophobic pollen coat [2]. In both cases, genetic and molecular studies confirm that these interactions involve novel classes of molecules and require extremely hydrophobic components. The immediate goals of this project are to elucidate the roles of pollen surface lipids and proteins in pollen hydration.
