We are dissecting the mechanisms that govern the initial cell—cell interactions during reproduction in the flowering plant, Arabidopsis thaliana. Unlike most animal cell—cell contacts, plant cells interact through elaborate cell walls and extracellular matrices. The initial contacts between the male pollen and female stigma cells include specific and selective binding of appropriate pollen by the stigma, rapid pollen-recognition, pollen hydration, and pollen tube formation. We have measured the first step in Arabidopsis reproduction, the adhesion of pollen to the stigma, at the single cell level—a binding that is surprisingly strong and specific. The pollen coat, a lipid- and protein-rich matrix that enshrouds pollen, does not mediate this specific adhesion, but instead enables pollen-stigma signalling and pollen hydration. Mutants that abolish this matrix have proven fruitful in elucidating the pollen coat's roles at the pollen-stigma interface. Pollen from such mutants adheres to the stigma at levels indistinguishable from wild type, indicating that binding molecules reside in the exine, but do not hydrate. Interestingly, a genetic screen for pollen adhesion mutants has defined one individual that exhibits aberrant exine. A reverse-genetic approach will further our understanding of the function of pollen coat proteins in coat integrity, signalling and hydration. Biochemical analyses will unravel the functions of the coat lipids. Experiments involving the application of exogenous purified lipids can bypass pollen hydration specificity, inducing heterologous grains to hydrate and germinate pollen tubes. These many approaches promise to enhance our understanding of the myriad of unique interactions that occur at the pollen-stigma interface.
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