Pollen-stigma adhesion in Arabidopsis: a species-specific interaction mediated by lipophilic molecules in the pollen exine

To investigate the nature and role of cell adhesion in plants, we analyzed the initial step of pollination in Arabidopsis: the binding of pollen grain s to female stigma cells. Here we show this interaction occurs within secon ds of pollination. Because it takes place prior to pollen hydration, it als o requires adhesion molecules that can act in a virtually dry environment. We developed assays that monitored adhesion of populations of pollen grains and individual cells. Adhesion between pollen and stigma cells is highly s elective - Arabidopsis pollen hinds with high affinity to Arabidopsis stigm as, while pollen from other species fails to adhere. Initial binding is ind ependent of the extracellular pollen coat (tryphine), indicating that adhes ion molecules reside elsewhere on the pollen surface, most likely within th e exine walls. Immediately after pollination, the stigma surface becomes al tered at the interface, acquiring a pattern that interlocks with the exine; this pattern is evident only with pollen from Arabidopsis and its close re latives. Purified exine fragments bind to stigma cells, and biochemical ana lyses indicate that this specific, rapid and anhydrous adhesion event is me diated by lipophilic interactions.