Dry stigmas,water and self-incompatibility in Brassica

The evolution of dry stigmas has been accompanied by the development — in the pollen — of mechanisms for accessing water from the stigmatic epidermis. Development of self- and cross-pollen on the stigmatic surface has been examined in Brassica oleracea, focusing on the hydration of the grains. Unlike self-compatible (SC) Arabidopsis thaliana, pollen hydration of self-incompatible (SI) Brassica oleracea is preceded by a latent period of between 30–90 min, which is significantly shortened by inhibition of protein synthesis in the stigma. Physiological experiments, some with isolated pollen coatings, indicate that during the latent period signals passing from the pollen to the sigma are responsible for readying the stigmatic surface for penetration and — after self-pollination — activation of the SI system. The changes at the stigma surface include the expansion of the outer layer of the cell wall beneath the grain. This expansion does not occur following self-pollination, when coating-derived signals stimulate a stigmatic response which interrupts hydration and arrests grain development. Cell manipulation studies suggest that self grains are not inhibited metabolically, but are physiologically isolated from the subjacent stigmatic papilla. This focusing of the SI response at the pollen-stigma interface ensures that a single papilla can simultaneously accept cross-pollen and reject self-grains. The evolution of this highly efficient SI system is disussed in the perspective of pathogen-defence mechanisms known also to be located in epidermal cells.