Flowering plants have developed a variety of strategies to control fertilization. Self-incompatibility is a mechanism that allows plants to distinguish and reject their own or genetically similar pollen. Thus, self-incompatible plants avoid inbreeding and promote genetical diversity. In Solanaceae and other families, pollen recognition occurs in the upper part of the style and involves a ribonuclease (S-RNAse) secreted by the transmitting tissue to the extracellular matrix and taken up by pollen tubes. Inside pollen tubes, S-RNases interact with the SLF protein, the recognition factor of male side. The consequence of this interaction is the rejection (incompatible reaction) or acceptance (compatible reaction) of pollen by the pistil. Pollen rRNA is selectively degraded in incompatible reactions suggesting that S-RNase has a cytotoxic effect during pollen rejection. Using immunocytochemistry and confocal microscopy, S-RNase was shown to be compartmentalized in the vacuole of pollen tubes. In compatible pollinations, the vacuole appears to be intact while in incompatible pollinations, vacuole is disrupted when pollen tubes are rejected, releasing S-RNase to the cytoplasm. This result suggests that compartmentalization plays a key role in the process of selecting the succesful pollen.
