Prevents self-polination

Did you know that roses are bisexual? Most plants are like this - they're hermaphrodites. With such an arrangement, one wonders what prevents plants from fertilising themselves. In fact, ubiquitin-mediated protein breakdown is involved: the plant recognises and rejects its own pollen! The exact mechanism is not yet fully clear, but enzyme E3 has been found and when a proteasome inhibitor has been added, the rejection has been noticeably impaired.