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.