How flowering plants discriminate between self and non-self pollen to prevent inbreeding

Flowering plants have evolved various genetic mechanisms to circumvent the tendency for self-fertilization created by the close proximity of male and female reproductive organs in a bisexual flower, One such mechanism is gametophytic self-incompatibility, which allows the female reproductive organ, the pistil, to distinguish between self pollen and non-self pollen; self pollen is rejected, whereas non-self pollen is accepted for fertilization, The Solanaceae family has been used as a model to study the molecular and biochemical basis of self/non-self-recognition and self-rejection, Discrimination of self and non-self pollen by the pistil is controlled by a single polymorphic locus, the S locus, The protein products of S alleles in the pistil, S proteins, were initially identified based on their cosegregation with S alleles, S proteins have recently been shown to indeed control the ability of the pistil to recognize and reject self pollen, S proteins are also RNases, and the RNase activity has been shown to be essential for rejection of self pollen, suggesting that the biochemical mechanism of self-rejection involves the cytotoxic action of the RNase activity. S proteins contain various numbers of N-linked glycans, but the carbohydrate moiety has been shown not to be required for the function of S proteins, suggesting that the S allele specificity determinant of S proteins lies in the amino acid sequence, The male component in self-incompatibility interactions, the pollen S gene, has not yet been identified, The possible nature of the pollen S gene product and the possible mechanism by which allele-specific rejection of pollen is accomplished are discussed.