HOW FLOWERING PLANTS DISCRIMINATE BETWEEN SELF AND NON-SELF POLLEN TOPREVENT 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 me chanism 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 m odel to study the molecular and biochemical basis of self/non-self-rec ognition and self-rejection, Discrimination of self and non-self polle n by the pistil is controlled by a single polymorphic locus, the S loc us, The protein products of S alleles in the pistil, S proteins, were initially identified based on their cosegregation with S alleles, S pr oteins have recently been shown to indeed control the ability of the p istil to recognize and reject self pollen, S proteins are also RNases, and the RNase activity has been shown to be essential for rejection o f self pollen, suggesting that the biochemical mechanism of self-rejec tion involves the cytotoxic action of the RNase activity. S proteins c ontain various numbers of N-linked glycans, but the carbohydrate moiet y has been shown not to be required for the function of S proteins, su ggesting 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 pos sible nature of the pollen S gene product and the possible mechanism b y which allele-specific rejection of pollen is accomplished are discus sed.