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.
Kao
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Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park 16802, USA.
McCubbin
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D000483:Alleles; D005838:Genotype; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D010944:Plants / Q000235:genetics / Q000276:immunology*; D011058:Pollen / Q000276:immunology*; D021382:Protein Sorting Signals / Q000235:genetics / Q000502:physiology; D012098:Reproduction / Q000235:genetics / Q000276:immunology; D012260:Ribonucleases / Q000378:metabolism
D010940:Plant Proteins; D021382:Protein Sorting Signals; D012260:Ribonucleases