The most widespread SI system is genetically controlled at the chromosomal region called the S locus, which contains numerous alleles that control recognition specificity. These genes code for S-locus ribonucleases (S-RNases), enzymes that degrade RNA. S-RNases are expressed in the stigma, style and ovary and concentrate in the style ECM where they interact with pollen tubes. The recognition factors on the pollen side are the SLF (S-locus, F-box) alleles. If the S alleles of the pollen match those in the stylar ECM, the pollen is rejected. If they are different, the pollen tubes are allowed to grow toward the ovary.
The exact nature of the interactions is still unclear, but the work of Cruz-Garcia and other scientists suggests that S-RNases are taken up into the pollen tubes and sequestered there in vacuoles. If the pollen is too closely related and therefore incompatible, the vacuolar compartments are somehow disrupted, and the S-RNases released to the cytosol begin to destroy the RNA of the pollen tubes, in effect, killing them. However, S-specific matching between S-RNase and SLF, by itself is not sufficient for rejection of incompatible pollen. Other factors not linked to the S locus are also required in the genera Nicotiana (tobacco), Solanum (tomato) and Petunia. Three other glycoproteins, proteins with attached carbohydrate components, have been found to be associated with pollen tubes. These proteins, transmitting-tract-spec
|Contact: Dr. Felipe Cruz-Garcia|
American Society of Plant Biologists