Self-Incompatibility
The failure of gametes from the same plant to form a viable embryo is known as self-incompatibility (SI). The process was initially studied in ornamental tobacco, where it was noticed that pollen grains that fell on the stigma of the same plant failed to grow down the style. However, the pollen could grow successfully down the style of other plants of the same species. In addition to tobacco, SI mechanisms have evolved in many diverse angiosperms families, with different families utilizing different proteins to accomplish the same goal. SI increases outcrossing and heterozygosity and prevents inbreeding.
SI systems operate prior to fertilization during interactions between the male gametophyte (pollen or pollen tube) and the female sporophytic tissue (pistil). Incompatibility can be achieved by prevention of pollen germination, retardation of the growth or disorientation of the pollen tube, or failure of nuclear fusion. These processes are usually controlled by a single genetic locus (the S locus) with multiple alleles. The S locus contains one or more genes expressed in the male or female reproductive tissues. Differences in proteins encoded by these genes are the basis for the recognition of self or nonself pollen; when the pollen genotype matches either of the two S alleles of the pistil, the pollen cannot grow.
The two major types of SI can be described as gametophytic and sporophytic SI systems. Gametophytic SI (GSI), in which the behavior of the pollen is determined by the haploid pollen genotype at the S locus, is the more common system, found in more than 60 families of plants. In many cases of GSI, the pollen tube is able to initiate growth through the style before growth is arrested. GSI has been well studied in many species of the family Solanaceae, including petunia, tobacco, and tomato. The proteins expressed in the female tissues necessary for GSI have been identified and are ribonucleases. They were named the S-RNases because they are encoded by genes found on the S locus. The pollen-expressed male component that interacts with these S-RNases is still unknown, however, as is the mechanism by which S-RNases mediate the incompatibility response.
In contrast to GSI, the less common sporophytic SI (SSI) is determined by the genotype of the female tissue, not the pollen. SSI responses occur early in pollen-pistil interactions and often block hydration of the pollen grain or emergence of the pollen tube at the stigma surface. SSI has been studied in the family Brassicaceae, where the stigma proteins involved have been identified. They include a protein secreted into the cell wall of certain stigma cells and a second protein that appears to be a receptor. Both of these proteins are likely required in order to recognize the pollen SSI component, which has been shown to be a small peptide found on the surface of the pollen grain.
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