Breeding of Vegetable, Spice and Tuber Crops

Gametophytic Self-Incompatibility (GSI)

• This form of self-incompatibility is more common than SSI but not so well understood.

• It occurs in nearly one-half of all the families of angiosperms, including the Solanaceous crops (Potatoes, tomatoes [wild, not cultivated)

1. Beets (Beta vulgaris)
2. Buttercups (Ranunculus)
3. Lilies
4. Roses
5. Many grasses

The rules

• The S loci are (as in SSI plants) extremely polymorphic; that is, there is an abundance of multiple alleles in the population.
• Incompatibility is controlled by the single S allele in the haploid pollen grain.
• Thus a pollen grain will grow in any pistil that does not contain the same allele (so, as shown here and in contrast to what happens in SSI, S2 pollen from an S1S2 parent will grow down on S1S3 style.
• This appears to be the mechanism in tobacco:
• All pollen grains — incompatible as well as compatible — germinate forming pollen tubes that begin to grow down the style.
• However, growth of incompatible pollen tubes stops in the style while compatible tubes go on to fertilize the egg in the ovary.
• The block within incompatible pollen tubes is created by an S-locus-encoded ribonuclease (RNase), which is

• Synthesized within the style
• Enters the pollen tube and
• Destroys its RNA molecules
• Halting pollen tube growth.

• The RNase molecules contain a hyper variable region which is the basis for each S specificity (S1, S2, S3, etc.).
• The pollen tube expresses a molecule that binds RNase. This targeting molecule also exists is different S specificities (S1, S2, S3, etc.).
• In compatible ("nonself") tubes, the targeting molecule sequesters the RNase in a vacuole thus permitting RNAs in the cytosol to survive and growth to continue.
• In incompatible ("self") tubes the binding of, for example, the S1 targeting molecule to an S1 binding site on the S1 RNase releases the RNase into the cytosol so the RNAs of the pollen tube are destroyed and growth is halted.