Breeding of Vegetable, Spice and Tuber crops (2+1)

Sporophytic Self-Incompatibility (SSI)

• This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower
• In this system, rejection of self pollen is controlled by the diploid genotype of the sporophyte generation.
• mustard
• The control lies in the "S-locus", which is actually a cluster of three tightly-linked loci
• SLG (S-Locus Glycoprotein) which encodes part of a receptor present in the cell wall of the stigma
• SRK (S-Receptor Kinase), which encodes the other part of the receptor. Kinases attach phosphate groups to other proteins. SRK is transmembrane protein embedded in the plasma membrane of the stigma cell.
• SCR (S-locus Cysteine-Rich protein), which encodes a soluble, secreted ligand for the same receptor.
• Because the plants cannot fertilize themselves, they tend to be heterozygous; that is, carry a pair of different S loci (here designated S1 and S2).
• However, dozens of different S alleles may be present in the population of the species; that is; the S-locus in the species is extremely polymorphic (analogous to the major histocompatibility locus of vertebrates)
 


• The difference between the alleles is concentrated in certain "hyper variable regions" of the receptor (analogous to the hyper variable regions that provide the great binding diversity of antibodies).
 

The rules

• Pollen will not germinate on the stigma (diploid) of a flower that contains either of the two alleles in the sporophyte parent that produced the pollen.
• This holds true even though each pollen grain — being haploid — contains only one of the alleles.
• In the example shown here, the S2 pollen, which was produced by a S1S2 parent, cannot germinate on an S1S3 stigma.
• The explanation
• The S1S2 pollen-producing sporophyte synthesizes both SCR1 and SCR2 for incorporation in (and later release from) both S1 and S2 pollen grains.
• If either SCR molecule can bind to either receptor on the pistil, the kinase triggers a series of events that lead to failure of the stigma to support germination of the pollen grain.
• Among these events is the ubiquination of proteins targeting them for destruction in proteasomes.
• If this path is not triggered (e.g., pollen from an S1S2 parent on an S3S4 stigma, the pollen germinates successfully)