Self-Incompatibility in Brassica: The Nature and Role of Female Glycoproteins

Summary

The roles played by stigmatic glycoproteins in the self-incompatibility (SI) mechanism in Brassica oleracea have been investigated. In a physiological study, the suppression of protein synthesis was demonstrated to overcome SI, but also affect a number of other pollination-related events, such as regulated hydration of the pollen grain. Interestingly, inhibition of glycosylation appears to affect SI alone, leaving other processes to proceed normally. When stigmatic glycoproteins are used in bioassays, it is clear that they can regulate pollen tube growth in accordance with the S (incompatibility) genes present. However, the multiplicity of glycoproteins possessed by each genotype, and their presence in quantity indicates that they are unlikely to be simple intercellular signals. Database searches involving concensus sequences from the family of stigmatic glycoproteins have revealed strong homologies with domains in several classes of animal proteins, principally the von Willebrand factor and type VI collagen. These polypeptides, which are also heavily glycosylated, are held to be involved in protein-protein interactions and extracellular matrix formation — processes which may also be involved in the initial stages of pollination. Detailed modelling of these sequences indicate that, despite small differences in amino acid composition, these domains are strikingly similar in their three dimensional molecular architecture.