Control of pollen hydration in Brassica requires continued protein synthesis, and glycosylation in necessary for intraspecific incompatibility

Abstract

Pollen hydration and self-incompatibility (SI) in Brassica have been studied by using a combination of in vivo video-microscopy and experiments with metabolic inhibitors. Experiments with cycloheximide confirm earlier observations that pollen hydration is regulated through protein synthesis. No protein or glycoprotein has positively been identified with this event; however, it is unlikely that the total pool of any particular glycoprotein is involved, but rather a newly synthesized or otherwise activated fraction. Micromanipulation of pollen on the stigmatic papillae suggests that access to this hydration regulation system is limited to members of the Brassicaceae: pollen grains of other species-even those possessing dry stigmas-fail to hydrate. It is proposed that an interaction between enzymes of the stigma surface and the superficial layer of the pollen grain coating creates continuity between the content of the papillar wall and the grain protoplast. Inhibition of protein synthesis also overcomes SI, and since the advent of regulated hydration and synthesis of the so-called S-gene glycoproteins coincide with the acquisition of the SI system, there is strong circumstantial evidence that the same molecular species is involved in both processes. Experiments with tunicamycin, which prevents glycosylation of glycoproteins, indicate that the glycosyl groups of the S-gene glycoprotein are required for the operation of the SI system but not for the regulation of hydration. Further experiments suggest that pollen is positively inhibited on incompatible papillae but that this inhibition is biostatic. Recovery from the effects of the SI system appears to involve the metabolism of an inhibitor by the pollen. SI in Brassica thus emerges as a sophisticated process under dynamic control in both the female and male partners. The evolutionary advantages of such a system are discussed.