Pollen-wall proteins: localization and enzymic activity

Abstract

Cytochemical methods have been used to examine the distribution of acid phosphatase, ribonuclease, esterase, amylase and protease activity and protein in the walls of pollen grains and spores. Enzyme activity was detected in the walls of 50 angiosperm pollens, in pine, and in the spore wall of a species of Equisetum. Activity was absent from the 2 species of ferns examined. The survey covered all major structural pollen types. In all cases enzyme activity was associated principally with the intine, the inner cellulosic part of the wall. In all aperturate grains activity was concentrated in or around the apertural intine; that is, over or near the potential sites of emergence of the pollen tube. Protein concentrations in the intine at these sites could be demonstrated in several pollen types by staining methods and by absorption at 285 nm. Developmental study showed that the enzymes are incorporated in the intine during the early period of wall growth following the release of the spores from the meiotic tetrads. During this period, stratified ribosomal endoplasmic reticulum lies adjacent to the inner spore wall over the areas of incorporation. In Cosmos bipinnatus, a composite, the material is incorporated as ribbons or leaflets, which interleave with cellulose lamellae. In other species the wall protein may take the form of granules, tubules or vesicles, embedded in the intine cellulose. At maturity the intine is separated from the spore cytoplasm by an intact plasmalemma, so the wall enzymes are to be regarded as being extracellular. In some species enzyme activity was detected in materials on the surface of the pollen exine derived probably from the surrounding tapetal tissue; however, this was usually insignificant compared with that shown by the intine. The intine enzymes are very readily leachable, and their function is probably connected with the early nutrition of the pollen tube and the penetration of the stigma. It is likely that the wall proteins make up a very large proportion of mobile protein of the pollen grain, and they may therefore be important in pollen allergenicity.