Bundles of hexagonally arranged tubules in timothy grass pollen: Detection of a novel pollen component using anhydrous fixation and image analysis techniques in transmission electron microscopy

Abstract

Pollen from timothy grass (Phleum pratense L.) was subjected to various aqueous and non-aqueous fixation and preparation protocols for transmission electron microscopy. Only in the cytoplasm of anhydrously prepared pollen grains were conspicuous inclusions observed that range in size from less than 1 mum up to 8 or 10 mum. These bodies have so far not been described in the literature. Higher magnifications show that these inclusions consist of bundles of hexagonally arranged small tubules. In order to obtain details of the ultrastructure of this novel pollen component, TEM micrographs of ultrathin sections of hexagonally arranged tubules were analyzed using Fourier transform techniques of image analysis. It was found that the tubules form groups with quasi-periodic hexagonal arrangement, with an average centre-to-centre spacing between the neighbouring tubules of approximately 42 nm. Individual tubules are formed by 12 or 13 particles. The outer diameter of the tubules ranges between 22 and 24 nm. From our experiments, we conclude that the quasi-periodic hexagonally arranged tubules forming conspicuous cytoplasmic inclusions in dry timothy grass pollen grains are structurally similar to microtubules.