In vivo pollen tube cell of Arabidopsis thaliana I. Tube cell cytoplasm and wall

Pollen tubes are an excellent system for the study of polarized tip growth, cell movement, and cell-to-cell communication in plants. We report the first description at the transmission electron microscope level of cryofixed, in vivo grown pollen tubes. The tube cell of Arabidopsis thaliana growing in the style can be divided into four zones, similar to those reported for pollen tube cells of members of the family Solanaceae. Some distinct differences from data reported for other species include the morphology of the clear zone at the tip, the composition of the tip wall, and the apparent lack of arabinogalactan proteins in the pollen tube and surrounding transmitting tissue. The clear zone of A. thaliana is packed with vesicles which do not form the inverted cone found in other species and that fuse in clusters with the plasmalemma in the tip region. Immunocytochemical analyses revealed that the pollen tube has a primary, pectic wall composed solely of highly esterified homogalacturonans at the tip. This wall becomes increasingly thicker basipetally and is composed of both highly esterified and lowly esterified homogalacturonans behind the clear zone. A secondary callosic wall appears behind the clear zone and becomes thicker basipetally. Using a variety of probes, we have been unable to identify any arabinogalactan proteins in the pollen tube cell or in the transmitting tissue of the style. Unlike our earlier reports of the extracellular matrices of the septum transmitting tissue of the ovary, the stylar matrices were found to be rich in both highly and lowly esterified homogalacturonans.