The architecture and properties of the pollen tube cell wall

Abstract

The pollen tube wall differs in both structure and function from walls of vegetative plant cells. Cellulose represents only a small portion of the cell wall polymers, so an organized microfibrillar system has not been identified yet. The initial wall, formed by secretion at the growing tip, is mostly composed of methyl esterified pectins. During cell wall maturation, concomitant with its translocation from apex to shank, these are demethylated by pectin methylesterase to yield carboxyl groups which have the potential to bind calcium ions, adding mechanical strength to the gel. Callose synthase activity is established close to the growing tip, and builds a callose layer beneath the fibrous pectic layer. The mature wall also contains proteins, arabinogalactan proteins and pollen extensin-like proteins. The mature wall is a cylinder that resists turgor expansion, but is stronger at the base than the tip due to the presence of the callose layer and the gelation of pectin polymers in the shank. Permeability of the wall is essential, to allow passage of both ions and sporophytic proteins that determine compatibility in many species. Influx of calcium ions affects the tip cytoplasm, especially the cytoskeleton, and oscillatory changes in these fluxes are involved in the "pulsatile" mode of growth. This process deposits extra wall material during the "slow" growth phase, which generates rings of increased density in the walls that can be readily seen with appropriate antibodies. © Springer-Verlag Berlin Heidelberg 2006.