How does the vital interaction between pollen and pistil work?

Sexual reproduction in seed plants begins when the pollen grain (male gametophyte) lands on the surface of a compatible stigma, where it hydrates and resumes its metabolism. A pollen tube emerges through the pollen aperture, which is guided through the style by chemical clues from the female tissue to the ovary. Within the ovary, the pollen tube enters a receptive ovule through the micropyle, and delivers the male gametes within the embryo sac (female gametophyte) to carry out fertilization.

Pollen tubes are fast-growing cells exhibiting polar growth at their apex. Pollen tube growth is driven by the rapid and continuous secretion of Golgi-derived vesicles, which dock and fuse with the plasma membrane at the pollen tube tip, providing new plasma membrane and cell wall precursors. Pollen tube cell walls differ in both structure and function from those of somatic plant cells. At the tip, the pollen tube wall is formed by a primary wall, mainly composed of newly synthesized pectins. This primary wall forms the outer layer of the cell wall in the pollen tube shank, where a secondary callose wall is deposited adjacent to the plasma membrane. The content of cellulose in pollen tubes is remarkably low and the subcellular localization varies depending on the species.

The pollen tube wall dynamics are an important feature in the success of fertilization, displaying multiple functions: physical control of pollen tube shape, protection of male gametes against mechanical damage, adhesion of the pollen tube to the pistil transmitting tissue and resistance against the turgor pressure. But how, in molecular terms, does this vital interaction between pollen pollen and pistil work?

A new study in Annals of Botany provides novel information about the time-course and spatial distribution of several pectin and arabinogalactan protein epitopes in olive (Olea europaea L.) pollen at different stages of pollen germination and discusses their putative functions in the context of the pollen–pistil interaction. The authors suggest that galactans might provide mechanical stability to the pollen tube, reinforcing those regions that are particularly sensitive to tension stress and mechanical damage. On the other hand, arabinans and AGPs might be important in recognition and adhesion phenomena of the pollen tube and the stylar transmitting cells, as well as the egg and sperm cells.

Electrophoretic profiling and immunocytochemical detection of pectins and arabinogalactan proteins in olive pollen during germination and pollen tube growth. (2013) Annals of Botany 112 (3): 503-513 doi: 10.1093/aob/mct118
Cell wall pectins and arabinogalactan proteins (AGPs) are important for pollen tube growth. The aim of this work was to study the temporal and spatial dynamics of these compounds in olive pollen during germination. Immunoblot profiling analyses combined with confocal and transmission electron microscopy immunocytochemical detection techniques were carried out using four anti-pectin (JIM7, JIM5, LM5 and LM6) and two anti-AGP (JIM13 and JIM14) monoclonal antibodies. Pectin and AGP levels increased during olive pollen in vitro germination. (1 → 4)-β-D-Galactans localized in the cytoplasm of the vegetative cell, the pollen wall and the apertural intine. After the pollen tube emerged, galactans localized in the pollen tube wall, particularly at the tip, and formed a collar-like structure around the germinative aperture. (1 → 5)-α-L-Arabinans were mainly present in the pollen tube cell wall, forming characteristic ring-shaped deposits at regular intervals in the sub-apical zone. As expected, the pollen tube wall was rich in highly esterified pectic compounds at the apex, while the cell wall mainly contained de-esterified pectins in the shank. The wall of the generative cell was specifically labelled with arabinans, highly methyl-esterified homogalacturonans and JIM13 epitopes. In addition, the extracellular material that coated the outer exine layer was rich in arabinans, de-esterified pectins and JIM13 epitopes. Pectins and AGPs are newly synthesized in the pollen tube during pollen germination. The synthesis and secretion of these compounds are temporally and spatially regulated. Galactans might provide mechanical stability to the pollen tube, reinforcing those regions that are particularly sensitive to tension stress (the pollen tube–pollen grain joint site) and mechanical damage (the tip). Arabinans and AGPs might be important in recognition and adhesion phenomena of the pollen tube and the stylar transmitting cells, as well as the egg and sperm cells.