Programic Phase

Pollen function - the progamic phase 

The progamic phase of pollen development encompasses events which lead from pollination to fertilisation. Pollination occurs when the pollen grains are transferred by wind, insect or mammalian vectors to the stigma. Pollen grains are shed in a partially and often highly dehydrated condition and typically may contain only 15 % water. Dehydration begins within the anther and is completed upon anther opening (dehiscence) when pollen is directly exposed to the atmosphere. Exine and aperture structure and positioning facilitate structural remodelling during pollen dehydration and rehydration. Pollen grains subsequently hydrate and form the pollen tube which emerges through one of the apertures. During adhesion pollen wall proteins are released and a stable and close association of the pollen coat (consisting of proteins, lipids and pigments largely derived from the degenerated tapetal cell layer) with the stigma surface occurs. This step may provide signalling for the controlled released of water from the stigma to enable hydration and successful germination of the pollen grains.

Restriction of pollen hydration prevents pollen germination in incompatible pollen of Brassica species and is evidence of signalling between pollen and stigma. Both male and female components of the Brassica pollen-pistil signalling system have now been identified (Nasrallah & Nasrallah, 1993; Schopfer et al., 1999). These include small cysteine-rich proteins present in the pollen coat which are thought to interact with self incompatibility-locus (S-locus) specific glycoproteins (SLGs) and S-locus receptor-like kinases (SRKs) in the stigmatic surface, the first point of contact between pollen and pistil.

The pollen tube which grows by a process of tip extension penetrates the stigmatic surface initially growing either in between or within the cell walls of the stigmatic papillar cells. Genes encoding several classes of pectin degrading enzymes have been isolated which are often specifically expressed in mature and germinating pollen. Polygalacturonase activity is secreted by pollen tubes and the activities of other cell wall hydrolases including cutinases and ß galactosidases and glucuronidases are present in pollen of several species. Such enzymes are likely to assist in the degradation of stigmatic cell wall materials and the progressive invasive growth through the transmitting tissue of the style. In this regard the pollen-pistil interaction is reminiscent of the invasive growth of a pathogen such as a fungal hypha which involves complex signalling interactions between pathogen and host cells.

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