Callase enzyme secreted by the tapetum separates the tetrads to form free microspores

Callase enzyme secreted by the tapetum separates the tetrads to form free microspores (Scott et al., 2004). From flower stage 9 uni-nucleate microspores develop to meiocytes inside the loculus, which is surrounded by the tapetum providing nutrition for pollen development. Meiocytes undergo a mitotic division named as (Pollen Mitosis I; PMI) at this stage the pollen coat is synthesized. These polarize, with a large vacuole at one end and the nucleus at the other, preparing the microspore for an asymmetric cell division. The cell plate formed during pollen mitosis I has a distinctive hemispherical shape, and after division is completed the smaller generative cell detaches from the cell wall and is enveloped within the vegetative cell (Paupiere et al., 2014). The generative cell undergoes Pollen Mitosis II (PMII) of cell division to form the two sperm cells of the mature pollen grain (Figure 1.3). Pollen grains start to accumulate starch during their final stages of the development (Koonjul et al., 2005). Mature pollen is then released into the environment (dehiscence) The movement of pollen grains onto the stigma usually occurs by wind, water or insects. Germination then occurs by the formation of a pollen tube through a pore in the pollen wall until it reaches the embryo sac. One special feature of pollen grains is their wall which is different from any other plant cell. The pollen wall is multi-layered and consists of materials that is highly resistant to degradation, as important determine for pollen viability.

On the surface of the pollen wall a layer that called pollen coat (Taylor and Helper, 1997). This layer is sticky and contains lipids, proteins and phenolic compounds. It is believed to be important for pollen transfer and as well as protection against UV- radiation, and the lipids and proteins are thought to be important in pollen stigma interaction (Dickinson et al., 2000). After dehiscence, the pollen is dehydrated and metabolically inactive however some pollen grains are still partially hydrated and metabolically active (Nepi et al., 2001).

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