Pollen walls in angiosperms typically consist of an outer exine composed of sporopollenin and an inner intine composed of cellulose and pectin (Fig. 4) (Paxson-Sowders et al. 1997; Morant et al. 2007). Models of development have been proposed based on observations on numerous species, including Lilium and arabidopsis (e.g. Suzuki et al. 2008). Similar processes have been described in both these species.Fig. 4
Diagram of arabidopsis pollen wall structure. The inner intine and the various components of the outer exine are indicated. Note the pollen coat (tryphine and pollenkitt) filling the cavities of the exine sculpture. Taken from Suzuki et al. (2008).
Once again, prior to meiosis, the pollen mother cell is surrounded by a callose special cell wall (Blackmore et al. 2007). Immediately after meiosis, four microspores derived from the pollen mother cell form a tetrad. A callose special wall surrounds the microspores (Blackmore et al. 2007). A cellulose primexine then forms between the plasma membrane and callose wall of each microspore.
Both the callose wall and primexine are deposited at the surface of the microspore through processes mediated by the plasma membrane (Blackmore et al. 2007). A section of the primexine is then adapted to form column-like structures called the probaculae upon which sporopollenin, secreted by the microspore, will eventually accumulate and polymerize. Sporopollenin deposition and accumulation extend the probaculae, which form the baculae and the tectum (Heslop-Harrison 1963, 1968).
The callose wall then degrades and the developing baculae and tectum are exposed to the fluid of the locule and receive sporopollenin secreted by the tapetum. Wall formation is complete when the nexine and intine layers are formed and the primexine recedes and disappears (Suzuki et al. 2008). The mature pollen grain is then coated by tryphine and pollenkitt, which are synthesized by the tapetum (Dickinson and Lewis 1973; Blackmore et al. 2007).