4. Pollen and spores

Modes of sporopollenin deposition in spore and pollen walls

The basic mechanisms involved in the formation of the spore wall, and the deposition of sporopollenin in the exospore/exine, have been illuminated by numerous ultrastructural studies performed on extant and fossil species across the plant kingdom (Paxson-Sowders et al. 2001). Blackmore and Barnes (1987) proposed a number of sporopollenin deposition processes apparent in the spore wall. Firstly, they recognized the role of white-line-centred lamellae (WLCL) in this process.

The accumulation of sporopollenin on an array of WLCL is regarded as being the most primitive method of sporopollenin deposition and has been identified in a number of algal groups and most, if not all, embryophytes (Wellman 2004). These lamellae materialize at the plasma membrane with sporopollenin polymerizing out onto either side of the white line. They accumulate in a variety of ways to form the spore/pollen wall (Blackmore and Barnes 1987; Blackmore et al. 2000; Wellman 2004).

Another mode of exospore/exine formation involves the deposition of sporopollenin from the surrounding cells of the tapetum. Transmission electron microscopy has shown that the tapetal cells possess a highly active secretory system containing lipophilic globules, which are thought to contain the precursors of sporopollenin and are deposited onto the surface either directly contributing to the exospore/exine or forming extra-exosporal layers (Piffanelli et al. 1998).

‘ Blackmore et al. (2000) suggested that a tapetal contribution to the spore wall can take place in a variety of ways, including the addition to the layers formed by the WLCL or directly onto WLCL. Studies of pollen wall formation in angiosperms highlight the role that tapetal cells play in supplying nutrients and lipid components to developing microsporocytes and microspores (Scott et al. 1991; Ariizumi et al. 2004; de Azevedo Souza et al. 2009). Interestingly, the most basal extant land plants (liverworts) lack a tapetum, which is acquired in mosses and vascular plants.

An alternative deposition process involves centripetal accumulation of sporopollenin onto previously formed layers. Blackmore et al. (2000) noted that exospore formation may be achieved by sporopollenin accumulation below a pre-existing layer, either by WLCL accumulation or by the deposition of granular or unstructured sporopollenin. A further mode of deposition is observed in seed plants where sporopollenin accumulates within a pre-patterned cell surface glycocalyx referred to as the primexine (Blackmore and Barnes 1987; Blackmore et al. 2000; Wellman 2004), which is essentially an exine precursor.

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