Beyond the green: understanding the evolutionary puzzle of plant and algal cell walls.

Niklas (2000) defined plants as “photosynthetic eukaryotes,” thereby including brown, red, and green macroalgae and microalgae. These groups share several features, including the presence of a complex, dynamic, and polysaccharide-rich cell wall. Cell walls in eukaryotes are thought to have evolved by lateral transfer from cell wall-producing organisms (Niklas, 2004). Green and red algae originate from a primary endosymbiotic event with a cyanobacterium, which is thought to have occurred over 1,500 million years ago (Palmer et al., 2004). Even though extant cyanobacteria have cell walls that are based on a peptidoglycanpolysaccharide-lipopolysaccharide matrix and thus differ markedly from the polysaccharide-rich cell walls of plants, there is preliminary evidence that they may contain some similar polysaccharides (Hoiczyk and Hansel, 2000), and genes already involved in polysaccharide synthesis or those subsequently coopted into wall biosynthesis may have been transferred during endosymbiosis. Independent secondary endosymbiotic events subsequently gave rise to the Euglenozoa (which lack cell walls) and brown algae (which have cell walls; Palmer et al., 2004). Investigations of the diversity of wall composition, structure, and biosynthesis that include algae, therefore, may lend new insights into wall evolution (Niklas, 2004). Algal cell wall research, in common with that of land plants, has focused on commercially important species and polysaccharides; thus, the most well-described algal wall components include the commercially and ecologically important laminarans, carrageenans, fucans, and alginates (Mabeau and Kloareg, 1997; Campo et al., 2009). However, there are over 35,600 species of seaweed, and their cell wall components exhibit enormous diversity (for review, see Painter, 1983; Kloareg and Quatrano, 1988; De Reviers, 2002). Even though distinct suites of polysaccharides are known to occur in different taxa such that algal cell wall profiles can be used as taxonomic markers (Parker, 1970; Domozych et al., 1980), some wall components have a wider distribution and are also found in other organisms, including land plants. Renewed interest in plant and algal cell wall composition (Popper and Fry, 2003, 2004; Niklas, 2004; Vissenberg et al., 2005; Van Sandt et al., 2007; Fry et al., 2008a, 2008b; Popper, 2008; Sørensen et al., 2008), perhaps driven by potential industrial applications (Pauly and Keegstra, 2008) and a desire to better understand cell wall functions (Niklas, 2004), has been facilitated by the development of several techniques capable of screening cell wall polymers. Increased information has added detail to the diversity known to exist in cell wall composition, generated as organisms adapted to specific niches (Sarkar et al., 2009). However, it is also becoming apparent that similarities, as well as differences, exist between plant and algal cell walls. Therefore, examination of the patterns of occurrence of wall components suggests that existing diversity is likely to be the result of a variety of different evolutionary scenarios.