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 peptidoglycan-polysaccharide-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).peer-reviewe