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Lignin Biosynthesis Complications
Lignin is a polymer that lends its sturdy properties to wood and makes plant cell walls tougher, which creates problems for chemists converting cellulosic plant biomass into biofuels. Vanholme et al. (p. 1103, published online 15 August; see the cover) have identified a new step in the biosynthetic pathway of lignin in Arabidopsis in which caffeoyl shikimate esterase catalyzes synthesis of caffeate. Cellulose from mutant plants, which had reduced amounts of lignin, was more efficiently processed into glucose.
Lignin is a major component of plant secondary cell walls. Here we describe caffeoyl shikimate esterase (CSE) as an enzyme central to the lignin biosynthetic pathway. Arabidopsis thaliana cse mutants deposit less lignin than do wild-type plants, and the remaining lignin is enriched in p-hydroxyphenyl units. Phenolic metabolite profiling identified accumulation of the lignin pathway intermediate caffeoyl shikimate in cse mutants as compared to caffeoyl shikimate levels in the wild type, suggesting caffeoyl shikimate as a substrate for CSE. Accordingly, recombinant CSE hydrolyzed caffeoyl shikimate into caffeate. Associated with the changes in lignin, the conversion of cellulose to glucose in cse mutants increased up to fourfold as compared to that in the wild type upon saccharification without pretreatment. Collectively, these data necessitate the revision of currently accepted models of the lignin biosynthetic pathway.