Genes involved in the biosynthesis of lignin precursors in Arabidopsis thaliana

Lignin is a complex polymer assembled from monolignol precursors derived from phenylalanine after several hydroxylation and methylation steps of the aromatic ring and reduction of the lateral chain. Three main monolignols, the p-coumaryl, coniferyl and sinapyl alcohols, give rise, respectively, to the hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units of the polymer. A complete inventory of the genes potentially involved in the monolignol pathway in the model plant, Arabidopsis thaliana, is presented in this review. Genes encoding enzymes implicated in constitutive lignin synthesis were identified on the basis of their homology to monolignol biosynthesis genes of other plants and their high expression in lignified tissues (floral stems, roots). This overview shows that most of these genes belong to multigene families and that some (PAL, 4CL, CAD) are duplicated in this model plant. The genes encoding the cytochrome P450 monooxygenases (C4H, C3H, F5H) are unique except for F5H that has at least one homologue gene present in the complete genome. Mutants and transgenic Arabidopsis lines deregulated in the monolignol biosynthesis pathway are listed and the impact of the target gene deregulation on growth, and lignin content and structure are reported.