Regulation of syringyl to guaiacyl ratio in lignin biosynthesis

Recent findings, hypotheses and controversies on lignin biosynthesis and results of gene manipulations are reviewed. The distribution of the three main structural units of lignin, p-hydroxyphenyl (H)-, guaiacyl (G)- and syringyl (S) propane, in situ is described. New pathways that regulate the ratio of S to G moieties operating at the stages of cinnamoyl CoA, cinnamyl aldehyde and cinnamyl alcohol are introduced. The roles of monolignol glucoside in the lignification of tree xylem are discussed. The results of gene manupulations that alter the lignin structures are also introduced.