Stable Isotope Tracer Experiments Give Strong Evidence Supporting a Controversial Pathway in Lignin Biosynthesis

Abstract

Lignin is composed mainly of three structural units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) moieties. The composition of lignin varies with the species and type of tissue, cell, and wall layer. Since the S/G ratio has a great effect on the quality of lignin, many attempts have been made to alter this ratio by genetic engineering. However, the manipulation of genes encoding enzymes of the conventional lignin pathway established by Higuchi [1] has generated unexpected results, which have led the scientific community to re-evaluate lignin biosynthesis. Therefore, our understanding of lignin biosynthesis has progressed rapidly in the last decade. Currently, the lignin biosynthetic pathway is believed to be a metabolic grid, and the focus of interest in lignin biosynthesis is how G intermediates are hydroxylated and methylated to the syringyl monolignol in angiosperms [2]. Our group investigated a controversial pathway in the grid using stable isotope-labeled tracer experiments to clarify whether the pathway functions. Here we explain the efficiency of the stable isotope tracer method and discuss recent findings using this method.