A functional and comparative study of the evolution of phenylpropanoid metabolism in land plants.

摘要

Phenylpropanoid metabolism is a hallmark of land plants, and produces a spectrum of secondary metabolites, including lignin, sporopollenin, flavonoids and hydroxycinnamate conjugates. Many of these compounds are essential for plants' successful survival in a terrestrial environment. Since its origin, phenylpropanoid metabolism has been extensively exploited by plants in adaptation to ecological niches, and this interplay of metabolism, adaptation and ecology has provided a rich system for studying evolutionary biology. The research presented in this dissertation explores phenylpropanoid metabolism in Selaginella moellendorffii, a species that represents a lineage of vascular plants that diverged from the well-studied flowering plants over 400 million years ago. Functional characterization of several key enzymes in the pathway revealed that Selaginella has evolved new metabolic routes within the relatively conserved framework of phenylpropanoid metabolism, leading to the biosynthesis of syringyl lignin, a lignin type traditionally considered to be restricted in flowering plants. By comparing the metabolic pathway of syringyl lignin biosynthesis in Selaginella to the analogous pathway in flowering plants, we have found that the occurrence of syringyl lignin in these two distantly-related lineages is due to convergent evolution, in which distinct biochemical mechanisms were employed. This study not only advances our understanding of the evolution of plant phenylpropanoid metabolism in general, but also provides valuable tools for rerouting lignin biosynthesis and thus modifying lignocellulosic biomass in economically important crops.