Phenylpropanoids play important roles in plant structure, growth and development, as well as in plants' defenses against biotic and abiotic stresses. From a human perspective, phenylpropanoid metabolites influence both human health and the potential utility of plants in agricultural contexts. The most abundant phenylpropanoid product is lignin. By definition, the biosynthesis of lignin is a hallmark of vascular plants because it is in the xylem of the vasculature that lignin is deposited. In these cells, lignin functions to allow tracheary element cell walls to resist the tension generated during transpiration. Further, its deposition, particularly in plants in which secondary growth occurs, provides physical support to the plant body. Finally, many phenylpropanoids have been exploited by plants to serve as UV protectants. It is for these reasons that the evolution of the phenylpropanoid pathway is thought to have been critical to the colonization of land by tracheophytes. Despite the profound importance that lignin, and phenylpropanoid metabolism in general, had on the emergence of land plants, virtually nothing is known about the evolution of the pathway. The lignin heteropolymer is produced via the oxidative coupling of subunits termed monolignols. The polymerization of these subunits leads to the formation of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin, respectively. H subunits are generally minor components, and the degree to which G and S units are incorporated into the polymer varies widely among species, tissue types, and even within an individual cell wall. S lignin is generally considered to be restricted to flowering plants, but it is also deposited in members of the genus Selaginella. Members of the genus Selaginella are lycophytes, an ancient group of vascular plants that diverged 300 to 400 million years ago from the lineage that ultimately gave rise to the angiosperms. There are two fundamental questions to be addressed by this project. First, how has the ability to deposit S lignin has been conserved in, or reinvented by, such divergent taxa? Second, to what extent are the enzymes involved in S lignin synthesis similar or different in Selaginella and Arabidopsis? The approach to be taken involves isolating genes encoding critical enzymes in lignin biosynthesis from Selaginella moellendorffii and testing their function in Arabidopsis and yeast.
Broader Impacts: Although the phenylpropanoid metabolism produces many compounds of interest, a major goal of research on the pathway has been to improve our understanding of lignin biosynthesis. The extraction of lignin during the pulping process is both costly and damaging to the environment, thus the production of plants with more readily extractable lignin would be beneficial for both economic gain and for long-term environmental sustainability. Furthermore, the quantity and quality of lignin in forage species has been found to impact negatively their digestibility in ruminant animals. As a result, the application of similar strategies to crops used as animal feedstocks might be expected to lead to comparable gains. The ability to engineer lignin biosynthesis is critical to these efforts. The presence of syringyl lignin in Selaginella suggests that new methods to engineer lignin synthesis may emerge from the research to be completed in this project. In this context it is noteworthy that to date, efforts to engineer syringyl lignin synthesis in gymnosperms have failed. This may indicate that our understanding of lignin biosynthesis, particularly in plants other than angiosperms, is as yet incomplete. Studying this important pathway in Selaginella may provide new insights that will aid in metabolic engineering of a broader range of economically important plants. Finally, this project will provide training in biochemistry, genetics, and molecular biology to the students involved in the research. This project will also begin research into a new model organism that will soon be embraced by others because of its potential for putting plant biology into an evolutionary context, while at the same time deepening our explorations into the enzymes that dictate lignin monomer composition.
