Biosynthesis, Regulation and Engineering of C-Lignin

University Of North Texas, Denton TX

Investigators

Abstract

Lignin is one of earth's most abundant natural polymers, a key component of woody plant material, and a major waste product in bio-refineries that process plant material for production of liquid biofuels. The lead investigator and collaborators have discovered a new type of lignin, apparently restricted to the seed coats of a range of non-crop species. This new type of lignin has significant promise for conversion to carbon fibers. If this type of lignin could be engineered into bioenergy feedstocks, it would represent a high value co-product of biorefining. This project will determine the biochemical pathway by which this new type of lignin is synthesized, and the steps that will need to be taken to make it accumulate in the stems and leaves of plants. Understanding the limitations to the accumulation of different types of lignin in different plant tissue and cell types also addresses a fundamental problem in cell biology. This project uses a range of approaches from genetic mechanisms through metabolic engineering to production and testing of biomaterials. Outreach activities will target two demographically distinct groups; highly talented high school students at the Texas Academy of Mathematics and Science, and mathematics and science public school teachers with primarily Hispanic students who are Limited English Proficient (LEP). Teachers will develop curricular activities in Spanish and English to implement in their classrooms. The project will include informal learning activities through deployment of the projects to encourage K-12 girls into STEM through outreach with the Dallas Society of Women Engineers. During lignin biosynthesis, the monolignol precursors are functionalized by aromatic hydroxylation and O-methylation to generate, successively, hydroxyphenyl (H), catechyl (C), guaiacyl (G), 5-hydroxyguaiacyl (5-OH-G), and syringyl (S) units. Natural lignins in angiosperms consist of approximately equal amounts of G and S units, with less than 2% of H units and no C or 5-OH-G units; gymnosperm lignins are similar but lack S units. 5-OH-G units are incorporated into lignins in transgenic plants in which the second methylation step is blocked, but C-units have not been reported to accumulate in the lignin of angiosperms in which the first methylation step is similarly blocked. The Principal Investigators have shown that the seed coats of a wide range of dicot and monocot plants contain a novel type of lignin (C-lignin) derived totally from catechyl units. This linear polymer has advantageous properties for conversion to carbon fibers. However, C-lignin appears to be restricted to seed coats in nature. This award will address two major questions: what is the mechanism underlying the biosynthesis of C-lignin in plant seed coats, and why does C-lignin not occur naturally in the vegetative tissues of the plant? The researchers hypothesize that C-lignin biosynthesis involves loss of function of one or both methylation steps in classical monolignol biosynthesis, and will determine how this happens using transcriptome profiling in developing seeds of Cleome hassleriana, in which there is an abrupt transition from G lignin to C lignin biosynthesis at around 13 days post-pollination. Addressing the second question will involve attempting to introduce the C-lignin polymer into different tissues of the model legume Medicago truncatula, utilizing existing mutants in which enzymes of lignin methylation have been functionally disrupted. These studies aim to determine the potential for engineering accumulation of C-lignin in vegetative tissues in sufficient yields to enable industrial processing, and, in this context, improved methods for extraction of C-lignin from both seeds and vegetative tissues will be developed, and the physical properties of the polymer further analyzed.