| Lignin is one of the most abundant biopolymers in plants and plays an important role in plant structure and stress defense. Lignin is also considered to be a hallmark of vascular plants because of its crucial role in plant terrestrialization. However, lignin is an undesired component in the pulp and paper industry, bioethanol production, and forage digestibility. Thus, understanding the functions and the evolution of lignin biosynthesis genes can not only advance our knowledge of the evolution of land-adaptation for vascular plants but also help guide the effort to exploit the potential for genetic manipulation of lignin for desirable traits in economically important crops. In this dissertation I studied a cinnamyl alcohol dehydrogenase (CAD) gene family in an important basal angiosperm species Liriodendron tulipifera L. A total of seven LtuCAD homologs were identified from a comprehensive Liriodendron EST database. The comparison of amino acid sequences, phylogeny, expression analysis, and complementation in the Arabidopsis cad4 cad5 double mutant indicate that LtuCAD1 is involved in lignin biosynthesis in Liriodendron. This finding provides the opportunity to manipulate the expression of LtuCAD1 for lignin modification in Liriodendron and broadens our knowledge of lignin biosynthesis in basal angiosperms. I also investigated the impact of expressing a parsley tyrosine-rich peptide (TYR) in poplar. With an aim to facilitate lignin removal during the utilization of woody biomass as a biofuel feedstock, transgenic poplars carrying the TYR sequence were previously generated, and a number of transgenic lines released more polysaccharides following protease digestion and were more flexible than wild type plants. In the current study, it was found that expression of the parsley tyrosine-rich peptide did not compromise the plant stem growth, susceptibility to pathogen, nor cause significant wood chemistry alternation in the transgenic poplars. Only five transcripts were found differentially expressed in the transgenic plants, all with at least 4-fold decrease of transcript abundance relative to the wildtype. These five transcripts encode a sulfotransferase domain protein, a NB-ARC domain-containing disease resistance protein, and one putative protein, respectively. The results suggest that it is possible to increase cell wall digestibility and flexibility without compromising growth and pathogen resistance of the poplar plants expressing a tyrosine-rich peptide encoding sequence. |
