Transgenic down-regulation of the Pt4CL1 gene family encoding 4-coumarate:coenzyme A ligase (4CL) has been reported as a means for reducing lignin content in cell walls and increasing overall growth rates, thereby improving feedstock quality for paper and bioethanol production. Using hybrid poplar (Populus tremula 3 Populus alba), we applied this strategy and examined field-grown transformants for both effects on wood biochemistry and tree productivity. The reductions in lignin contents obtained correlated well with 4CL RNA expression, with a sharp decrease in lignin amount being observed for RNA expression below approximately 50% of the nontransgenic control. Relatively small lignin reductions of approximately 10% were associated with reduced productivity, decreased wood syringyl/guaiacyl lignin monomer ratios, and a small increase in the level of incorporation of H-monomers (p-hydroxyphenyl) into cell walls. Transgenic events with less than approximately 50% 4CL RNA expression were characterized by patches of reddish-brown discolored wood that had approximately twice the extractive content of controls (largely complex polyphenolics). There was no evidence that substantially reduced lignin contents increased growth rates or saccharification potential. Our results suggest that the capacity for lignin reduction is limited; below a threshold, large changes in wood chemistry and plant metabolism were observed that adversely affected productivity and potential ethanol yield. They also underline the importance of field studies to obtain physiologically meaningful results and to support technology development with transgenic trees.Composed of diverse layers of cellulose microfibrils and amorphous hemicelluloses within a matrix of pectins, proteins, and lignin, the secondary cell walls of plants are diverse in their morphology, chemistry, and physiological functions. Lignification is of particular interest, as it exhibits highly predictable temporal and spatial patterning and is the last major step in the structural reinforcement of cell walls before the protoplast is dissolved (Donaldson, 2001
Background:The plastid-localized arogenate dehydratase (ADT) gene family is hypothesized to differentially control carbon flux for lignin deposition, with lignin being the main contributor to lignocellulosic recalcitrance. Results: Single and multiple ADT knock-outs resulted in differential control over lignin content/composition. Conclusion: The first evidence for Phe upstream metabolism differentially controlling carbon flux into distinct secondary cell wall types was discovered. Significance: Upstream metabolic networks regulate secondary cell wall formation.
The Arabidopsis mutant Atomt1 lignin differs from native lignin in wild type plants, in terms of sinapyl (S) alcohol-derived substructures in fiber cell walls being substituted by 5-hydroxyconiferyl alcohol (5OHG)-derived moieties. During programmed lignin assembly, these engender formation of benzodioxane substructures due to intramolecular cyclization of their quinone methides that are transiently formed following 8-O-4' radical-radical coupling. Thioacidolytic cleavage of the 8-O-4' inter-unit linkages in the Atomt1 mutant, relative to the wild type, indicated that cleavable sinapyl (S) and coniferyl (G) alcohol-derived monomeric moieties were stoichiometrically reduced by a circa 2 : 1 ratio. Additionally, lignin degradative analysis resulted in release of a 5OHG-5OHG-G trimer from the Atomt1 mutant, which then underwent further cleavage. Significantly, the trimeric moiety released provides new insight into lignin primary structure: during polymer assembly, the first 5OHG moiety is linked via a C8-O-X inter-unit linkage, whereas subsequent addition of monomers apparently involves sequential addition of 5OHG and G moieties to the growing chain in a 2 : 1 overall stoichiometry. This quantification data thus provides further insight into how inter-unit linkage frequencies in native lignins are apparently conserved (or near conserved) during assembly in both instances, as well as providing additional impetus to resolve how the overall question of lignin macromolecular assembly is controlled in terms of both type of monomer addition and primary sequence.
Single and multiple T-DNA knockouts of genes encoding arogenate dehydratases (ADTs) in Arabidopsis were obtained in homozygous form. These were analyzed for potential differences in lignin contents and compositions, as well as for distinct phenotypes over growth and development. Of these different lines, distinct reductions in lignin contents were obtained, with those having different G:S ratios depending upon the combination of ADT genes being knocked out. Results from pyrolysis GC/MS analyses indicated that differential carbon flux occurred into the vascular bundles (vb) and interfascicular fibers (if). These results provide additional new insight into factors controlling lignin heterogeneity and configuration.
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