Douglas D. Stokke

Repression of Lignin Biosynthesis Promotes Cellulose Accumulation and Growth in Transgenic Trees

Because lignin limits the use of wood for fiber, chemical, and energy production, strategies for its downregulation are of considerable interest. We have produced transgenic aspen (Populus tremuloides Michx.) trees in which expression of a lignin biosynthetic pathway gene Pt4CL1 encoding 4-coumarate:coenzyme A ligase (4CL) has been downregulated by antisense inhibition. Trees with suppressed Pt4CL1 expression exhibited up to a 45% reduction of lignin, but this was compensated for by a 15% increase in cellulose. As a result, the total lignin–cellulose mass remained essentially unchanged. Leaf, root, and stem growth were substantially enhanced, and structural integrity was maintained both at the cellular and whole-plant levels in the transgenic lines. Our results indicate that lignin and cellulose deposition could be regulated in a compensatory fashion, which may contribute to metabolic flexibility and a growth advantage to sustain the long-term structural integrity of woody perennials.

Characterization of the Cellulosic Cell Wall

Cell differentiation was induced in callus cultures of Pinus radiata D. Don by culturing on a medium containing activated charcoal but no phytohormones. Tracheids and sclereids, both with lignified cell walls, differentiated in callus derived from xylem strips; the tracheids had reticulate or pitted secondary cell wall patterns. Only tracheids differentiated in callus derived from hypocotyl segments. These tracheids had helical, scalariform, reticulated or pitted secondary cell wall patterns. The lignin in the cell walls was identified histochemically and by pyrolysis gas chromatography–mass spectrometry, and was quantified as thioglycolic acid lignin. Acid hydrolysates of cell walls isolated from the differentiated xylem-derived calli contained higher proportions of glucose and mannose than those of cell walls from the undifferentiated calli. This is consistent with the presence of greater proportions of glucoand/or galactogluco-mannans in the secondary cell walls of the differentiated cells. Transgenic cell lines of xylem-derived cultures were established following Biolistic® particle bombardment with a plasmid containing the coding region of the nptII gene and the coding region of the cad gene from P. radiata. Expression of the nptII gene in transgenic lines was confirmed by an NPTII-ELISA. Overexpression of cad in the transgenic lines led to a down-regulation of CAD (EC 1.1.1.195) expression.

PROPORTIONS OF WOOD ELEMENTS IN STEM, BRANCH, AND ROOT WOOD OF BLACK OAK (QUERCUS VELUTINA)

The volumetrie proportions of vessel elements, fibres, rays, and axial parenchyma plus vasicentric tracheids were determined for the stern, branch and root wood of threc black oak (Quercus velutina Lam.) trees. There were statistical differences in the proportions of wood elements between locations within the trees sampled, i.e., branches, sterns, roots, oblique roots, and lateral roots. Branches had the highest proportion of vesseI elements, whereas sterns had the greatest proportion ofaxial parenchyma plus vasicentric tracheids. The highest proportions of rays were found in root wood. Fibre proportion was greatest in the stern and branch wood.