Abstract:
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.
