Modified Lignin and Delignification with a CAD-Deficient Loblolly Pine
MacKay J., Presnell T., Jameel H., Taneda H., O'Malley D., Sederoff R.
Lignin composition and lignin removal were investigated in loblolly pines (Pinus taeda L.) deficient in the monolignol biosynthesis enzyme, cinnamyl alcohol dehydrogenase (CAD). The CAD-deficient pines were homozygous for the cad-n1 allele, a mutant form of the gene encoding CAD. We show that lignin from CAD-deficient seedlings is more easily removed by mild alkali at room temperature. The nature of the phenolics recovered from this treatment link the increased lignin solubility to a build up of aromatic aldehyde lignin subunits. We carried out a retrospective genetic analysis to identify a 12 year-old CAD-deficient tree from which we isolated Milled Wood Lignin (MWL) and obtained wood chips for pulping studies. The UV absorption spectra of MWL and the effect of sodium borohydride reduction of MWL showed that the CAD-deficient tree had substantially higher incorporation of aldehyde subunits. The CAD-deficient wood was delignified more extensively than the wild type in soda pulping but not in kraft conditions. More extensive lignin removal, both in mild alkali treatment and in soda pulping, indicate that suppression of CAD in softwood trees may hold promise to produce woods well suited for "milder" pulping conditions that consume less chemicals and generate less waste.