Lignin structure in a mutant pine deficient in cinnamyl alcohol dehydrogenase.

Cinnamyl alcohol dehydrogenase (CAD) activity is deficient in loblolly pine (Pinus taeda L.) harboring a mutated allele of the cad gene (cad-n1). We compared lignin structure of CAD-deficient and wild-type pines, both types segregating within full-sib families obtained by controlled crosses. The type and frequency of lignin building units and distribution of interunit bonds were determined from the GC-MS analysis of thioacidolysis monomers and dimers. While the lignin content was only slightly reduced, the lignin structure was dramatically modified by the mutation in both mature and juvenile trees. Lignins from CAD-deficient pine displayed unusually high levels of coniferaldehyde and dihydroconiferyl alcohol. In addition, biphenyl and biphenyl ether bonds were in large excess in these abnormal lignins. These results suggest that the CAD-deficient pines efficiently compensate for the shortage in normal lignin precursors by utilizing nontraditional wall phenolics to construct unusual lignins particularly enriched in resistant interunit bonds.