Effects of Coumarate 3-Hydroxylase Down-regulation on Lignin Structure

Down-regulation of the gene encoding 4-coumarate 3-hydroxylase (C3H) in alfalfa massively but predictably increased the proportion of p-hydroxyphenyl (P) units relative to the normally dominant guaiacyl (G) and syringyl (S) units. Stem levels of up to approximately 65% P (from wild-type levels of approximately 1%) resulting from down-regulation of C3H were measured by traditional degradative analyses as well as two-dimensional 13C-1H correlative NMR methods. Such levels put these transgenics well beyond the P:G:S compositional bounds of normal plants; p-hydroxyphenyl levels are reported to reach a maximum of 30% in gymnosperm severe compression wood zones but are limited to a few percent in dicots. NMR also revealed structural differences in the interunit linkage distribution that characterizes a lignin polymer. Lower levels of key beta-aryl ether units were relatively augmented by higher levels of phenylcoumarans and resinols. The C3H-deficient alfalfa lignins were devoid of beta-1 coupling products, highlighting the significant differences in the reaction course for p-coumaryl alcohol versus the two normally dominant monolignols, coniferyl and sinapyl alcohols. A larger range of dibenzodioxocin structures was evident in conjunction with an approximate doubling of their proportion. The nature of each of the structural units was revealed by long range 13C-1H correlation experiments. For example, although beta-ethers resulted from the coupling of all three monolignols with the growing polymer, phenylcoumarans were formed almost solely from coupling reactions involving p-coumaryl alcohol; they resulted from both coniferyl and sinapyl alcohol in the wild-type plants. Such structural differences form a basis for explaining differences in digestibility and pulping performance of C3H-deficient plants.