Variation in lignin and cell wall digestibility in caffeic acid O-methyltransferase down-regulated maize half-sib progenies in field experiments

Lignin content and structure were examined in seven caffeic acid O-methyltransferase antisense (COMT-AS) maize progenies and their corresponding normal inbred lines in relation to cell wall digestibility. The seven parental inbreds were chosen for their highly divergent in vitro wall digestibility. Maize plants were grown under field conditions to determine (i) if the positive effect of COMT down-regulation on wall chemistry and digestibility was similar to that previously observed for COMT-AS maize grown in the greenhouse and (ii) to what extent the genetic background was a factor in determining the effect of the transgene. All COMT-AS progenies displayed a significant reduction in endogenous COMT activity (14-43% residual activity). In all but one genetic background (F4), the COMT-AS gene resulted in an expected increase in wall digestibility accompanied by changes in lignin composition. These effects varied greatly among parental lines, and independently of the inherent digestibility values in the corresponding non-transformed lines. Curiously, in the highly digestible F4 background, the typical decrease in syringyl (S) unit lignin and a higher frequency of 5-OH guaiacyl lignin resulting from the introduction of the COMT-AS transgene were not observed. Our results indicate that COMT down-regulation via an antisense strategy is an efficient tool for forage maize improvement in the field.