Transgenic down-regulation of caffeic acid <i>O</i> -methyltransferase (COMT) led to improved digestibility in tall fescue ( <i>Festuca arundinacea</i> )

Abstract

Dry matter digestibility is one of the most important characteristics of forage. The major constraint on ruminant digestion of forage cell walls is lignin. Sequences of cDNA encoding a key lignin biosynthetic enzyme, caffeic acid O-methyltransferase (COMT), was cloned from the widely grown monocot forage species tall fescue (Festuca arundinacea Schreb.). Enzymatic properties of recombinant COMT protein expressed in E. coli were determined using six substrates. The preferred substrates for tall fescue recombinant COMT were 5-hydroxyferulic acid and caffeoyl aldehyde. Transgenic tall fescue plants carrying either sense or antisense COMT gene constructs were obtained by microprojectile bombardment of single-genotype-derived embryogenic suspension cells. Consistent and closely related molecular and biochemical data demonstrated that two co-suppressed transgenic lines were down-regulated in their lignin biosynthesis. These COMT down-regulated transgenic tall fescue plants showed substantially reduced levels of transcripts, significantly reduced enzymatic activities, significantly decreased lignin content, apparently altered lignin composition and significantly increased (9.8-10.8%) digestibility.