RNAi‐suppression of barley caffeic acid O‐methyltransferase modifies lignin despite redundancy in the gene family

• Published in: Plant biotechnology journal 2019-03, Vol.17 (3), p.594-607
• Main Authors: Daly, Paul, McClellan, Christopher, Maluk, Marta, Oakey, Helena, Lapierre, Catherine, Waugh, Robbie, Stephens, Jennifer, Marshall, David, Barakate, Abdellah, Tsuji, Yukiko, Goeminne, Geert, Vanholme, Ruben, Boerjan, Wout, Ralph, John, Halpin, Claire
• Format: Article
• Language: English
• Subjects: Animal feed; Barley; barley (Hordeum vulgare); BASIC BIOLOGICAL SCIENCES; Biofuels; Biomass energy; Biosynthesis; brown‐midrib; Caffeic acid; caffeic acid O-methyltransferase (COMT), lignin, brown-midrib, barley (Hordeum vulgare), straw, RNAi, biofuels, biorefineries; caffeic acid O‐methyltransferase (COMT); Cell walls; Chemical and Process Engineering; Coumaric acid; Digestibility; Engineering Sciences; Enzymatic activity; Enzyme activity; Feeds; Food engineering; Gene Expression Regulation, Plant - genetics; Genes, Plant - genetics; Genomes; Genomics; Grain; Hordeum - enzymology; Hordeum - genetics; Hordeum - metabolism; Life Sciences; Lignin; Lignin - metabolism; Methyltransferase; Methyltransferases - metabolism; Phenotypes; Phylogeny; Plant Proteins - genetics; Plant Proteins - metabolism; Proteins; Redundancy; Renewable energy; RNA Interference; RNA-mediated interference; Sorghum; Straw; Transferases; Vegetal Biology; Wheat
• ISSN: 1467-7644; 1467-7652
• DOI: 10.1111/pbi.13001

Summary Caffeic acid O‐methyltransferase (COMT), the lignin biosynthesis gene modified in many brown‐midrib high‐digestibility mutants of maize and sorghum, was targeted for downregulation in the small grain temperate cereal, barley (Hordeum vulgare), to improve straw properties. Phylogenetic and expression analyses identified the barley COMT orthologue(s) expressed in stems, defining a larger gene family than in brachypodium or rice with three COMT genes expressed in lignifying tissues. RNAi significantly reduced stem COMT protein and enzyme activity, and modestly reduced stem lignin content while dramatically changing lignin structure. Lignin syringyl‐to‐guaiacyl ratio was reduced by ~50%, the 5‐hydroxyguaiacyl (5‐OH‐G) unit incorporated into lignin at 10‐–15‐fold higher levels than normal, and the amount of p‐coumaric acid ester‐linked to cell walls was reduced by ~50%. No brown‐midrib phenotype was observed in any RNAi line despite significant COMT suppression and altered lignin. The novel COMT gene family structure in barley highlights the dynamic nature of grass genomes. Redundancy in barley COMTs may explain the absence of brown‐midrib mutants in barley and wheat. The barley COMT RNAi lines nevertheless have the potential to be exploited for bioenergy applications and as animal feed.