Abstract
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.
Highlights
The properties of plant biomass are largely determined by its composition and in particular by the amount and structure of lignin
caffeic acid O-methyltransferase (COMT) genes cannot be identified by phylogeny alone; the closely related genes CbCOMT1 and CbIEMT of Clarkia breweri, encode O-methyltransferases with distinct substrate specificities, and only one is a COMT (Wang and Pichersky, 1999)
The encoded proteins all contained 12 out of 13 of the conserved residues and the genes were annotated as HvCOMT1, HvCOMT2, HvCOMT3 and BdCOMT
Summary
The properties of plant biomass are largely determined by its composition and in particular by the amount and structure of lignin. In the C4 grasses maize (Zea mays) and sorghum (Sorghum bicolor), mutations in certain lignin biosynthesis genes, including caffeic acid O-methyltransferase (COMT), give rise to a phenotype of brown midribs that is associated with lower lignin content and higher digestibility (Bout and Vermerris, 2003; Vignols et al, 1995). Such bm or bmr mutants are marketed in the USA as superior forage and silage cultivars and some are reported to increase bioethanol yields (Dien et al, 2009). The lignin pathway is generally better characterised in dicots than monocots (Anterola and Lewis, 2002), COMT’s main role in both types of plant appears to be to methylate 5-hydroxyconiferaldehyde on the route to the synthesis of S units (Osakabe et al, 1999)
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