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

Paul Daly, Christopher McClellan, Marta Maluk, Helena Oakey, Catherine Lapierre, Robbie Waugh, Jennifer Stephens, David Marshall, Abdellah Barakate, Yukiko Tsuji, Geert Goeminne, Ruben Vanholme, Wout Boerjan, John Ralph, Claire Halpin

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Abstract

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.

Original languageEnglish
Pages (from-to)594-607
Number of pages14
JournalPlant Biotechnology Journal
Volume17
Issue number3
Early online date2 Oct 2018
DOIs
Publication statusPrint publication - Mar 2019
Externally publishedYes

Bibliographical note

© 2018 The Authors. Plant Biotechnology Journal published by Society for Experimental Biology and The Association of Applied Biologists and John Wiley & Sons Ltd.

Keywords

  • Biofuels
  • barley (Hordeum vulgare)
  • brown-midrib
  • caffeic acid O-methyltransferase (COMT)
  • lignin
  • straw

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    Daly, P., McClellan, C., Maluk, M., Oakey, H., Lapierre, C., Waugh, R., Stephens, J., Marshall, D., Barakate, A., Tsuji, Y., Goeminne, G., Vanholme, R., Boerjan, W., Ralph, J., & Halpin, C. (2019). RNAi-suppression of barley caffeic acid O-methyltransferase modifies lignin despite redundancy in the gene family. Plant Biotechnology Journal, 17(3), 594-607. https://doi.org/10.1111/pbi.13001