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Sci Rep. 2019 Aug 12;9(1):11597. doi: 10.1038/s41598-019-47957-0.

OsCAldOMT1 is a bifunctional O-methyltransferase involved in the biosynthesis of tricin-lignins in rice cell walls.

Author information

1
Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan.
2
School of Biological Sciences, The University of Hong Kong, Pokfulam, Hong Kong, China.
3
Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan. ytobimatsu@rish.kyoto-u.ac.jp.
4
U.S Department of Energy Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI, 53726, USA.
5
École polytechnique Fédérale de Lausanne, EPFL, 1015, Lausanne, Switzerland.
6
Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan.
7
Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan. tumezawa@rish.kyoto-u.ac.jp.
8
Research Unit for Development of Global Sustainability, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan. tumezawa@rish.kyoto-u.ac.jp.

Abstract

Lignin is a phenylpropanoid polymer produced in the secondary cell walls of vascular plants. Although most eudicot and gymnosperm species generate lignins solely via polymerization of p-hydroxycinnamyl alcohols (monolignols), grasses additionally use a flavone, tricin, as a natural lignin monomer to generate tricin-incorporated lignin polymers in cell walls. We previously found that disruption of a rice 5-HYDROXYCONIFERALDEHYDE O-METHYLTRANSFERASE (OsCAldOMT1) reduced extractable tricin-type metabolites in rice vegetative tissues. This same enzyme has also been implicated in the biosynthesis of sinapyl alcohol, a monolignol that constitutes syringyl lignin polymer units. Here, we further demonstrate through in-depth cell wall structural analyses that OsCAldOMT1-deficient rice plants produce altered lignins largely depleted in both syringyl and tricin units. We also show that recombinant OsCAldOMT1 displayed comparable substrate specificities towards both 5-hydroxyconiferaldehyde and selgin intermediates in the monolignol and tricin biosynthetic pathways, respectively. These data establish OsCAldOMT1 as a bifunctional O-methyltransferase predominantly involved in the two parallel metabolic pathways both dedicated to the biosynthesis of tricin-lignins in rice cell walls. Given that cell wall digestibility was greatly enhanced in the OsCAldOMT1-deficient rice plants, genetic manipulation of CAldOMTs conserved in grasses may serve as a potent strategy to improve biorefinery applications of grass biomass.

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