Yuri Takeda,Yuki Tobimatsu, Steven D. Karlen, Taichi Koshiba, Shiro Suzuki, Masaomi Yamamura, Shinya Murakami, Mai Mukai, Takefumi Hattori, Keishi Osakabe, John Ralph, Masahiro Sakamoto, Toshiaki Umezawa

Abstract

Downregulation of p‐Coumaroyl ester 3‐hydroxylase (C3ˊH) is a key enzyme involved in the biosynthesis of lignin, a phenylpropanoid polymer that is the major constituent in vascular plants’ secondary cell walls. Although the crucial role of C3ˊH in lignification and its manipulation to upgrade lignocellulose have been investigated in eudicots, limited information is available in monocotyledonous grass species despite their potential as biomass feedstocks. Here we address the pronounced impacts of C3ˊH‐deficiency on the structure and properties of grass cell walls. C3ˊH‐knockdown lines generated via RNAi‐mediated gene silencing, with ~0.5% of residual expression levels, reached maturity and set seeds, whereas, in contrast, C3ˊH‐knockout rice mutants generated via CRISPR/Cas9‐mediated mutagenesis were severely dwarfed and sterile. Cell wall analysis of the mature C3ˊH‐knockdown RNAi lines revealed that their lignins were largely enriched in p‐hydroxyphenyl (H) units while substantially reduced in the normally dominant guaiacyl (G) and syringyl (S) units. Interestingly, however, the enrichment of H units was limited to being within the non‐acylated lignin units, with grass‐specific γ‐p‐coumaroylated lignin units remaining apparently unchanged. Suppression of C3ˊH also resulted in relative augmentation in tricin residues in lignin as well as substantial reduction in wall cross‐linking ferulates. Collectively, our data demonstrate that C3ˊH expression is an important determinant, not only of lignin content and composition, but also of the degree of cell wall cross‐linking. We also demonstrated that C3ˊH‐suppressed rice displays enhanced biomass saccharification.

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See https://onlinelibrary.wiley.com/doi/abs/10.1111/tpj.13988