Modification of xylan in secondary walls alters cell wall biosynthesis and wood formation programs

Abstract

Wood of broad-leaf tree species is a valued source of renewable biomass for biorefinery and a target for genetic improvement efforts to reduce its recalcitrance. Glucuronoxylan (GX) plays a key role in recalcitrance through its interactions with cellulose and lignin. To reduce recalcitrance, we modified wood GX by expressing GH10 and GH11 endoxylanases from Aspergillus nidulans in hybrid aspen (Populus tremula L. x tremuloides Michx.) and targeting the enzymes to cell wall. The xylanases reduced tree height, modified cambial activity by increasing phloem and reducing xylem production, and reduced secondary wall deposition. Xylan molecular weight was decreased, and the spacing between acetyl and MeGlcA side chains was reduced in transgenic lines. The transgenic trees produced hypolignified xylem having thin secondary walls and deformed vessels. Glucose yields of enzymatic saccharification without pretreatment almost doubled indicating decreased recalcitrance. The transcriptomics, hormonomics and metabolomics data provided evidence for activation of cytokinin and ethylene signaling pathways, decrease in ABA levels, transcriptional suppression of lignification and a subset of secondary wall biosynthetic program, including xylan glucuronidation and acetylation machinery. Several candidate genes for perception of impairment in xylan integrity were detected. These candidates could provide a new target for uncoupling negative growth effects from reduced recalcitrance. In conclusion, our study supports the hypothesis that xylan modification generates intrinsic signals and evokes novel pathways regulating tree growth and secondary wall biosynthesis.

Integrity of xylan backbone affects plant responses to drought

“…The lines with different transgene expression levels are indicated by letters: from a – the strongest to c – the weakest. The phenotypes of xylanase-expressing lines are described in detail in the accompanying paper ( Sivan et al., 2024 ).…”

“…Hybrid aspen ( P. tremula L. × tremuloides Michx., clone T89) was transformed by A. tumefaciens to obtain WP::GH10 and WP::GH11 lines as described in the accompanying paper ( Sivan et al., 2024 ). Two or three lines with the highest expression were selected out of approx.…”

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“…Hybrid aspen (P. tremula L. × tremuloides Michx., clone T89) was transformed by A. tumefaciens to obtain WP::GH10 and WP:: GH11 lines as described in the accompanying paper (Sivan et al, 2024). Two or three lines with the highest expression were selected out of approx.…”

“…The lines with different transgene expression levels are indicated by letters: from athe strongest to cthe weakest. The phenotypes of xylanase-expressing lines are described in detail in the accompanying paper (Sivan et al, 2024).…”