Field and saccharification performances of poplars severely downregulated in CAD1

Abstract

Summary Lignin is one of the main factors causing lignocellulosic biomass recalcitrance to enzymatic hydrolysis. Glasshouse‐grown poplars severely downregulated for CINNAMYL ALCOHOL DEHYDROGENASE 1 (CAD1), the enzyme catalysing the last step in the monolignol‐specific branch of lignin biosynthesis, have increased saccharification yields and normal growth. Here, we assess the performance of these hpCAD poplars in the field under short rotation coppice culture for two consecutive rotations of 1 yr and 3 yr. While 1‐yr‐old hpCAD wood had 10% less lignin, 3‐yr‐old hpCAD wood had wild‐type lignin levels. Because of their altered cell wall composition, including elevated levels of cinnamaldehydes, both 1‐yr‐old and 3‐yr‐old hpCAD wood showed enhanced saccharification yields upon harsh alkaline pretreatments (up to +85% and +77%, respectively). In contrast with previous field trials with poplars less severely downregulated for CINNAMYL ALCOHOL DEHYDROGENASE (CAD), the hpCAD poplars displayed leaning phenotypes, early bud set, early flowering and yield penalties. Moreover, hpCAD wood had enlarged vessels, decreased wood density and reduced relative and free water contents. Our data show that the phenotypes of CAD‐deficient poplars are strongly dependent on the environment and underpin the importance of field trials in translating basic research towards applications.

Whole genome duplication of wild-type and CINNAMYL ALCOHOL DEHYDROGENASE1-downregulated hybrid poplar reduces biomass yield and causes a brittle apex phenotype in field-grown wild types

“…The diploid hpCAD line showed a biomass yield penalty, but no shift in lignin content compared with its WT counterpart when grown in the greenhouse. Similar results were obtained when this hpCAD line was grown for 3 years in the field (De Meester et al, 2022), whereas in another study no yield penalty and a 10% decrease in lignin content was obtained when this hpCAD line was grown under greenhouse conditions (Van Acker et al, 2017). Independent of these contradictory results, all these studies support a link between growth speed and lignin content in CAD1-downregulated poplars.…”

“…The plants appeared normal under standard conditions in the greenhouse, but were dwarfed when grown at 30 • C (Zhao et al, 2013). Together, these studies demonstrate that caution is required when interpreting, comparing or extrapolating published results from lignin mutants, because the environment might influence biomass yield and composition, as also discussed by De Meester et al (2022).…”

Lignin engineering in forest trees: From gene discovery to field trials

“…(2020) P. tremula × P. alba CAD RNAi yes East Flanders, Belgium 3 240 plants per line 4 years 1 year ↓10% to ↓8% [↓10% to ↓6%] ↓S/G [↓S/G] ↑ [↑] n.d. [n.d.] ↓ [WT] De Meester et al. (2022b) [ Van Acker et al., 2017 ] P. tremula × P. alba CAD RNAi yes East Flanders, Belgium 3 240 plants per line 4 years 3 years (4 y/o roots) WT [↓10% to ↓6%] WT [↓S/G] ↑ [↑] n.d. [n.d.] ↓ [WT] De Meester et al. (2022b) [ Van Acker et al., 2017 ] P. deltoides × P. euramericana LTF1 fiber-specific suppression yes Shanghai, China 2 n.a.…”

“…First, LMID in lignin-modified trees could be caused by impaired functioning of the tracheary elements (TEs; vessels in hardwood, tracheids in softwood) as a result of (i) vascular collapse ( Leplé et al., 2007 ; Coleman et al., 2008 ; Wagner et al., 2009 ; Zhou et al., 2018 ; De Meester et al., 2020 ; de Vries et al., 2021a ), (ii) blockage of TEs with tyloses and phenolic deposits ( Kitin et al., 2010 ), or (iii) enlarged vessels (which generally enhance xylem vulnerability to embolism) combined with reductions in relative water content and free water available to support the hydraulic network ( De Meester et al., 2022b ). Consequently, the stem sap flow and/or hydraulic conductance may be perturbed, potentially leading to enhanced susceptibility to drought stress and/or impaired tree growth ( Cochard et al., 2004 ; Coleman et al., 2008 ; Cao et al., 2020 ; White, 2021 ; De Meester et al., 2022b ).…”

“…First, LMID in lignin-modified trees could be caused by impaired functioning of the tracheary elements (TEs; vessels in hardwood, tracheids in softwood) as a result of (i) vascular collapse ( Leplé et al., 2007 ; Coleman et al., 2008 ; Wagner et al., 2009 ; Zhou et al., 2018 ; De Meester et al., 2020 ; de Vries et al., 2021a ), (ii) blockage of TEs with tyloses and phenolic deposits ( Kitin et al., 2010 ), or (iii) enlarged vessels (which generally enhance xylem vulnerability to embolism) combined with reductions in relative water content and free water available to support the hydraulic network ( De Meester et al., 2022b ). Consequently, the stem sap flow and/or hydraulic conductance may be perturbed, potentially leading to enhanced susceptibility to drought stress and/or impaired tree growth ( Cochard et al., 2004 ; Coleman et al., 2008 ; Cao et al., 2020 ; White, 2021 ; De Meester et al., 2022b ). Impaired TEs have been observed in growth-impaired angiosperm trees with reduced expression of HCT ( Zhou et al., 2018 ), C3′H ( Coleman et al., 2008 ; Zhou et al., 2018 ), 4CL ( Kitin et al., 2010 ; Voelker et al., 2010 , 2011 ), CSE ( de Vries et al., 2021a ), CCR ( Leplé et al., 2007 ; De Meester et al., 2020 ), and CAD ( De Meester et al., 2022b ), as well as growth-impaired gymnosperm trees with reduced expression of 4CL ( Wagner et al., 2009 ).…”