Table 1.

Overview of greenhouse-grown trees with modified expression of lignin biosynthesis genes published since mid-2019.

Species	Gene	Method	Effect on total lignin	Effect on lignin composition	Saccharification efficiency	Pulping efficiency	Biomass yield	Reference
P. trichocarpa	C4H1	suppression via RNAi	↓13%	↓S/G	↑	n.d.	↓	Kim et al. (2020)
P. trichocarpa	C3H3	suppression via RNAi	↓42%	↑S/G	↑	n.d.	↓	Kim et al. (2020)
P. trichocarpa	C4H1/C4H2/C3H3	suppression via RNAi	↓50%	↑S/G	↑	n.d.	↓	Kim et al. (2020)
Populus deltoides × Populus euramericana	4CL1	fiber-specific suppression	↓21%	n.d.	n.d.	n.d.	WT	Cao et al. (2020)
P. deltoides × P. euramericana	4CL1	vessel-specific suppression	↓26%	n.d.	n.d.	n.d.	WT	Cao et al. (2020)
P. tremula × P. alba	4CL1	CRISPR-Cas9	↓19%	↓S/G	WT	n.d.	WT	Tsai et al. (2020)
P. tremula × P. alba	4CL5	CRISPR-Cas9	WT	WT	n.d.	n.d.	WT	Tsai et al. (2020)
P. tremula × P. alba	CSE1	CRISPR-Cas9	WT	n.d.	n.d.	n.d.	WT	de Vries et al. (2021a)
P. tremula × P. alba	CSE2	CRISPR-Cas9	WT	n.d.	n.d.	n.d.	WT	de Vries et al. (2021a)
P. tremula × P. alba	CSE1 and CSE2	CRISPR-Cas9	↓35%	↓S/G	↑	n.d.	↓	de Vries et al. (2021a)
P. alba × P. glandulosa	CSE1	CRISPR-Cas9	↓16%	n.d.	n.d.	n.d.	WT	Jang et al. (2021)
P. alba × P. glandulosa	CSE2	CRISPR-Cas9	↓16%	n.d.	n.d.	n.d.	WT	Jang et al. (2021)
P. alba × P. glandulosa	HCT	suppression via RNAi	↓11%	↓S/G	n.d.	n.d.	n.a.	Su et al. (2021)
P. tremula × P. alba	CCR2	CRISPR-Cas9	↓10%	WT S/G, incorporation of ferulic acid	↑	n.d.	WT	De Meester et al. (2020)
P. tremula × P. alba	CCR2	CRISPR-Cas9 knockout + vessel- and ray-specific overexpression of AtCCR1	↓18%	WT S/G, incorporation of ferulic acid	↑	n.d.	↓	De Meester et al. (2021)
P. trichocarpa	CCR2	suppression via RNAi	↓32%	↓S/G, incorporation of ferulic acid	n.d.	n.d.	↓	Yan et al., 2019
P. tomentosa	mF5H2a	overexpression	n.d.	↑S/G	↑	n.d.	n.d.	Fan et al. (2020)
P. tremula × P. alba	AtF5H1	overexpression	WT	↑S/G	↑	n.d.	WT	Lapierre et al. (2021)
P. trichocarpa	CAD1	suppression via RNAi	↓9%	↑S/G, incorporation of aldehyde components	n.d.	n.d.	WT	Yan et al., 2019
P. deltoides × P. euramericana	LTF1	fiber-specific suppression	↓43%	↓S/G	n.d.	n.d.	WT	Gui et al. (2020)
P. deltoides × P. euramericana	LTF1	vessel-specific suppression	↓16%	↑S/G	n.d.	n.d.	↓	Gui et al. (2020)
P. tremula × P. alba	ClDCS and ClCURS2	overexpression	↑23%	↓S/G	WT	n.d.	↓	De Meester et al. (2022a)
P. alba × Populus grandidentata	MdCHS3	overexpression	↓10%	WT S/G	↑	n.d.	WT	Mahon et al. (2022)
P. tremula × P. alba	PHBMT1	CRISPR-Cas9	WT	WT S/G, no detectable p-hydroxybenzoates	n.d.	n.d.	WTb	Zhao et al. (2021a); Zhao et al. (2021b)
P. tremula × P. alba	PHBMT1	overexpression	WT	WT S/G, increased p-hydroxybenzoates	n.d.	n.d.	WT	Zhao et al. (2021a); Zhao et al. (2021b)
P. alba × P. grandidentata	PHBMT1	overexpression	WT	WT S/G, increased p-hydroxybenzoates	n.d.	n.d.	WT	de Vries et al. (2022)
P. alba × P. grandidentata	CPL	overexpression	↓5%	↓S/G, increased p-hydroxybenzoates	↑	n.d.	↓	Mottiar et al. (2022)
P. alba × P. glandulosa	miR393	suppression via STTM	slightly ↑	n.d.	n.d.	n.d.	↑	Chu et al. (2021)
P. tremula × Populus tremuloides	HpSK	overexpression	WT	↑S/G, increased H units	↑	n.d.	↓	Hu et al. (2022)
P. alba × P. grandidentata	QsuB	overexpression	↓33%	↑S/G	↑	n.d.	WT	Unda et al. (2022)
P. tremula × P. alba	BdPMT1	overexpression	WT	WT S/G, incorporation of p-coumarates	↑	n.d.	WT	Lapierre et al. (2021)
P. tremula × P. alba	AtF5H1 and BdPMT1	overexpression	WT	↑S/G, incorporation of p-coumarates	↑	n.d.	WT	Lapierre et al. (2021)
P. trichocarpa	HSFB3-1	CRISPR-Cas9	↓17%	n.d.	n.d.	n.d.	↑	Liu et al. (2021a)
P. trichocarpa	MYB092	CRISPR-Cas9	↓27%	n.d.	n.d.	n.d.	WT	Liu et al. (2021a)
P. trichocarpa	HSFB3-1	overexpression	↑10%	n.d.	n.d.	n.d.	↓	Liu et al. (2021a)
P. trichocarpa	MYB092	overexpression	↑18%	n.d.	n.d.	n.d.	↓	Liu et al. (2021a)
P. tomentosa	miR828	overexpression	↓13%	n.d.	n.d.	n.d.	↓	Wang et al. (2022)
P. tomentosa	miR828	suppression via STTM	↑15%	n.d.	n.d.	n.d.	↑	Wang et al. (2022)
P. tomentosa	MYB171	overexpression	↑12%	n.d.	n.d.	n.d.	↑	Wang et al. (2022)
P. tomentosa	miR6443	overexpression	WT	↓S/G	↓	n.d.	WT	Fan et al. (2020)
P. tomentosa	miR6443	suppression via STTM	WT	↑S/G	↑	n.d.	WT, lodging phenotype	Fan et al. (2020)
Populus alba × P. glandulosa	MYB120	dominant suppression via SDRX	↓59%	↓S/G	n.d.	n.d.	↓	Kim et al. (2021)
P. deltoides	EPSPSc	overexpression	ectopic lignificationd	n.d.	n.d.	n.d.	n.d.	Xie et al. (2018)
P. deltoides	EPSPSc	suppression via RNAi	n.d.	n.d.	n.d.	n.d.	n.d.	Xie et al. (2018)
P. tomentosa	LAC14	overexpression	↑15%	↓S/G	n.d.	n.d.	↓	Qin et al. (2020)
P. tomentosa	LAC14	CRISPR-Cas9	↓7%	↑S/G	↑	n.d.	↑	Qin et al. (2020)

For an overview of lignin-engineering results in greenhouse-grown trees achieved up to mid-2019, see Wang et al. (2018) and Chanoca et al. (2019). Since mid-2019, only lignin-engineering results from greenhouse-grown trees of Populus species have been published. n.d., not determined; S/G, syringyl/guaiacyl ratio; effect on total lignin is given as a percentage of the total lignin analyzed via different methods; see the corresponding reference for specific information. If multiple lines were tested, only the line with the most extreme change in lignin is shown. For abbreviations of gene names, see the manuscript text. RNAi, RNA interference; STTM, short tandem target mimic; SRDX refers to the protein domain LDLELRL; CRISPR, clustered regularly interspaced short palindromic repeats; At, Arabidopsis thaliana; Md, Malus domestica; Bd, Brachypodium distachyon; Cl, Curcuma longa.

^amF5H2 is an engineered miR6443-resistant form of F5H2.

^bphbmt1 mutants showed a decrease in biomass until 2 months, when the plantlets were transferred from tissue culture to soil. When they grew for 3 or more months in soil, their growth appeared normal, although some mutant lines had a twisted trunk.

^cEPSPS is named EPSP in Xie et al. (2018) and EPSP-TF in Xie et al. (2020).

^dOverexpression under control of the CaMV35S promoter resulted in the ectopic deposition of lignin in cell walls of epidermis, phloem fiber, and pith cells as judged from microscopy sections stained with phloroglucinol-HCl. Lignin levels in the xylem were not quantified.