TABLE 3.

Targeted genetic modification of lignin biosynthesis in plants using specific promoters and/or sophisticated genome editing technique.

Gene category	Gene name	Carrying promoter	Transgene techniques	Lignin content and H/S/G composition	Effects on plants	Host species and references
General phenylpropanoid pathway	MsC4H ↓ a	Vascular-specific promoter bean PAL2	Antisense-expressed	Reduced and S/G ratio ↓	Normal growth phenotype	Medicago sativa; Reddy et al. (2005)
	AtC4H ↑	Vessel-specific promoter VND6	Ectopic complementation of mutants	Reduced and not reported	Complete restoration of growth defect of the mutant	Arabidopsis c4h-2; Yang et al. (2013)
	Pto4CL1 ↑	Xylem-specific promoter GRP1.8	Heterologous-overexpressed	Increased and not reported	Lignin content was increased in the stem, not in the leaves	Nicotiana tabacum; Lu et al. (2003)
	Pv4CL1 ↓	Rice OsUbi2	CRISPR/Cas9	Reduced and S/G ratio ↑	Normal growth phenotype and improved sugar release	Panicum virgatum; Park et al. (2017)
	AtHCT ↓	Fiber-specific promoter, pNST3	CRISPR/Cas9	Reduced and not reported	Normal growth phenotype	Arabidopsis thaliana; Liang et al. (2019)
	MsC3H ↓	Vascular-specific promoter bean PAL2	Antisense-expressed	Reduced and S/G ratio ↑, H units ↑	Normal growth phenotype	Medicago sativa; Reddy et al. (2005)
	AtCSE ↑	Vessel-specific promoter VND7	Ectopic complementation of mutants	Increased and not reported	Restored the vasculature integrity resulting in improved growth parameters	Arabidopsis cse-2; Vargas et al. (2016)
	PaCSE1 ↓, PaCSE2 ↓	Arabidopsis U6	CRISPR/Cas9	Reduced and not reported	Normal growth phenotype and improved sugar release	Populus alba; Jang et al. (2021)
Monolignol branch pathway	AtF5H ↑	Lignifying cell-specific promoter C4H	Heterologous-overexpressed	Increased and S units ↑	Normal growth phenotype	Nicotiana tabacum; Populus tremula; Franke et al. (2000)
	BnF5H ↓	CaMV 35S	CRISPR/Cas9	Reduced and S/G ratio ↓	Improved disease resistance and increased stem strength	Brassica napus; Cao et al. (2022)
	HvCOMT1 ↓	wheat U6	CRISPR/Cas9	Reduced and S/G ratio ↓	Normal growth phenotype and improved sugar release	Hordeum vulgare; Lee et al. (2021)
	PtoCCoAOMT ↓	Lignifying cell-specific promoter C4H	Antisense-expressed	Reduced and not reported	Normal growth phenotype	Nicotiana tabacum; Zhao et al. (2005)
	AtCCR1 ↑	Vascular-specific promoter bean ProSNBE	CRISPR/Cas9 complementation of mutants	Reduced and S/G ratio ↓ H units ↑	Normal growth phenotype and improved saccharification efficiency	Arabidopsis ccr1; De Meester et al. (2021)
Lignin oxidation and polymerization	PrxA3a ↓	Lignifying cell-specific promoter prxA3a	Antisense-expressed	Reduced and S units ↑	Normal growth phenotype	Populus sieboldii; Li et al. (2003)
	PtoLAC14 ↓	CaMV 35S	CRISPR/Cas9	Reduced and S units ↑	Normal growth phenotype with enhanced biomass saccharification	Populus tomentosa; (Qin et al., 2020)
TFs in lignin regulatory networks	PdLTF1 ↑	Fibre-specific promoter PdDUF579-9	Ectopic overexpression	Reduced and S/G ratio ↓	Normal growth phenotype	Populus deltoides; Gui et al. (2020)
	PtrMYB221 ↑	Xylem-specific promoter DX15	Ectopic overexpression	Reduced and not reported	Normal growth phenotype	Populus tremula; De Meester et al. (2021)
	OsWRKY36 ↓; OsWRKY102 ↓	CaMV 35S	CRISPR/Cas9	Increased and G units ↑	Altered culm morphology	Oryza sativa; Miyamoto et al. (2020)

^aArrows indicate up- or down-regulation of genes, and increases or decreases in lignin composition.