Table 3.

Biomass, lignin amount, and lignin composition of lignin-engineered trees grown in the field compared with greenhouse conditions.

Species	Gene	Method	Tested in GH?	Location	# Lines	Field size	Field duration	Age of the stems at harvest	Effect on total lignin	Effect on lignin composition	Saccharification efficiency	Pulping efficiency	Biomass yield	References
E. urophylla × E. grandis	C4H	RNAi	no	FL, USA	9	n.a.	2 years	2 years	↓40% to ↓30%	↓S/G	↑	n.d.	↓	Sykes et al. (2015)
E. urophylla × E. grandis	C3′H	RNAi	no	FL, USA	6	n.a.	2 years	2 years	↓28% to ↓25%	↓S/G	↑	n.d.	↓	Sykes et al. (2015)
P. tremula × P. alba	4CL	antisense	no	OR, USA	14	10–15 plants per line	2 years	2 years	↓10% to WT	↓S/G to ↑S/G	WT	n.d.	↓	Voelker et al. (2010)
P. tomentosa Carr.	4CL	sense and antisense	no	Hebei, China	5	60 plants per line	6 years	1 year	↓28% to ↓5%	↑S/G	n.d.	n.d.	↑	Tian et al. (2013)
P. nigra × Populus maximowiczii	4CL	antisense	yes	mountain, NC, USA	3	n.a.	3 years	2 years	↓8% to WT [↓47% to ↓8%]	↑S/V [↑S/V]	↑ [n.d.]	n.d. [n.d.]	n.d. [n.d.]	Xiang et al. (2017) [Xiang et al., 2017]
P. nigra × P. maximowiczii	4CL	antisense	yes	mountain, NC, USA	3	n.a.	3 years	3 years	WT [↓47% to ↓8%]	↓S/V to ↑S/V [↑S/V]	↑ [n.d.]	n.d. [n.d.]	n.d. [n.d.]	Xiang et al. (2017) [Xiang et al., 2017]
P. nigra × P. maximowiczii	4CL	antisense	yes	coastal plain, NC, USA	3	n.a.	3 years	2 years	↓7% to WT [↓47% to ↓8%]	↓S/V to ↑S/V [↑S/V]	WT [n.d.]	n.d. [n.d.]	n.d. [n.d.]	Xiang et al. (2017) [Xiang et al., 2017]
P. nigra × P. maximowiczii	4CL	antisense	yes	coastal plain, NC, USA	3	n.a.	3 years	3 years	↓9% to WT [↓47% to ↓8%]	↑S/V [↑S/V]	↑ [n.d.]	n.d. [n.d.]	n.d. [n.d.]	Xiang et al. (2017) [Xiang et al., 2017]
P. trichocarpa	4CL	antisense	yes	coastal plain, NC, USA	12	149 plants per line	2 years	1 year (2 y/o roots)	↓37% to WT [↓46% to ↓5%]	↓S/G to ↑S/G [↓S/G and ↑S/G]	n.d. [n.d.]	n.d. [n.d.]	↓90% to ↑83% [n.d.]	Stout et al. (2014) [Stout et al., 2014]
P. trichocarpa	4CL	antisense	yes	Piedmont, NC, USA	12	149 plants per line	2 years	1 year (2 y/o roots)	↓33% to WT [↓46% to ↓5%]	↓S/G to ↑S/G [↓S/G and ↑S/G]	n.d. [n.d.]	n.d. [n.d.]	↓92% to ↓14% [n.d.]	Stout et al. (2014) [Stout et al., 2014]
P. trichocarpa	4CL	antisense	yes	mountain, NC, USA	12	78 plants per line	2 years	1 year (2 y/o roots)	↓24% to WT [↓46% to ↓5%]	↓S/G to ↑S/G [↓S/G and ↑S/G]	n.d. [n.d.]	n.d. [n.d.]	↓92% to ↑24% [n.d.]	Stout et al. (2014) [Stout et al., 2014]
P. tomentosa	4CL	antisense	yes	Beijing, China	2	n.a.	5 years	5 years	↓8% to ↓6% [↓42% to ↓26%]	n.d. [n.d.]	WT [n.d.]	n.d. [n.d.]	WT [WT]	Wang et al. (2012) [Jia et al., 2004]
P. deltoides × P. euramericana	4CL	fiber-specific suppression	yes	Shanghai, China	3	30 plants per line	1 year	1 year	↓∼25% [↓21%]	n.d. [n.d.]	n.d. [n.d.]	n.d. [n.d.]	WT [WT]	Cao et al. (2020) [Cao et al., 2020]
P. deltoides × P. euramericana	4CL	vessel-specific suppression	yes	Shanghai, China	3	30 plants per line	1 year	1 year	↓∼20% [↓26%]	n.d. [n.d.]	n.d. [n.d.]	n.d. [n.d.]	↓ [WT]	Cao et al. (2020) [Cao et al., 2020]
P. alba × P. glandulosa	CSE1	CRISPR-Cas9	yes	Republic of Korea	1 or 3a	n.a.	1 year	8 months or 1 yearb	↓ ∼24% [↓ up to 16%]	n.d. [n.d.]	↑ [n.d.]	n.d. [n.d.]	WT [WT]	Jang et al. (2021) [Jang et al., 2021]
P. alba × P. glandulosa	CSE2	CRISPR-Cas9	yes	Republic of Korea	1 or 3a	n.a.	1 year	8 months or 1 yearb	↓29% [↓ up to 16%]	n.d. [n.d.]	↑ [n.d.]	n.d. [n.d.]	WT [WT]	Jang et al. (2021) [Jang et al., 2021]
P. tomentosa	CCoAOMT	antisense	yes	Beijing, China	2	n.a.	5 years	5 years	↓10% to ↓9% [↓16%]	n.d. [n.d.]	↑ [n.d.]	n.d. [n.d.]	WT [WT]	Wang et al. (2012) [Jing et al., 2004]
P. tremula × P. alba	CCoAOMT	antisense	no	n.a.	1	n.a.	3 years	3 years	↓13%	↑S/G	n.d.	↑	WT	Wei et al. (2008)
P. tremula × P. alba	CCR	sense and antisense	yes	Orléans,France	4	10 plants per line	8 years	2, 4, and 5 yearsc	↓47% to WT [↓30% to ↓22%]	↓S/G, incorporation of FA [↓S/G, incorporation of FA]	n.d. [n.d.]	↑ [n.d.]	↓ [WT]	Leplé et al. (2007) [Leplé et al., 2007]
P. tremula × P. alba	CCR	sense	yes	East Flanders, Belgium	2	120 plants per line	5 years	10 months	↓8% to ↓5% [↓30% to ↓22%]	WT S/G, incorporation of FA [↓S/G, incorporation of FA]	↑ [↑]	n.d. [n.d.]	↓ [WT]	Van Acker et al. (2014) [Leplé et al., 2007]
P. tremula × P. alba	CCR	sense and antisense	yes	Orléans, France	2	120 plants per line	5 years	2 years	↓24% to ↓9% [↓30% to ↓22%]	↓S/G to WT, incorporation of FA [↓S/G, incorporation of FA]	↑ [↑]	n.d. [n.d.]	↓ [WT]	Van Acker et al. (2014) [Leplé et al., 2007]
P. tremula × P. alba	COMT	antisense	yes	Orléans, France	2	10 plants per line	5 years	2 years	WT [WT]	↓S/G, incorporation of 5-OHG [↓S/G, incorporation of 5-OHG]	n.d. [n.d.]	n.d. [↓]	WT [WT]	Pilate et al. (2002) [Lapierre et al., 1999; Van Doorsselaere et al., 1995]
P. tremula × P. alba	COMT	antisense	yes	Orléans, France	2	10 plants per line	5 years	2 years (4 y/o roots)	WT [WT]	↓S/G, incorporation of 5-OHG [↓S/G, incorporation of 5-OHG]	n.d. [n.d.]	↓ [↓]	WT [WT]	Pilate et al. (2002) [Lapierre et al., 1999; Van Doorsselaere et al., 1995]
P. tremula × P. alba	COMT	antisense	yes	Berkshire, England	2	12 plants per line	4 years	4 years	WT [WT]	↓S/G, incorporation of 5-OHG [↓S/G, incorporation of 5-OHG]	n.d. [n.d.]	↓ [↓]	WT [WT]	Pilate et al. (2002) [Lapierre et al., 1999; Van Doorsselaere et al., 1995]
P. tremula × P. alba	COMT	antisense	yes	n.a.	2	n.a.	2 years	2 years	WT [WT]	↓S/G, incorporation of 5-OHG [↓S/G, incorporation of 5-OHG]	n.d. [n.d.]	↓ [↓]	WT [WT]	Lapierre et al. (1999) [Lapierre et al., 1999; Van Doorsselaere et al., 1995]
P. tremula × P. alba	CAD	sense and antisense	yes	n.a.	3	n.a.	2 years	2 years	↓4% to WT [↓4% to WT]	WT S/G, ↑ free phenolic G and S units [WT S/G]	n.d. [n.d.]	↑ [↑]	WT [WT]	Lapierre et al. (1999) [Lapierre et al., 1999]
P. tremula × P. alba	CAD	antisense	yes	Orléans, France	2	10 plants per line	5 years	2 years	Slightly ↓ [WT]	WT S/G, ↑ free phenolic G and S units [WT S/G]	n.d. [n.d.]	n.d. [↑]	WT [WT]	Pilate et al. (2002) [Baucher et al., 1996]
P. tremula × P. alba	CAD	antisense	yes	Orléans, France	2	10 plants per line	5 years	2 years (4 y/o roots)	Slightly ↓ [WT]	WT S/G, ↑ free phenolic G and S units [WT S/G]	n.d. [n.d.]	↑ [↑]	WT [WT]	Pilate et al. (2002) [Baucher et al., 1996]
P. tremula × P. alba	CAD	antisense	yes	Berkshire, England	2	12 plants per line	4 years	4 years	Slightly ↓ [WT]	WT S/G, ↑ free phenolic G and S units [WT S/G]	n.d. [n.d.]	↑ [↑]	WT [WT]	Pilate et al. (2002) [Baucher et al., 1996]
Morus alba	CAD	natural mutantd	no	Tsukuba, Japan	1	n.a.	n.a.	1 year	Lighter color in Mäule staining	n.d.	↑	↑	n.d.	Ikeda et al. (2021)
Morus alba	CAD	natural mutantd	no	Tsukuba, Japan	1	n.a.	n.a.	n.a.	↓33%	↓S/G	n.d.	n.d.	n.d., lodging phenotype	Yamamoto et al. (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.	1 year	1 year	↓ [↓43%]	n.d. [↓S/G]	↑ [n.d.]	n.d. [n.d.]	WT [WT]	Gui et al. (2020) [Gui et al., 2020]
P. deltoides × P. euramericana	LTF1	vessel-specific suppression	yes	Shanghai, China	2	n.a.	1 year	1 year	↓ [↓16%]	n.d. [↑S/G ]	↓ [n.d.]	n.d. [n.d.]	↓ [↓]	Gui et al. (2020) [Gui et al., 2020]

# lines, number of lines with the intended transgenic construct or mutant allele (thus not including control lines); age of stem at harvest, if not explicitly stated otherwise, the age of the root is equal to that of the stem; n.d., not determined; n.a., not available; y/o, year-old; S/G, syringyl/guaiacyl ratio; S/V, syringaldehyde/vanillin ratio; FA, ferulic acid; 5-OHG, 5-hydroxyguaiacyl. Text in square brackets [ ] refers to data from greenhouse-grown trees of the same transgenic line, if available. For saccharification efficiency, see the corresponding reference for specific information about biomass pretreatments. Effect on total lignin is given as a percentage of the total lignin analyzed via different methods; see the corresponding reference for specific information.

^aOne line for lignin amount determination, three lines for biomass determination.

^bEight-month-old trees for total lignin determination, 12-month-old trees for biomass determination.

^cTwo-year-old trees for total lignin and lignin composition determination, 4-year-old trees for biomass yield determination, 5-year-old trees for chemical pulping experiments.

^dThe data presented are for the natural mutant Sekizaisou in comparison with other mulberry lines.