Table 2.

Summary of reported genetic modifications within the phenylpropanoid pathway and their effects on saccharification yield, total lignin content, lignin composition/structure (and/or the effect on metabolites) and plant phenotype; TG (transgenic), M (mutation), ND (not determined).

Entry	Species	Gene alteration	Type	Compositional/structural change in lignin or metabolites	Effect on lignin content	Saccharif‐ ication yield[a]	Phenotypic effect	Ref.
1	Arabidopsis thaliana	PAL deficient	M	Phe overaccumulation; S/G↑ flavonol glycosides↓	↓	ND	No change	54
2	Brachypodium distachyon	PAL downregulation	TG	S/G↑; ferulate↓	↓	2×↑	Delayed development; root growth↓	99
3	Capsicum annum, C. chinense and Solanum tuberosum	C4H upregulation	TG	S/G↑	↓	ND	Curled leaves, dwarfism, or no change	55
4	Arabidopsis ref3	C4H deficient	M	S/G↑; New cinnamoylmalate	↓	ND	Dwarfism, male sterility, collapsed vasculature	56
5	Populus alba x grandidentata	C3H downregulation	TG	H ≈100×↑; S/G↓	50 %↓	ND	No change	100
6	Lolium perenne	CCR downregulation	TG	No change	↓	↑	No change	101
7	Arabidopsis thaliana	CCR deficient	M	Ferulic acid‐coniferyl alcohol ether dimers in metabolites; sinapoyl malate ≈4×↓	25‐35 %↓	ND	Dwarfism, delayed senescence	102
8	Pinus radiata	CCR downregulation	TG	Increase of p‐coumaroyl hexose, caffeic acid hexoside and ferulic acid hexoside; H↓ and G↓	ca. 50 %↓	↑	ND	67
9	Maize	CCR deficient	M	H‐units strongly decreased; S/G slightly↑	Slight↓	↑	No change	103
10	Nicotiana tabacum	CCR downregulation	TG	S/G↑; β‐O‐4 units↓; introduction of ferulic acid and sinapic acid	↓	↑	Orange xylem; Less severe: no change; more severe: dwarfism, collapsed vessels	104
11	Populus tremula x Populus alba	CCR downregulation	TG	Ferulic acid incorporated into lignin	5‐24 %↓	15 %↑	Orange xylem; Less severe: no change; More severe: dwarfism	105
12	Populus tremula x Populus alba	CCR downregulation	TG	Oligolignols↓	↓	≈20 %↑	Orange‐coloured wood	105, 106, 107
13	Medicago truncatula	CAD deficient	M	≈95 % Sinapaldehyde‐ and conifer‐ aldehyde‐derived	Increase in wall‐bound lignin moieties	ND	None at 22 °C; Dwarfed at 30 °C	43
14	Maize	CAD downregulation	TG	S/G↓	No change	↑	No change	108
15	Triticum sinskajae	CAD downregulation	TG	H↓; S/G↑	No change	ND	Slight dwarfism	109
16	Brachypodium distachyon	CAD deficient	M	Increase in β‐O‐4‐ and 4‐O‐5‐coupled sinapaldehyde units; increase in free phenolic groups	↓	↑	No change	110
17	Pinus taeda	CAD deficient	M	Incorporation of coniferaldehyde and dihydroconiferyl alcohol; increase in vanillin, ferulic acid, p‐coumaric acid, coniferaldehyde and p‐hydroxybenz‐ aldehyde	↑	↑	Dark‐brown wood	91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111
18	Panicum virgatum	CAD downregulation	TG	Hydroxycinnamaldehydes↑	Lignin↓ and cutin↓	↑	No change	112
19	Koshihikari x Chugoku 117 rice	CAD deficient	TG, M	ND	↓	↑	No change	113
20	Arabidopsis thaliana	HCT deficient	M	H↑; S≈0; G≈0	↓	ND	Severe dwarfism	57
21	Populus nigra	HCT downregulation	TG	H 17×↑	No change	ND	Dwarfism	114
22	Medicago sativa	CCoAOMT deficient	TG	Incorporation of 5OH‐CA producing novel 5OH‐G units as benzodioxanes	≈20 % decrease	≈10 % increase in cellulose	No change	42
23	Brown midrib‐3 Maize	COMT deficient	M	S/G↓; incorporation of 5OH‐CA producing benzodioxanes	↓	Improved	No change	115
24	Arabidopsis thaliana	COMT deficient	M	S/G↓; trimeric moiety 5OHG‐5OHG‐G; benzodioxane linkages	No change	ND	No change	58
25	Saccharum officinarum cv. CP88‐1762	COMT deficient	M	S↓; p‐coumarate (on lignin)↓	6‐12 %↓	28‐32 %↑	No change	116
26	Arabidopsis thaliana	F5H deficient	M	S 70‐75 %↓; G↑	No change	ND	Lack of 2° wall structure in inter‐fascicular and xylem fibres	65
27	Arabidopsis thaliana	F5H overexpression	TG	≈100 % S (i.e., G↓, S↑)	↓	No change	Decreased plant stiffness	117, 118
28	Arabidopsis thaliana	F5H deficient	M	≈100 % G (i.e., G↑, S↓)	No change	Decrease	Decreased plant stiffness	117
29	Brassica napus	COMT, C4H, C3H, F5H	TG	No change	26‐40 % decrease in the seeds	ND	No change	102
30	Arabidopsis thaliana	F5H upregulation; COMT downregulation	TG	Lignin>70 % 5OH‐CA‐derived; ≈90 % benzodioxane units	↓	ND	Dwarfism; male sterility	60, 119
31	Arabidopsis thaliana	CCR and CAD deficient	TG	Increase in interunit bonding; incorporation of coniferaldehyde, sinap‐ aldehyde, ferulic acid	50 %↓	↓	Dwarfism; male sterility	62
32	Arabidopsis thaliana	CSE deficient	M	H↑	↓	4×↑	No change	66
33	Populus alba x grandidentata	FMT introduced	TG	Ester linkages introduced into lignin backbone; S/G↑	Little change	78 %↑	No change	44
34	M. sativa	C3H downregulation	TG	H 65×↑	50 %↓	ND	Dwarfism	120
35	Populus tremula x alba	F5H upregulation	TG	97.4 % S; (i.e., G↓, S↑)	Little change	Pulp yield higher, pulp brighter	No change	121, 122

[a] Measured at low conversion extent; yields at full conversion will not differ.123