Table 1.
Effects of engineering expression of genes encoding proteins affecting non-cellulosic polysaccharides on enzymatic saccharification and cell wall composition of woody species.a
| Gene | Approach | Species | Protein | Protein family | Promoter | Role of protein | Saccharification efficiency | Phenotypic effect | Effect on cell wall composition | Reference |
|---|---|---|---|---|---|---|---|---|---|---|
| PtxtPME1 | Down- or up-regulation | Hybrid aspen | Pectin methyl esterase (PME) | CE8 | 35S | Removal of methyl ester groups from HG | EH(+), AEH(-) in down-regulated lines | In down-regulated lines: Height(ND) diameter(+) | Up-regulation resulted in (-) in DM while downregulation resulted in (+) DM and (-) in the amount of galacturonan. | Siedlecka et al., 2008; Escamez et al., 2017 |
| PoGT47C | RNAi down-regulation | Hybrid poplar | Glycosyl transferase | GT47 | 35S | Probably biosynthesis of RES in X | EH(+) | Growth(ND) | S/G(+), Xyl content(-), Glu content(-), GX(-), thickness of cell walls(-), RES of GX (-) | Lee et al., 2009 |
| AaXEG2, HvXYL1, Tv6GAL, AtCel1 | Over-expression | Poplar (P. alba) | Xyloglucanase, xylanase, galactanase, and glucanase | GH5, GH10, GH35, and GH9 | 35S | Hydrolysis of XG, X, Ga, and GM | EH(+) for AaXEG2, HvXYL1 and AtCel1, EH(-) for Tv6GAL | Growth(+) for AaXEG2. | L content (ND) for HvXYL1 and AtCel1, (-) for AaXEG2. | Park et al., 2004; Kaida et al., 2009 |
| AaXEG2 | Over-expression | Acacia mangium | Xyloglucanase | GH5 | 35S | Hydrolysis of XG | EH(+) | Growth(+) | Cellulose(+), L(-),wall bound XG in the xylem(-) | Hartati et al., 2011; Kaku et al., 2011 |
| PtxtXET16A/PtxtXTH34 | Up-regulation | Hybrid aspen | Xyloglucan endotransglucosylase (XET) | GH16 | 35S | Hydrolysis and re-ligation of XG | EH(ND), AEH(ND) | Growth(ND) | Loosely bound XG fraction(-), tightly bound XG fraction(+), PCW XG(+) | Nishikubo et al., 2011; Escamez et al., 2017 |
| PoGT43B, PtxtGT43B and PtxtGT43C | RNAi down-regulation | Poplar (P. trichocarpa), hybrid aspen | Glycosyl transferases | GT43 | 35S and GT43B | Scaffolding in X synthase complex, X backbone biosynthesis | EH(+), AEH(ND) | Growth(+) for GT43B promoter | Xyl content relative to RES (Xyl:RES) in X(-). Cell wall thickness(-), wood density(-), MFA(-), S/G(-). | Lee et al., 2011; Ratke et al., 2018 |
| PtxtPL1-27 | Up-regulation | Hybrid aspen | Pectate lyase (PEL) | PL1 | 35S | Cleavage of HG backbone | EH(+) (for release of Xyl), AEH(+) | Growth(-) | L(+), M(-), TFA soluble Glu (+), solubility of pectin, X, and XG(+) | Biswal et al., 2014 |
| PtGAUT12-1/PtGT8D-1, PtGAUT12-2/PtGT8D-2 | RNAi down-regulation | Poplar (P. trichocarpa, P.deltoides) | Galacturonosyl transferase | GT8 | 35S | Biosynthesis of pectin and RES in X | EH(+) | Growth(ND or +) | L (+), mechanical strength (-), S/G(+), Xyl(-), GalA(-), pectin(-), X(-), Rha(-) | Lee et al., 2011; Li et al., 2011; Biswal et al., 2015 |
| PcGCE | Over-expression | Hybrid aspen | Glucuronoyl esterase | CE15 | 35S | Hydrolysis of ester linkages between MeGlcA and L | EH(-), AEH(-), Cellulose conversion (+) | Growth(-) | L(+), C(-), S/G(+), Ara(-), Rha(-), MeGlcA(-). Compositional changes of pectin. | Gandla et al., 2015 |
| PtxtXyn10A | Antisense down-regulation | Hybrid aspen | Xylan endotransglycosylase | GH10 | 35S | Hydrolysis and re-ligation of X chains | EH(ND), AEH(ND) | Growth(+) | Ga(-), X(+), TW content in stem(-), MFA(-) | Derba-Maceluch et al., 2015; Escamez et al., 2017 |
| PtrDUF579-3 | Antisense down-regulation | Poplar (P. trichocarpa) | GX methyl transferase (GXM3) | GT8 | 35S | Methylation of GlcA in GX biosynthesis | AEH(+) | Growth(ND) | Mechanical strength of stem(-), methylation of GlcA in GX(-) | Song et al., 2016 |
| PdDUF231A | Up-regulation | Poplar (P. deltoides) | Acyl transferase/esterase | DUF231 | UBI3 | Probably acetylation of X | EH(+) | Growth(+) | L(-), Cellulose(+),acetylated X at O-3(+) | Yang et al., 2017 |
| PtxtRWA (AB and CD clades) | RNAi suppression of RWA family | Hybrid aspen | RWA-A, RWA-B, RWA-C, and RWA-D | RWA | GT43B | Probably transport of acetyl-CoA to Golgi | EH(+), AEH(+) | Growth(ND) | Total acetylation and X acetylation at position O2(-), MeGlcA(+) | Pawar et al., 2017b |
| AnAXE1 | Over-expression | Hybrid aspen | Acetylxylan esterase (AX) | CE1 | 35S | Deacetylation of X and possibly other polymers | EH(+) AEH(+) | Growth(ND) | Total acetylation and acetylation of X at position O2 (-). X chain length and size of L polymer(-). L solubility (+), S/G(-) | Pawar et al., 2017a |
| PdGAUT4 | RNAi down-regulation | Poplar (P. deltoides) | α-1,4-Galacturonosyl transferase | GT8 | 35S | Probably synthesis of HG | HWEH(+) (Glu yield) | Growth(+) | S/G(+), HG(-), RGII(-), GalA(-), Calcium and boron in the cell wall(-). HG and RGII cross-link in cell wall(-) | Biswal et al., 2018 |
aAEH, saccharification efficiency of enzymatic hydrolysis after acid pretreatment; Ara, arabinose; C, carbohydrate; DM, degree of methylation; EH, saccharification efficiency of enzymatic hydrolysis without pretreatment; Ga, galactan; GalA, galacturonic acid; Glu, glucose; GlcA, glucuronic acid; GM, glucomannan; GX, glucuronoxylan; Hybrid aspen, Populus tremula ×tremuloides; HG, homogalacturonan; Hybrid poplar, Populus alba ×tremula; HWEH, saccharification efficiency of enzymatic hydrolysis after hot water pretreatment; L, lignin; M, mannan; MeGlcA, 4-O-methylglucuronic acid; MFA, microfibril angle; ND, No detectable effect compared to WT; RGII, Rhamnogalacturonan II; Rha, rhamnose; RES, reducing end sequence of xylan; S/G, syringyl:guaiacyl ratio; TFA, trifluoroaceticacid; TW, tension wood; X, xylan; Xyl, xylose; XG, xyloglucan; (+), increase compared to WT; (–), decrease compare to WT.