Fig. 5

Model of GAUT4 function in cell wall porosity and biomass recalcitrance. a Representative chemical structures of select major wall polymers (cellulose, hemicellulose⁵¹, lignin⁶⁴, and the pectins HG¹¹ and RGII⁶⁵ with modification).(b Model depicting reduction of GAUT4-synthesized HG in GAUT4-KD versus wild-type (WT) biomass and hypothesized consequences on wall porosity and lignin migration during hydrothermal pretreatment. A hypothesis: a reduction of HG/RGII in GAUT4-KD switchgrass leads to a reduction in cross-linking of polymers between polysaccharides and/or lignin (i.e., reduced FA-based linkage between lignin and hemicellulose (yellow wavy line), and reduced ionic salt bridges and borate diester bonds between HG and RGII (blue wavy line), respectively). This reduced cross-linking results in (1) increased wall porosity and (2) reduced LCC linkages that together lead to easier lignin migration during LHW pretreatment and reduced biomass recalcitrance. FA: ferulate; S: syringyl unit; G: guaiacyl unit; HG: homogalacturonan; RGII: rhamnogalacturonan II; LHW: liquid hot water