Fig. 5: Model of action for callose integration in lignocellulosic biomass.

Callose acts as a cell wall spacer subsequently impacting biomass porosity and enzymatic access through an increase in hygroscopicity. a, Model of a poplar cellulose microfibril without (left) and with callose deposition (right). Callose is represented as a cross section of a helical structure with central bound water surrounded by mobile water, which partially increases the distance of lignin from cellulose. Bound and mobile water around callose explain the absence of short-range interactions between callose and other secondary cell wall polymers. b, Model of a poplar macrofibril without (left) and with callose deposition (right). Callose is inserted in between coalesced microfibril groups. The deposition of callose negatively impacts the subsequent polymerization of lignin surrounding the macrofibril. For representation convenience, we decided to represent the higher end of the poplar macrofibril size (30 nm) according to ref. ⁵³. c, Model of poplar macrofibril assembly without (left) and with callose deposition (right). Callose self-aggregates in between macrofibrils, which explains the observed increase in secondary cell wall porosity. The range of pore size affected is 4–30 nm, which is in the size range of hydrolytic enzymes. As such, callose is believed to act as a hydrophilic spacer of secondary cell wall polymer, further promoting access to hydrolytic enzymes for subsequent saccharification. LPMOs, lytic polysaccharide monooxygenases.