A simulation study of the water ordering effect of callose

Callose, a polysaccharide closely related to cellulose, is a minority component of the plant cell wall, playing a crucial role, among others, in plants' development and resistance to environmental stress. These functions are often attributed to the enhancement by callose of the mechanical properties of semi-ordered assemblies of cellulose nanofibres, that represent the majority component of the plant cell wall. A recent study (Gensler, W. {\it Plant Signal. Behav.} {\bf 2019}, {\it 14}, e1548878), however, suggested that the enhancement of mechanical properties by callose might be partly due to its ability to order neighboring water molecules, resulting in the formation, up to room temperature and beyond, of solid-like water-callose domains. This hypothesis is tested by atomistic molecular dynamics simulations, using ad-hoc models consisting of callose and cellulose chains forming networks (gels) in water, spanning a wide range of polysaccharide-water concentrations. The simulation results highlight systematic differences in the coordination and H-bonding of callose and cellulose by water. These structural differences are reflected in the different dynamical properties of water in samples made of callose or cellulose in water, to some extent validating the water-ordering hypothesis. However, mechanical properties, characterised by the computation of the Young's modulus of the polysaccharide / water gels, are nearly the same in the callose/water and in the cellulose/water samples, lending support to the view that callose's ability to link cellulose nanofibres into networks is the main mechanism underlying the strengthening of the plant cell wall.