Examination of biological hotspot hypothesis of primary cell wall using a computational cell wall network model

Computational modeling reveals that a cell wall network whose mechanical integrity is dominated by a small mass fraction of “hotspot” linkers between microfibrils can sit close to a percolation threshold, across which mechanical integrity is very sensitive to the number of hotspots. In the model, the mechanical properties of cell wall fragments consisting of cellulose microfibrils and xyloglucan linkers with different levels of disorder were examined under progressive decimation of the network, modeling enzymatic degradation. The percolation limit so obtained is close to mass fraction of xyloglucan that must be removed to induce creep experimentally. Greater disorder in the interconnectivity of the network raises the number of hotspot linkers per fibril necessary to reach the percolation threshold. To maintain the required mechanical stiffness with a sparse network of hotspot connections, either each xyloglucan linker must be much stiffer than a single polymeric strand or an additional cell wall component, i.e. pectin, must carry substantial load with a sensitive non-linear mechanical response, such as that associated with a glass transition.