B. Clair, T. Alm'eras, G. Pilate, D. Jullien, J. Sugiyama, C. Riekel

An efficient motor system based on the development of mechanical stress during the maturation of wood cell walls allows trees to achieve successful height growth and adaptive reorientations. In some hardwood species, a tensile stress is generated by a specialized wall layer, the G-layer, made of a polysaccharide hydrogel reinforced by cellulose microfibrils aligned with the cell axis.
The mechanism generating this tension has long been a matter of debate. It seems doubtful that stress is induced in crystalline microfibrils after their deposition, because of their high stiffness and chemical stability. Alternative hypotheses involve cellulose re-crystallisation or interaction between the G-layer and other layers.
In vivo investigation on the state of stress of these components was necessary to elucidate this mechanism. Here we provide experimental evidence that tension appears in crystalline microfibrils of the G-layer shortly after their deposition.
Using microbeam synchrotron X-ray diffraction, we measured how the lattice spacing of cellulose changes from the cambium to the mature wood of poplar trees, and found it to increase in the G-layer but not in other layers. We conclude that maturation stress originates in the swelling pressure of the hydrogel and is directly transmitted to the microfibrils within the G-layer. By contrast with previous models, this new hypothesis for the origin of maturation stress fits well with all current knowledge on the mechanical behavior and the biological function of the G-layer.