Résumé
Tension wood (TW) is produced by temperate hardwood trees in order to support theirincreasing weight, orient their axes and cope with environmental cues such as wind. Poplar TWfibres harbour a supplemental layer, the G-layer, rich in crystalline cellulose, containing matrixpolysaccharides but no lignin. The tensile force responsible for the specific mechanicalproperties of TW originates from the G-layer and is transmitted to cellulose microfibrils soonafter their deposition, during G-fibre maturation (Clair et al, 2011). This force is likely tooriginate from physical changes in the high porosity hydrogel recently identified in the G-layer.RG-I type pectins appear as good candidate molecules responsible for the formation of this gel.Indeed, during G-layer maturation, LM5 labelling (specific to RG-I side chains) decreased,while RU1 labelling (specific to RG-I backbone) increased (Guedes et al, 2017). This suggesteda hydrolysis of the RG-I side chains during G-fibre maturation possibly by a β-galactosidase asdemonstrated in flax phloem fibers (Roach et al, 2011). Flax phloem fibres and TW G-fibresexhibit many similar features and, in flax, it has been shown that the hydrolysis of the sidechains of RG-I type pectins was associated to the very peculiar mechanical properties of bastfibres.In order to determine if RG-I pectins were effectively involved in the building of the G-layertensile force, we carried out different measurements on a common sampling during G-fibredevelopment: i) β-galactosidase activities using a histochemical test, ii) the evolution of RG-Iimmunolabelling profiles using LM5 and RU1 as probes and iii) the stiffening of the differentcell wall layers using Atomic Force Microscopy (AFM). We found a good correlation betweenβ-galactosidase activities and RG-I immunological labelling but we failed to establish a directassociation between RG-I hydrolysis and cell wall stiffening.