Angiosperm trees are able to reorient their axes thanks to their capacities to differentiate tension wood on the upper side of stems and branches. In poplar, tension wood fibres develop an extra cell wall layer, named G-layer, responsible for the peculiar mechanical properties of tension wood. G-layer is composed of highly crystalline cellulose microfibrils embedded in a polysaccharide/glycoprotein matrix devoid of lignin. During the G-layer deposition, cellulose microfibrils get oriented parallel to the fibre axis and placed under the state of high tensile stress. Chitinase-like proteins (CTL) have been proposed to play different roles in plant growth and development. Genes encoding CTL have been associated to cellulose synthesis and their mutation causes ectopic deposition of lignin. Transcripts of CTL2 gene were shown to be highly abundant in differentiating tension wood (Déjardin et al., 2004). Its homologue in Arabidopsis is specifically expressed in stems and co-expressed with the CesA genes involved in cellulose synthesis in secondary cell walls (Persson et al., 2005). In our study, we investigated potential function of CTL2 in the formation and mechanical properties of tension wood. Expression of CTL2 was quantified in different tissues and the effects of CTL2 down-regulation have been characterized in GM poplar trees. CTL2 gene appeared highly expressed in xylem with no difference in expression between tension and opposite wood. Downregulation of CTL2 did not seem to impact growth and development of trees in greenhouse conditions. However, stems appeared to be highly breakable in the transverse direction. Anatomical observations of xylem showed altered formation of Glayers in vessel-surrounding fibres in transgenic poplars. Further analyses indicated a substantial increase in wood lignin content. Cellulose crystallinity, microfibril angle and stem mechanical properties are currently assessed. Results will be presented and discussed in the light of present hypothesis regarding CTL2 function in plant development.