ln response to a mechanical stress, trees are able to reorientate their axes thanks to the differentiation of a special wood named tension wood. ln poplar, tension wood fibres are mainly characterized by an additional secondary layer, named G-layer mostly constituted of highly crystalline cellulose microfibrils orientated almost parallel to the fibre axis. These specific features account for the particular mechanical properties of tension wood. ln order to understand the mechanisms underlying G-layer formation, different cDNA libraries have been prepared from cambium, developing and mature xylem sam pied in wood area with or without tension wood. 10,062 EST were sequenced from these libraries and annotated. Genes specifically or preferentially expressed during tension wood formation were identified from EST distribution in the different libraries. ln addition, a poplar microarray was used to determine global gene expression pattern during tension wood formation. Among other genes, many fasciclin-like arabinogalactan proteins (PopFLAs) appeared highly expressed in the differentiating xylem, once secondary cell wall synthesis occurred. Ten of these PopFLAs were specifically expressed in tension wood. Further immunocytochemical studies indicated that some PopFLAs accumulated at the inner side of the G layer suggesting a specific function for these proteins in the building of the G layer.