In response to environmental stimuli such as wind, snow, slope or asymmetric crown shape, leaning woody plants form a special tissue, named reaction wood, which allows tree axis reorientation. In angiosperm trees, this reaction wood is associated with tensile strains and consequently has been named tension wood (TW). It occurs at the upper face of leaning stems and branches, and can be obtained in a controlled way on young trees by simply tilting the stem. Inpoplar, TW is mainly characterized by xylem fibres which are poorly lignified and have an extra thick gelatinous secondary layer, named G layer, in the secondary cell wall. This G layer contains almost completely pure crystalline cellulose with microfibrils oriented nearly parallel to the axis of the fibre. Different models have been proposed in order to link these biochemical and ultrastructural features to the mechanical properties of TW. To assess the validity of these models we need to find out the molecular mechanisms involved in the formation of the G layer. With the aim to identify such mechanisms we looked for genes differentially expressed between TW and OW. We have previously shown that l0 fasciclin-like arabinogalactan (FLAs) poplar genes were highly expressed in the xylem of tension wood [1, 2]. Five other popFLAs (l I to l5) were shown to be expressed in xylem but not differentially between TW and ow t2l. These 15 popFLAs belong to the class of arabinogalactan-proteins (AGPs) containing one fasciclin-like domain surounded by two AGP- like domains. While the fasciclin-like domain appears well conserved among the different proteins, we find more variability in the AGPlike domains which carry the arabinogalaotosylation sites. Thanks to the recent publication of the poplar genome sequence we identified 41 additional poplar genes encoding putative fasciclin-like protein. Two-thirds of the predicted genes contain one FASI (fasciclin-like) domain as defined by the reference "IPR000782" in InterPro database, the other 17 genes carrying 2 FAS1 domains. In addition, 70% of the sequences are predicted to be anchored to a glycosyl-phosphatidylinositol (GpI). It has been postulated that this dual presence of fasciclin-like domain and GPI-anchor could support a putative function of these proteins in cell adhesion and signaling. In regard to the phylogenic analysis of this large gene family we investigated the expression profiling of these 56 poplar FLAs. This has been done through electronic northern and from genome-wide expression data collected on Affymetrix poplar array experiments. Electronic northern were done from 200.199 expressed sequence tags (ESTs) available in public databases representing 37 different non normalized cDNA libraries. Genome-wide expression patterns were investigated in tension wood in response l) to nyctemeral variation and 2) to re-watering after water shortage. The aim of thefirst experiment was to see if nyctemer variation of FLAs gene could be correlated with expression pattern of other cell-wall related genes. In the second experiment, re-watering induces xylem re-growth together with a significant increase of differentiation of vessel elements. Expression analyses of these FLAs in these experiments will be presented and discussed.