Summary Fifteen poplar cDNA encoding fasciclin‐like arabinogalactan proteins (PopFLAs) were finely characterized, whereas the presence of arabinogalactan proteins (AGPs) was globally assessed during wood formation. PopFLAs transcript accumulation was analysed through EST distribution in cDNA libraries, semi‐quantitative RT‐PCR, microarray experiment and Northern blot analysis. Similarly, AGPs contents were globally quantified by rocket electrophoresis. AGPs accumulation was further examined by Western blotting and immunocytolocalization. Ten PopFLAs were specifically expressed in tension wood (TW) and not expressed in the cambial zone. Rocket electrophoresis revealed important AGPs accumulation in TW xylem. An anti‐AGPs specific antibody recognized two proteins preferentially present in the cell wall‐bound fraction from TW. Immunocytochemistry revealed a strong labelling close to the inner part of the G‐layer of TW fibres. PopFLAs are expressed in xylem and many are up‐regulated in TW. It is suggested that some PopFLAs accumulating at the inner side of the G‐layer may have a specific function in the building of this layer. PopFLAs expression may therefore be linked to the specific mechanical properties of TW.

The short-term influence of adequate and high nitrogen fertilization on poplar lignification was investigated. The high nitrogen supply decreased lignin staining in the newly formed secondary xylem, indicating that lignin deposition was affected. Acetyl bromide determinations gave a 9-10% decrease in lignin content; however, Klason lignin content was unchanged. Thioacidolysis showed that elevated N supply affected lignin structure such that there was a reduced frequency of lignin units involved in beta-O-4 bonds, a reduced syringyl/guaiacyl ratio, an increased frequency of p-hydroxyphenyl lignin units, more guaiacyl units with free phenolic groups, and more p-hydroxybenzoic acid ester-linked to poplar lignins. These features suggest that lignins from poplars grown under high N bear structural similarities to lignins formed during early stages of wood development. The findings also indicate that a gravitational stimulus inducing the formation of tension wood and high N availability lead to similar and additive effects on lignin content and structure.

Previous studies indicated that high nitrogen fertilization may impact secondary xylem development and alter fibre anatomy and composition. The resulting wood shares some resemblance with tension wood, which has much thicker cell walls than normal wood due to the deposition of an additional layer known as the G-layer. This report compares the short-term effects of high nitrogen fertilization and tree leaning to induce tension wood, either alone or in combination, upon wood formation in young trees of Populus trichocarpa (Torr. & Gray) × P. deltoides Bartr. ex Marsh. Fibre anatomy, chemical composition and transcript profiles were examined in newly formed secondary xylem. Each of the treatments resulted in thicker cell walls relative to the controls. High nitrogen and tree leaning had overlapping effects on chemical composition based on Fourier transform infrared analysis, specifically indicating that secondary cell wall composition was shifted in favour of cellulose and hemicelluloses relative to lignin content. In contrast, the high-nitrogen trees had shorter fibres, whilst the leaning trees had longer fibres that the controls. Microarray transcript profiling carried out after 28 days of treatment identified 180 transcripts that accumulated differentially in one or more treatments. Only 10% of differentially expressed transcripts were affected in all treatments relative to the controls. Several of the affected transcripts were related to carbohydrate metabolism, secondary cell wall formation, nitrogen metabolism and osmotic stress. RT-qPCR analyses at 1, 7 and 28 days showed that several transcripts followed very different accumulation profiles in terms of rate and level of accumulation, depending on the treatment. Our findings suggest that high nitrogen fertilization and tension wood induction elicit largely distinct and molecular pathways with partial overlap. When combined, the two types of environmental cue yielded additive effects.