Immunolocalization of fasciclin-like arabinogalactan proteins in the G-layers of poplar tension wood fibers

In hardwood trees, tension wood (TW) is an adaptive mechanism used by trees to orient their stems and branches, withstand their own weight, and improve wind resistance. In many species, such as poplar, TW fiber cell walls exhibit a supplemental layer, named the G-layer, which is responsible for the mechanical properties of TW. However, the molecular mechanisms involved still need to be clarified. The synthesis of a number of fasciclin-like arabinogalactan proteins (FLA) has been shown to be highly upregulated during tension wood formation in poplar and is potentially associated with the outstanding mechanical properties of tension wood. Three polyclonal antibodies directed against different poplar TW-specific FLA epitopes were produced and used to assess the presence of these FLAs in differentiating and mature tension wood fibers. Using immunohistochemistry, FLAs were detected at early stages of G-layer differentiation, specifically at the inner side of G-fiber cell walls, whereas a weaker signal was detected in mature G-fibers. However, western blot analyses of protein extracts from differentiating and mature tension wood revealed increased levels of FLA in mature TW fibers, suggesting that these FLAs remained present in mature G layers but were not accessible to anti-FLA antibodies in TW histological sections. Overall, specific FLAs involved in secondary cell wall construction are located at the inner side of the G-layer and are likely actors in the unique mechanical properties of TWs.