In response to environmental variation, xyloglucan could fix the microfibrils to the inner surface of the wall to withstand the tensile stress generated within the G-layer. This would explain why the basal regions of stems of transgenic poplars overexpressing xyloglucanase could not bend upward. This finding has ramifications for the production of bioethanol, which requires tree cellulose to be enzymatically hydrolyzed. The level of cellulose degradation with enzymes was markedly increased in the xylem overexpressing xyloglucanase. We propose that xyloglucan serves as a key hemicellulose and a tightening tether of cellulose microfibrils in the secondary walls.