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In order to create trees in which cellulose, the most abundant component in biomass, can be enzymatically hydrolyzed highly for the production of bioethanol, we examined the saccharification of xylem from several transgenic poplars, each overexpressing either xyloglucanase, cellulase, xylanase, or galactanase. The level of cellulose degradation achieved by a cellulase preparation was markedly greater in the xylem overexpressing xyloglucanase and much greater in the xylems overexpressing xylanase and cellulase than in the xylem of the wild-type plant. Although a high degree of degradation occurred in all xylems at all loci, the crystalline region of the cellulose microfibrils was highly degraded in the xylem overexpressing xyloglucanase. Since the complex between microfibrils and xyloglucans could be one region that is particularly resistant to cellulose degradation, loosening xyloglucan could facilitate the enzymatic hydrolysis of cellulose in wood.

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. [ABSTRACT FROM AUTHOR]