Plant xyloglucans play an important role in the processes of cell wall extension, determine their mechanical properties, thus affecting growth and morphology of individual cells and whole organs. Being one of the main components of hemicellulose, xyloglucans play a particular physiological role in woody plants. To study xyloglucan physiological role, transgenic aspen (Populus tremula L.) plants with a recombinant sp-Xeg gene from the fungus Penicillium canescens were produced. Constitutive expression of this gene in the heterologous surrounding was confirmed by RT-PCR method. The analysis of protein extracts from the leaves of greenhouse-grown plants and microshoots grown in vitro showed activation of xylogluconase in transgenic lines. The strongest activation (1.6-fold) was observed in the leaf extracts (clone PtXVXeg1b) and in vitro microshoots (clone PtXVXeg1c). In transgenic plants, the relative content of pentosans in the wood was declined. In control plants (Pt genotype), it was equal to 148 mg/g dry wt, whereas in tested clones (PtXVXeg1a, PtXVXeg1b, and PtXVXeg1c), it varied from 100 to 140 mg/g dry wt. The strongest decrease (by 31%) in the content of pentosans was observed for the line PtXVXeg1c; the content was equal to 102.1 ± 1.5 mg/g dry wt. A comparative analysis of leaf morphology revealed an increase in the length of petiole and a decrease in the length of the main vein in transgenic lines. In control plants, the ratio of the petiole length to the length of the main vein was equal to 0.49, whereas in transgenic plants, it varied from 0.51 to 0.66. A significant increase of this index was observed in 12 from 14 transgenic lines. © 2012 Pleiades Publishing, Ltd.