Tissue Metabolic Responses to Artificial Bending and Gravitation Stimuli in Betula platyphylla

Betula platyphylla Suk (Asian white birch) is an economically important tree species in the paper-pulping and biofuel industries. To investigate the mechanism of wood formation at the metabolic level, we evaluated metabolic responses associated with tension-wood formation. Four-year-old trees were subjected to artificial bending treatment for 6 weeks. The xylem growth rate of tension wood (TW) was significantly faster than that of opposite wood (OW), and it exhibited a higher cellulose content. Metabolomics analysis was performed on metabolites of TW, OW and normal wood (NW), and 183 metabolites were identified, of which levels of 142 were altered between groups. Metabolites related to fatty-acid and amino-acid metabolism, the glycolytic pathway, and the metabolism of fructose, mannose and starch sucrose were abundant in TW. Glucose 1-phosphoric acid, fructose and mannose associated with tension-wood development were elevated. Levels of xylitol and ribosol (related to the conversion of glucose), coniferol (the main monomer of lignin) and shikimic acid (an intermediate in lignin synthesis) were decreased in TW. These metabolites are likely involved in xylem development.