Poplar leaf bud resin metabolomics: seasonal profiling of leaf bud chemistry in <i>Populus trichocarpa</i> provides insight into resin biosynthesis

Trees in the genus Populus synthesize sticky and fragrant resins to protect dormant leaf buds during winter. These resins contain diverse phenolic metabolites, in particular, hydroxycinnamate esters and methylated flavonoids. Populus trichocarpa leaf bud resin is characterized by methylated dihydrochalcone aglycones. To determine how the resin profile is influenced by seasonal changes, P. trichocarpa lateral leaf bud extracts and secreted surface resin were collected monthly over a 1-year cycle. The dihydrochalcones in both sets of extracts were quantified using ultrahigh pressure liquid chromatography-mass spectrometry (UPLC-MS), and other chemical changes were monitored using nontargeted metabolomics by UPLC-high-resolution MS (UPLC-HRMS). The results indicate that the dihydrochalcone content changes over the seasons and that biosynthesis occurs concomitant with bud development in the summer months. Nontargeted metabolomics data confirmed a pattern of dramatic changes in the summer and further suggested additional periods of substantive biochemical change in the resin. While overall patterns of surface-extracted resin matched those of whole bud extracts, some of the dynamics were shifted in the surface resin samples. This study provides the basis for the use of dihydrochalcones and other identified resin components as metabolic markers for more detailed investigations of resin biosynthesis, secretion, and movement to the bud surface.