Populus root salicinoid phenolic glycosides are not mobilized to support metabolism and regrowth under carbon-limited conditions

Remobilization of carbon storage compounds in trees is crucial for the resilience to disturbances, stress and the requirements of their perennial lifestyle, all of which can impact photosynthetic carbon gain. Trees contain abundant non-structural carbohydrates in the form of starch and sugars for long-term carbon storage, yet questions remain about the ability of trees to remobilize non-conventional carbon compounds under stress. Aspens, like other members of the genus Populus, have abundant specialized metabolites called salicinoid phenolic glycosides, which contain a core glucose moiety. In this study, we hypothesized that the glucose-containing salicinoids could be remobilized as an additional carbon source during severe carbon limitation. We made use of genetically modified hybrid aspen (Populus tremula L.x Populus albaL.) with minimal salicinoid content and compared these with control plants with high salicinoid content during resprouting (suckering) in dark (carbon-limited) conditions. As salicinoids are abundant anti-herbivore compounds, identification of such a secondary function for salicinoids may provide insight into the evolutionary pressures that drive their accumulation. Our results show that salicinoid biosynthesis is maintained during carbon limitation and suggests that salicinoids are not remobilized as a carbon source for regenerating shoot tissue. However, we found that salicinoid-producing aspens had reduced resprouting capacity per available root biomass when compared with salicinoid-deficient aspens. Therefore, our work shows that the constitutive salicinoid production in aspens can reduce the capacity for resprouting and survival in carbon-limited conditions.