Tuma, T. T.; Nyamdari, B.; Hsieh, C.; Chen, Y.-H.; Harding, S. A.; Tsai, C.-J.

Non-structural carbohydrate reserves of stems and roots underpin overall tree fitness as well as productivity under short-rotation management practices such as coppicing for bioenergy. While both sucrose and starch comprise the predominant carbohydrate reserves of Populus, utilization is understood primarily in terms of starch turnover. The tonoplast sucrose transport protein SUT4 modulates sucrose export to distant sinks, but the possibility of its involvement in sink tissue carbohydrate remobilization has not been explored. Here, we used PtaSUT4-knockout mutants of Populus tremula x alba (INRA 717-1B4) in winter and summer glasshouse coppicing experiments to strain carbon demand and test for SUT4 involvement in reserve utilization. We show that epicormic bud emergence was delayed and subsequent growth reduced in sut4 mutants following winter but not summer coppicing. Reserve depletion during post-coppice regrowth was not impaired in the sut4 mutants under winter or summer glasshouse conditions. Interestingly, xylem hexose increased during post-coppice growth exclusively in the winter when osmoprotection is critical, and the increase was attenuated in sut4 mutants. Accrual of abundant defense metabolites, including salicinoids, chlorogenic acids, and flavonoid products was prioritized in the summer, but conspicuously lower in sut4 mutants than controls. Together, our results point to shifting priorities for SUT4 function from support for osmoprotection in winter to chemical defense in summer. Delayed bud release and growth following winter but not summer coppicing in the sut4 mutants demonstrate the importance of SUT4 in modulating trade-offs between growth and the other priorities during reserve utilization in Populus.