Sucrose or starch? The influence of tonoplast sucrose transporter perturbation on carbon partitioning for growth, defense, and winter protection in coppiced poplar

Abstract

AbstractNon-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 ofPopulus, 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 usedPtaSUT4-knockout mutants ofPopulus tremula × 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 insut4mutants following winter but not summer coppicing. Reserve depletion during post-coppice regrowth was not impaired in thesut4mutants 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 insut4mutants. Accrual of abundant defense metabolites, including salicinoids, chlorogenic acids, and flavonoid products was prioritized in the summer, but conspicuously lower insut4mutants 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 thesut4mutants demonstrate the importance of SUT4 in modulating trade-offs between growth and the other priorities during reserve utilization inPopulus.