Systemic and Local Regulation of Root Growth by Vascular Trehalose 6-Phosphate is Correlated With Re-allocation of Primary Metabolites Between Shoots and Roots.

Trehalose 6-phoshpate (Tre6P) is a key signalling molecule that reflects carbon status and integrates it with developmental decision making. Tre6P has been demonstrated to regulate various developmental processes, including vegetative growth, shoot branching, flowering, and root branching. Here, we investigate how vasculature-derived Tre6P influences root system architecture by expressing heterologous Tre6P synthase or Tre6P phosphatase (TPP) specifically in the plant vasculature. Plants with elevated vascular-derived Tre6P levels had smaller root systems, reduced sucrose levels, and lower root metabolite levels, whereas vascular TPP overexpression had the opposite effect. Using reciprocal grafting experiments, we identified shoot-derived vascular Tre6P to be the main driver of these systemic responses. In lines with increased Tre6P in the shoot vasculature, the shoot-to-root metabolite ratios were consistently increased, indicating that Tre6P modulates metabolite allocation and utilisation to balance carbon partitioning between shoot and root growth. Our data further suggest that vascular Tre6P contributed to optimising the carbon-to-nitrogen ratio to support growth and fitness. Besides this systemic function, this study shows that root-derived Tre6P also plays a critical local role in modulating root development. Collectively, our results demonstrate that Tre6P in the vasculature functions both systemically and locally to coordinate metabolic status with root growth and architecture.