Functional features of TREHALOSE-6-PHOSPHATE SYNTHASE1 - an essential enzyme in Arabidopsis thaliana.

In Arabidopsis, TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1) catalyzes the synthesis of the sucrose-signaling metabolite trehalose 6-phosphate (Tre6P) and is essential for embryogenesis and normal post-embryonic growth and development. To understand its molecular functions, we transformed the embryo-lethal tps1-1 null mutant with various forms of TPS1 and with a heterologous TPS (OtsA) from Escherichia coli, under the control of the TPS1 promoter, and tested for complementation. TPS1 protein localized predominantly in the phloem-loading zone and guard cells in leaves, root vasculature and shoot apical meristem, implicating it in both local and systemic signaling of sucrose status. The protein is targeted mainly to the nucleus. Restoring Tre6P synthesis was both necessary and sufficient to rescue the tps1-1 mutant through embryogenesis. However, post-embryonic growth and the sucrose-Tre6P relationship were disrupted in some complementation lines. A point mutation (A119W) in the catalytic domain or truncating the C-terminal domain of TPS1 severely compromised growth. Despite having high Tre6P levels, these plants never flowered, possibly because Tre6P signaling was disrupted by two unidentified disaccharide-monophosphates that appeared in these plants. The non-catalytic domains of TPS1 ensure its targeting to the correct subcellular compartment and its catalytic fidelity, and are required for appropriate signaling of sucrose status by Tre6P.