In mammals, blood sugar levels are tightly controlled by two hormones – insulin lowers blood sugar levels, while glucagon raises them. In flowering plants, a comparable regulatory mechanism exists, mediated by the sugar-signalling molecule trehalose 6-phosphate (Tre6P). Tre6P plays a central role in sensing and responding to changes in sucrose availability, the primary transport sugar in plants. Much like insulin, Tre6P acts as a negative feedback regulator: its levels closely mirror those of sucrose, and it functions to prevent excessive accumulation by modulating carbon metabolism. Due to this pivotal role, Tre6P is often described as the "plant’s insulin". Expressing tagged forms of TREHALOSE-6-PHOSPHATE SYNTHASE1 (TPS1), the main Tre6P-synthesizing enzyme in Arabidopsis, revealed that Tre6P synthesis occurs predominantly in the vasculature, especially the phloem, and meristematic tissues of the whole plant. The vascular expression of TPS1 on both sides of the apoplastic barrier (in phloem parenchyma as well as in companion cells and sieve elements) ideally places Tre6P at the interface between source and sink and at a highly strategic site for systemic signalling of sucrose status. It follows that beyond its role in sugar homeostasis, Tre6P is a key regulator of plant development modulating key developmental transitions and plant architecture across the green lineage.