Brassinosteroids promote sugar synthesis by inhibiting BIN2 phosphorylation of phosphoenolpyruvate carboxykinase.

Sugar is both an essential energy source and the major substrate for cell wall biosynthesis during plant growth, yet how growth-promoting hormones regulate sugar synthesis remains unclear. Here we show that the brassinosteroids (BRs) promote gluconeogenic and photosynthetic sugar synthesis by activating phosphoenolpyruvate carboxykinase (PCK), which catalyses the conversion of oxaloacetate to phosphoenolpyruvate, a central step in primary metabolism. Arabidopsis BR-deficient mutants display reduced PCK1 activity and elevated phosphorylation at conserved Ser-62 and Thr-66 residues. BR treatment induces PCK1 dephosphorylation and activation, whereas the GSK3-like kinase BIN2 phosphorylates these sites, altering quaternary structure and inhibiting PCK1. Phospho-blocking mutations of Ser-62/Thr-66 confer BR-independent PCK1 activity and enhance seedling growth, while phosphomimetic mutations reduce PCK1 activity and impair seedling growth and establishment. BRs also promote PCK dephosphorylation and activation in photosynthetic leaves of maize and sorghum. Our study demonstrates that BRs regulate primary metabolism via GSK3/BIN2-mediated phosphorylation of PCK, thereby promoting gluconeogenesis and photosynthesis.