Coumarins link iron deficiency to TOR inhibition in plants.

The target of rapamycin (TOR) is a conserved master regulator of growth1,2 that integrates nutrient availability with developmental programs. While TOR inhibition under nutrient limitation is generally attributed to upstream signaling pathways, endogenous small-molecule regulators of TOR have not been defined in eukaryotes. Here, we show that plants use coumarins to modulate TOR activity. In Arabidopsis thaliana, we discovered that iron deficiency-induced coumarin accumulation represses TOR-S6K signaling and restricts root growth. Structural modeling and kinase activity analyses support a mechanism consistent with competition between the coumarin esculetin and ATP for binding to the catalytic pocket. Together, these findings identify specialized metabolites as modulators of TOR signaling, linking nutrient-responsive metabolism to growth control.