Systemic Iron Signaling via OPT3 Influences Reductive Uptake and Coumarin Secretion

Iron (Fe) availability is frequently restricted in soils, particularly under alkaline conditions, resulting in Fe-deficiency chlorosis and reduced plant growth. In Strategy I plants, Fe acquisition relies on ferric chelate reductase (FCR) activity and the secretion of Fe-mobilizing compounds like coumarins; however, how these pathways are integrated through systemic Fe signaling remains poorly understood. In this study, coordination between reductive Fe uptake, coumarin biosynthesis, external pH, and systemic signaling was investigated in Arabidopsis thaliana. Physiological readouts, including chlorophyll content, root FCR activity, and coumarin fluorescence, were compared among wild-type Col-0, the systemic signaling mutant opt3-2, and the coumarin-deficient mutant f6h1-1 under Fe-sufficient, alkaline, and Fe-free conditions. Under alkaline Fe limitation, opt3-2 sustained higher chlorophyll levels, persistent FCR activity, and strongly enhanced coumarin secretion, whereas f6h1-1 exhibited severe chlorosis accompanied by compensatory FCR induction. Time-resolved analyses showed that external pH restricts FCR responsiveness, while loss of OPT3-mediated systemic signaling profoundly alters the timing and amplitude of coumarin deployment. Co-cultivation assay further demonstrated that coumarins released by opt3-2 partially alleviate Fe-deficiency symptoms in f6h1-1. Together, these results identify systemic signaling as a key integrator of reductive Fe uptake and coumarin-mediated Fe mobilization, establishing a unified framework for Fe homeostasis.