The Arabidopsis <scp>NPF7</scp> .2 mediates coumarin uptake for root iron acquisition

Iron (Fe) is a transition metal necessary for achieving essential physiological processes throughout the plant lifecycle. In Arabidopsis thaliana, secreting Fe-mobilizing coumarins (FMC) is a key mechanism enabling roots to acquire nonbioavailable Fe present in soils. Here, we unveil the pivotal role played by NRT1/PTR FAMILY 7.2 (NPF7.2) in coumarin secretion by conducting phenotypic analyses of NPF7.2 loss-of-function mutants, NPF7.2 expression-localization analyses and direct transport assay of NPF7.2 protein with coumarin compounds using yeast cells. NPF7.2 loss-of-function impairs coumarin secretion and Fe acquisition in Arabidopsis seedlings, and the corresponding mutants are hypersensitive to Fe deficiency. NPF7.2 protein colocalizes with the main coumarin exporter, PDR9 (PLEIOTROPIC DRUG RESISTANCE PROTEIN 9), in both cortex and epidermis cell layers in root areas where coumarin secretion occurs. A site-specific accumulation of fraxin, a storage form of fraxetin, in root peripheral tissues is disturbed in the mutants under Fe deficiency. When expressed in yeast cells, NPF7.2 protein has an uptake activity for fraxetin and scopoletin, a major FMC and its precursor, respectively. However, fraxin and scopolin, their storage form, are not transported. We propose that NPF7.2 facilitates PDR9-mediated FMC secretion from the epidermis into the rhizosphere by the cellular loading of coumarins from apoplastic spaces, leading to optimal Fe acquisition.