OPT3 plays a central role in both copper and iron homeostasis and signaling due to its ability to deliver copper as well as iron to the phloem in Arabidopsis

Copper and iron are micronutrients but are toxic when they accumulate in cells in excess. Crosstalk between copper and iron homeostasis in Arabidopsis thaliana has been documented and includes iron accumulation under copper deficiency and vice versa . However, molecular components of this crosstalk are not well understood. Iron concentration in the phloem has been suggested to act systemically, negatively regulating iron uptake to the root. Consistently, systemic iron signaling is disrupted in A. thaliana mutants lacking the phloem companion cell-localized iron transporter, AtOPT3, and opt3 mutants hyperaccumulate iron. Here, we report that in addition to iron, AtOPT3 transports copper and mediates copper loading to the phloem for delivery from sources to sinks. As a result of this function, the opt3-3 mutant accumulates less copper in the phloem, roots, developing leaves and embryos compared to wild type, is sensitive to copper deficiency, and mounts transcriptional copper deficiency response. Because copper deficiency has been shown to stimulate iron accumulation, we propose that reduced copper concentration in the phloem of the opt3-3 mutant and its constitutive copper deficiency contribute to iron overaccumulation in its tissues. Our data assign new transport capabilities to AtOPT3 and increase understanding of copper - iron interactions and signaling.