Medicago truncatula Iron-chaperone 1 (ICHAP1) is required for symbiotic nitrogen fixation

Hundreds of proteins in the cell require iron (Fe) or Fe-containing cofactors to function. However, how Fe ²⁺ or Fe ³⁺ are specifically allocated to each of these proteins in plant cells remains largely unknown. It has been proposed that Fe metalation could be driven by specific interactions with Fe-shuttling proteins known as Fe-chaperones. Here, we present the first family of plant Fe ²⁺ -chaperones (ICHAPs) with orthologues in dicots and monocots. The role of these proteins in Fe distribution to Fe-dependent metabolic processes has been illustrated using symbiotic nitrogen fixation in Medicago truncatula root nodules. ICHAP1 is a soluble Fe ²⁺ -binding protein that interacts with plasma membrane Fe ²⁺ transporter NRAMP1, but not with symbiosome Fe ²⁺ -transporters. ICHAP1 mutants present altered Fe distribution in cells and they cannot fix nitrogen. A second family member, ICHAP2 is required to target Fe ²⁺ to symbiosomes, as it accepts Fe ²⁺ from ICHAP1 and interacts with symbiosome Fe ²⁺ -importer VTL8, but not with NRAMP1. These results indicate a path for Fe ²⁺ allocation from the plasma membrane to the symbiosome through specific protein-protein interactions and Fe ²⁺ exchange from NRAMP1 to ICHAP1, to ICHAP2, and to VTL8.