Four plasma membrane-localized MGR transporters mediate xylem Mg loading for root-to-shoot Mg translocation in Arabidopsis.

Magnesium (Mg), an essential structural component of chlorophyll, is absorbed from the soil by roots and transported to shoots to support photosynthesis in plants. However, the molecular mechanisms underlying root-to-shoot Mg translocation remain largely unknown. We describe here the identification of four plasma membrane (PM)-localized transporters, named Mg release transporters (MGRs), that are critical for root-to-shoot Mg transport in Arabidopsis. Functional complementation assays in a Mg-uptake-deficient bacterial strain confirmed that these MGRs conduct Mg transport. PM-localized MGRs (MGR4, MGR5, MGR6, and MGR7) were expressed primarily in root stellar cells and participated in the xylem loading step of the long-distance Mg transport process. In particular, MGR4 and MGR6 played a major role in shoot Mg homeostasis, as their loss-of-function mutants were hypersensitive to low Mg but tolerant to high Mg conditions. Reciprocal grafting analysis further demonstrated that MGR4 functions in the root to determine shoot Mg accumulation and physiological phenotypes caused by both low- and high-Mg stress. Taken together, our study has identified the long-sought transporters responsible for root-to-shoot Mg translocation in plants.