Four plasma membrane-localized MGR transporters mediate xylem Mg 2+ loading for root-to-shoot Mg 2+ translocation in Arabidopsis.

Magnesium (Mg ²⁺ ), 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 ²⁺ translocation remain largely unknown. We describe here the identification of four plasma membrane (PM)-localized transporters, named Mg ²⁺ release transporters (MGRs), that are critical for root-to-shoot Mg transport in Arabidopsis. Functional complementation assays in a Mg ²⁺ -uptake-deficient bacterial strain confirmed that these MGRs conduct Mg ²⁺ 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 ²⁺ 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 ²⁺ but tolerant to high Mg ²⁺ conditions. Reciprocal grafting analysis further demonstrated that MGR4 functions in the root to determine shoot Mg ²⁺ accumulation and physiological phenotypes caused by both low- and high-Mg ²⁺ stress. Taken together, our study has identified the long-sought transporters responsible for root-to-shoot Mg ²⁺  translocation in plants.
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