Four plasma membrane-localized MGR transporters mediate xylem Mg2+ loading for root-to-shoot Mg2+ translocation in Arabidopsis

Abstract

Magnesium (Mg²⁺), 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 Mg²⁺ translocation remain largely unknown. We describe here the identification of four plasma membrane (PM)-localized transporters, named Mg²⁺ release transporters (MGRs), that are critical for root-to-shoot Mg transport in Arabidopsis. Functional complementation assays in a Mg²⁺-uptake-deficient bacterial strain confirmed that these MGRs conduct Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ but tolerant to high Mg²⁺ conditions. Reciprocal grafting analysis further demonstrated that MGR4 functions in the root to determine shoot Mg²⁺ accumulation and physiological phenotypes caused by both low- and high-Mg²⁺ stress. Taken together, our study has identified the long-sought transporters responsible for root-to-shoot Mg²⁺ translocation in plants.

Keywords: functional conservation; long-distance transport; magnesium; metal nutrient; phenotyping.