Magnesium (Mg) is an essential metal for chlorophyll biosynthesis and other metabolic processes in plant cells. Mg is largely stored in the vacuole of various cell types and remobilized to meet cytoplasmic demand. However, the transport proteins responsible for mobilizing vacuolar Mg2+ remain unknown. Here, we identified two Arabidopsis (Arabidopsis thaliana) Mg2+ transporters (MAGNESIUM TRANSPORTER 1 and 2; MGT1 and MGT2) that facilitate Mg2+ mobilization from the vacuole, especially when external Mg supply is limited. In addition to a high degree of sequence similarity, MGT1 and MGT2 exhibited overlapping expression patterns in Arabidopsis tissues, implying functional redundancy. Indeed, the mgt1 mgt2 double mutant, but not mgt1 and mgt2 single mutants, showed exaggerated growth defects as compared to the wild type under low-Mg conditions, in accord with higher expression levels of Mg-starvation gene markers in the double mutant. However, overall Mg level was also higher in mgt1 mgt2, suggesting a defect in Mg2+ remobilization in response to Mg deficiency. Consistently, MGT1 and MGT2 localized to the tonoplast and rescued the yeast (Saccharomyces cerevisiae) mnr2Δ (manganese resistance 2) mutant strain lacking the vacuolar Mg2+ efflux transporter. In addition, disruption of MGT1 and MGT2 suppressed high-Mg sensitivity of calcineurin B-like 2 and 3 (cbl2 cbl3), a mutant defective in vacuolar Mg2+ sequestration, suggesting that vacuolar Mg2+ influx and efflux processes are antagonistic in a physiological context. We further crossed mgt1 mgt2 with mgt6, which lacks a plasma membrane MGT member involved in Mg2+ uptake, and found that the triple mutant was more sensitive to low-Mg conditions than either mgt1 mgt2 or mgt6. Hence, Mg2+ uptake (via MGT6) and vacuolar remobilization (through MGT1 and MGT2) work synergistically to achieve Mg2+ homeostasis in plants, especially under low-Mg supply in the environment.

中文翻译：

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两个转运蛋白从液泡储存中调动镁，使植物适应镁缺乏。

镁 (Mg) 是植物细胞叶绿素生物合成和其他代谢过程的必需金属。镁主要储存在各种细胞类型的液泡中并重新动员以满足细胞质的需求。然而，负责动员液泡 Mg2+ 的转运蛋白仍然未知。在这里，我们鉴定了两种拟南芥 (Arabidopsis thaliana) Mg2+ 转运蛋白（镁转运蛋白 1 和 2；MGT1 和 MGT2），它们促进液泡中 Mg2+ 的动员，特别是当外部镁供应有限时。除了高度的序列相似性之外，MGT1 和 MGT2 在拟南芥组织中表现出重叠的表达模式，这意味着功能冗余。事实上，在低镁条件下，与野生型相比，mgt1 mgt2 双突变体（而非 mgt1 和 mgt2 单突变体）表现出夸大的生长缺陷，与双突变体中镁饥饿基因标记的较高表达水平一致。然而，mgt1 mgt2 中的总体镁水平也较高，表明响应镁缺乏的 Mg2+ 再动员存在缺陷。一致地，MGT1和MGT2定位于液泡膜并拯救了缺乏液泡Mg2+外排转运蛋白的酵母（酿酒酵母）mnr2Δ（锰抗性2）突变株。此外，MGT1 和 MGT2 的破坏抑制了钙调神经磷酸酶 B 样 2 和 3 (cbl2 cbl3) 的高镁敏感性，这是一种液泡 Mg2+ 隔离缺陷的突变体，表明液泡 Mg2+ 流入和流出过程在生理背景下是拮抗的。我们进一步将 mgt1 mgt2 与 mgt6 杂交，mgt6 缺乏参与 Mg2+ 吸收的质膜 MGT 成员，并发现三重突变体比 mgt1 mgt2 或 mgt6 对低镁条件更敏感。因此，Mg2+ 吸收（通过 MGT6）和液泡再动员（通过 MGT1 和 MGT2）协同作用，以实现植物中的 Mg2+ 稳态，特别是在环境中镁供应较低的情况下。