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

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.

Conserved mechanism for vacuolar magnesium sequestration in yeast and plant cells.

Magnesium (Mg2+) is an essential nutrient for all life forms. In fungal and plant cells, the majority of Mg2+ is stored in the vacuole but mechanisms for Mg2+ transport into the vacuolar store are not fully understood. Here we demonstrate that members of ancient conserved domain proteins (ACDPs) from Saccharomyces cerevisiae and Arabidopsis thaliana function in vacuolar Mg2+ sequestration that enables plant and yeast cells to cope with high levels of external Mg2+. We show that the yeast genome (as well as other fungal genomes) harbour a single ACDP homologue, referred to as MAM3, that functions specifically in vacuolar Mg2+ accumulation and is essential for tolerance to high Mg. In parallel, vacuolar ACDP homologues were identified from Arabidopsis and shown to complement the yeast mutant mam3Δ. An Arabidopsis mutant lacking one of the vacuolar ACDP homologues displayed hypersensitivity to high-Mg conditions and accumulated less Mg in the vacuole compared with the wild type. Taken together, our results suggest that conserved transporters mediate vacuolar Mg2+ sequestration in fungal and plant cells to maintain cellular Mg2+ homeostasis in response to fluctuating Mg2+ levels in the environment.

The relationship between Cd-induced autophagy and lysosomal activation in WRL-68 cells.

This study shows that Cd induces autophagy in the human's embryonic normal liver cell line (WRL-68). The expression of LC3B-II and the mature cathepsin L were analyzed by Western blotting. The autophagosomes and lysosomes were directly visualized by electron microscopy and confocal microscopy analysis in Cd-exposed WRL-68 cells. In this study, we first found that autophagy induced the activation of lysosomal function in WRL-68 cells. The lysosomal activation was markedly decreased when the cells were co-treated with 3-MA (an inhibitor of autophagy). Secondly, we provided the evidence that the activation of lysosomal function depended on autophagosome-lysosome fusion. The colocalization of lysosome-associated membrane protein-2 (LAMP2) and GFP-LC3 was significantly reduced, when they were treated with thapsigargin (an inhibitor of autophagosome-lysosome fusion). We demonstrated that deletion or blockage of the autophagosome-lysosome fusion process effectively diminished lysosomal activation, which suggests that lysosomal activation occurring in the course of autophagy is dependent on autophagosome-lysosome fusion. Thirdly, we provided evidence that the activation of lysosomal function was associated with lysosomal acid. We investigated the relationship between autophagosome-lysosome fusion and pH in acidic compartments by visualizing fusion process in WRL-68 cells. This suggests that increasing pH in acidic compartments in WRL-68 cells inhibits the autophagosome-lysosome fusion. Finally, we found that the activation of lysosomal function was associated with Ca(2+) stores and the intracellular Ca(2+) channels or pumps were possibly pH-dependent.