Magnesium transporters, MGT2/MRS2-1 and MGT3/MRS2-5, are important for magnesium partitioning within Arabidopsis thaliana mesophyll vacuoles.

• Magnesium accumulates at high concentrations in dicotyledonous leaves but it is not known in which leaf cell types it accumulates, by what mechanism this occurs and the role it plays when stored in the vacuoles of these cell types. • Cell-specific vacuolar elemental profiles from Arabidopsis thaliana (Arabidopsis) leaves were analysed by X-ray microanalysis under standard and serpentine hydroponic growth conditions and correlated with the cell-specific complement of magnesium transporters identified through microarray analysis and quantitative polymerase chain reaction (qPCR). • Mesophyll cells accumulate the highest vacuolar concentration of magnesium in Arabidopsis leaves and are enriched for members of the MGT/MRS2 family of magnesium transporters. Specifically, AtMGT2/AtMRS2-1 and AtMGT3/AtMRS2-5 were shown to be targeted to the tonoplast and corresponding T-DNA insertion lines had perturbed mesophyll-specific vacuolar magnesium accumulation under serpentine conditions. Furthermore, transcript abundance of these genes was correlated with the accumulation of magnesium under serpentine conditions, in a low calcium-accumulating mutant and across 23 Arabidopsis ecotypes varying in their leaf magnesium concentrations. • We implicate magnesium as a key osmoticum required to maintain growth in low calcium concentrations in Arabidopsis. Furthermore, two tonoplast-targeted members of the MGT/MRS2 family are shown to contribute to this mechanism under serpentine conditions.