Magnesium transport and function in plants: the tip of the iceberg

The maintenance of Mg²⁺ homeostasis in the plant is essential for viability. This review describes Mg²⁺ functions and balancing in plants, with special focus on the existing knowledge of the involved transport mechanisms. Mg²⁺ is essential for the function of many cellular enzymes and for the aggregation of ribosomes. Mg²⁺ concentrations also modulate ionic currents across the chloroplast and the vacuolar membranes, and might thus regulate ion balance in the cell and stomatal opening. The significance of Mg²⁺ homeostasis has been particularly established with regard to Mg²⁺'s role in photosynthesis. Mg²⁺ is the central atom of the chlorophyll molecule, and fluctuations in its levels in the chloroplast regulate the activity of key photosynthetic enzymes. Relatively little is known of the proteins mediating Mg²⁺ uptake and transport in plants. The plant vacuole seem to play a key role in Mg²⁺ homeostasis in plant cells. Physiological and molecular evidence indicate that Mg²⁺ entry to the vacuole is mediated by Mg²⁺/H⁺ exchangers. The Arabidopsis vacuolar Mg²⁺/H⁺ exchanger, AtMHX, is highly transcribed at the vascular tissue, apparently most abundantly at the xylem parenchyma. Inclusion of Mg²⁺ ions into the vacuoles of this tissue may determine their partitioning between the various plant organs. Impacts of Mg²⁺ imbalance are described with respect for both plant physiology and for its nutritional value to animal and human.