Magnesium (Mg) was recognized as an essential nutrient by 1925, and is essential for conformational stabilization of macromolecules such as nucleic acids, proteins, cell membranes, and walls; maintenance of enzyme activities such as of H+-ATPase, kinases and polymerases; and homeostasis of reactive oxygen species (ROS) under Al stress. Mg also serves as a regulator of cation–anion balance in cells and as an osmotically active ion regulating cell turgor together with K. Mg is particularly important to plants, with some 75% of leaf Mg involved in protein synthesis and 15–20% of total Mg associated with chlorophyll pigments, acting mainly as a cofactor of a series of enzymes involved in photosynthetic carbon fixation and metabolism.
Recent studies have shown that Mg contents in historical cereal seeds have markedly declined over time, and two thirds of people surveyed in developed countries received less than their minimum daily Mg requirement. People in many developed countries were commonly deficient in Mg, but this deficiency was not serious in developing or poor countries, indicating that loss of Mg by refining of food poses severe problems for human Mg uptake.
Although magnesium (Mg) is one of the most important nutrients, involved in many enzyme activities and the structural stabilization of tissues, its importance as amacronutrient ion has been overlooked in recent decades by botanists and agriculturists, who did not regard Mg deficiency (MGD) in plants as a severe health problem.
The declines in Mg, Zn, Fe, and I may also have some correlation with long-term unbalanced crop fertilization with nitrogen, phosphorus, and potassium (NPK) over the last decades. Grass tetany or paresis (milk fever) is a serious disorder in grazing animals, resulting from Mg decreases in grasses due to heavy application of potassium to soil; K+ is an antagonist for Mg2+ absorption in plants. These results suggest that more attention should be paid to crop MGD and to the problems left to us by the Green Revolution.
Thus, the mechanisms of response to MGD and ways to increase Mg contents in plants are two urgent practical problems. In current review it is discussed several aspects of MGD in plants, including phenotypic and physiological changes, cell Mg2+ homeostasis control by Mg2+ transporters, MGD signaling, interactions between Mg2+ and other ions, and roles of Mg2+ in plant secondary metabolism.
Both genotypic analysis of B. Oleracea and transcriptomic analysis of Arabidopsis suggest that complex systems in plants and other unknown transporters play key roles in MGD response. Interactions between Mg2+ and other ions, including toxic and nutrient ions, also appear complex under MGD conditions such as Mg deprivation and competitive Mg2+ deficiency.