Once in the cytoplasmic space of root cells Mg, along with the other cations, is probably transported radially into the stele and the vascular tissue. From the cells surrounding the xylem the ions are released or pumped into the xylem and carried up through the plant. In the case of Mg, which is highly mobile in both the xylem and phloem, the ions will be transported to the top of the plant and back down again in a continuous cycle of replenishment. Hence, uptake and release from vascular cells is probably a key part of whole plant Mg homeostasis. Figure 1 shows how few processes have been connected to their molecular mechanisms (only vacuolar uptake has been associated with a transport protein, AtMHX).
The diagram shows a schematic of a plant and the putative processes of Mg transport at the root and leaf where Mg is loaded and unloaded from the vascular tissues. Mg is taken up into the root cell wall space (1) and interacts with the negative charges associated with the cell walls and membranes. Mg may be taken up into cells immediately (symplastic pathway) or may travel as far as the Casparian band (4) before being absorbed into cells (apoplastic pathway; 2). The concentration of Mg in the root cells is probably buffered by storage in root cell vacuoles (3). Note that cells in the root tip do not contain vacuoles. Once in the root cell cytoplasm, Mg travels toward the centre of the root by plasmodesmata, where it is loaded into the xylem (5) for transport to the upper parts of the plant. When the Mg reaches the leaves it is unloaded from the xylem into cells (6) and again is buffered in vacuoles (7). Whether cycling of Mg into the phloem occurs via general cells in the leaf (8) or directly from xylem to phloem via transfer cells (9) is unknown. Mg may return to the roots in the phloem sap.
When a Mg ion has been absorbed by a cell requiring it for metabolic processes, it is generally assumed that the ion stays in that cell for as long as the cell is active. In vascular cells, this is not always the case; in times of plenty, Mg is stored in the vacuole, takes no part in the day-to-day metabolic processes of the cell (Stelzer et al., 1990), and is released at need. But for most cells it is death by senescence or injury that releases Mg and many of the other ionic constituents, recycling them into healthy parts of the plant. In addition, when Mg in the environment is limiting, some species are able to mobilise Mg from older tissues. These processes involve the release of Mg from its bound and stored states and its transport back into the vascular tissue, where it can be distributed to the rest of the plant. In times of growth and development, Mg is also remobilised within the plant as source and sink relationships change.
The homeostasis of Mg within single plant cells is maintained by processes occurring at the plasma membrane and at the vacuole membrane (see Figure 2). The major driving force for the translocation of ions in plant cells is ΔpH. H-ATPases pump H ions against their concentration gradient to maintain the pH differential that can be used for the transport of other ions and molecules. H ions are pumped out of the cytoplasm into the extracellular space or into the vacuole. The entry of Mg into cells may occur through one of two pathways, via channels using the ΔΨ (negative inside) across this membrane or by symport with H ions. To transport the Mg ion into the vacuole requires a Mg/H antiport transporter (such as AtMHX). The H-ATPases are dependent on Mg (bound to ATP) for activity, so that Mg is required to maintain its own homeostasis.
A schematic of a plant cell is shown including the four major compartments currently recognised as interacting with Mg. H-ATPases maintain a constant ΔpH across the plasma membrane and the vacuole membrane. Mg is transported into the vacuole using the energy of ΔpH (in A. thaliana by AtMHX). Transport of Mg into cells may use either the negative ΔΨ or the ΔpH. The transport of Mg into mitochondria probably uses ΔΨ as in the mitochondria of yeast, and it is likely that chloroplasts take Mg by a similar system. The mechanism and the molecular basis for the release of Mg from vacuoles and from the cell is not known. Likewise, the light-regulated Mg concentration changes in chloroplasts are not fully understood, but do require the transport of H ions across the thylakoid membrane.