Mechanisms of intracellular and short-distance ion transport in a plant

There are three types of ion transport for mineral nutrition elements: intracellular, short-distance, and long-distance.

Intracellular ion transport in the plant

Ions from the soil solution first enter the free space of the cell wall. Then, a portion of them is transported across the plasmalemma into the protoplast with the help of transport proteins. Transport proteins act over a very short distance – 80 nm.

The movement of substances within a single cell is carried out as a result of the circular motion of the cytoplasm and diffusion directed across this motion, which can achieve almost complete mixing of substances in the hyaloplasm.

The circulatory movement of the cytoplasm, i.e., cyclosis, occurs at a rate represented in the table below, involving contractile proteins of the actomyosin type (salt-soluble protein consisting of myosin and actin).

In higher plants, the movement of the cytoplasm occurs with the help of microtubules and microfilaments. The rate of cytoplasmic movement depends on the physiological state of the cell and, primarily, on the amount of ATP.

Substances that cause the destruction of microfilaments and the disassembly of microtubules also affect intracellular ion transport. The channels of the endoplasmic reticulum and vesicles of the Golgi apparatus also participate in the intracellular transport of substances.

Hyaloplasm is the matrix, a part of the cytoplasm of plant and animal cells, which contains all organelles and products of intracellular metabolism.

Mechanisms of short-distance substance transport

Roots transfer over 80% of absorbed ions to above-ground organs. This movement includes radial transport, xylem loading, and long-distance transport of substances through xylem vessels. Short-distance (radial) transport is the movement of ions from the root epidermal cells to the tracheal elements of the xylem (Fig. 47; Barber S.A., 1988). Short-distance transport is carried out through the cells of tissues not specialized for substance transport via the apoplast – the totality of intercellular spaces and interfibrillar cavities of cell walls; the symplast – the totality of protoplasts of cells connected by plasmodesmata; and the vacuome – the discrete system of cell vacuoles. The ratio of flows between these two compartments differs for various ions and depends on their concentration in the solution. At low availability of mineral nutrition elements, typical for the soil environment, the majority of ions absorbed in the root hair zone enter the symplast directly in the rhizodermis. At high concentrations in the medium, a significant portion of ions (up to 50% of those absorbed) moves through the apoplast (calcium and boron are translocated primarily via the apoplast). Symplastic translocation of ions from cell to cell occurs via plasmodesmata, and the efficiency of movement depends on the density of plasmodesmata penetrating the cell wall. The number of plasmodesmata connecting even the smallest cells reaches 20,000–30,000, and sometimes 100,000; the cross-sectional area of plasmodesmata accounts for 0.2–0.8% of the cell surface. The number of plasmodesmata per unit of cell surface area varies depending on the tissue type and the orientation of the wall. In the epidermis, the greatest number of plasmodesmata is found in the walls of trichoblasts. Moreover, there are more plasmodesmata on the tangential side than on the radial side. In hairless cells, the number of plasmodesmata in the walls decreases with distance from the trichoblast. Within the symplast, ions and their organic derivatives move from cell to cell via plasmodesmata without overcoming membranes at a speed of 1.5–2.0 cm/h. The structure of a plasmodesma is such that it not only connects the protoplasts of adjacent cells but also unites the endoplasmic reticulum of the cells into a single system via a desmotubule. Upon entering the protoplast, an ion can then enter the interior of the endoplasmic reticulum. Since each plasmodesma contains its channel, an ion that has entered it can be transported through the symplast from cell to cell without entering the cytosol. Desmotubules can expand, narrow, and close. In the latter case, transport through them ceases, and substances move through the plasmodesma only via the cytosol. Consequently, the transport of substances through plasmodesmata is regulated. Ion transport via the symplast is linked to metabolism, i.e., ion transport through the cell protoplasm to the xylem is an active process.

Fig. 47. Cross-section of root cells and tissues involved in ion absorption:

Arrows indicate the direction of ion movement through the selected row of cells:

1 – root hair; 2 – epidermis; 3 – cortex; 4 – endodermis; 5 – Casparian strips;

6 – pericycle; 7 – xylem; 8 – phloem

Vacuoles play an important role in the transport of substances via the symplast*. Ions do not pass through the vacuoles, but vacuoles and the root's conducting tissues compete with each other for the absorption of substances. The result of this competition is determined by the degree of saturation of plant tissues with salts. In plants whose vacuoles are saturated with ions, the majority of newly absorbed ions moves into the vessels, bypassing the vacuoles encountered. In plants grown on diluted nutrient solutions, a large part of the absorbed ions accumulates in the root cell vacuoles, which excludes them from direct transport to the vessels for a long time. The duration of an ion's residence in the cell sap is influenced by its concentration in the cell solution. If the external environment contains an optimal amount of an ion, the vacuole does not influence its transport via the symplast. If the external environment lacks the ion or it is absent entirely, it enters the cytosol from the vacuole and then into the symplastic pathway. If there is an excess of the ion in the external environment, its accumulation in the vacuole occurs. Consequently, vacuoles maintain the concentration of a given ion in the symplast at a constant level. The absorption of ions by vacuoles reduces their concentration in the symplast and ensures the creation of a concentration gradient necessary for their transport via the symplast. The entry of ions into vacuoles can occur both against the concentration gradient and against the electrochemical gradient, i.e., with the help of tonoplast transport proteins. Thus, from what has been said about the influence of vacuoles on substance transport, it can be concluded that the root is capable of performing a distributive function, directing deficient nutrition elements to the shoots and retaining excess or harmful ones in the vacuoles.

The movement of ions along the apoplast occurs due to diffusion and exchange adsorption along a concentration gradient and is accelerated by water flow. However, the diffusion of ions and molecules along the apoplast is interrupted at the level of the endodermis. Casparian strips serve as an insurmountable barrier to the movement of substances along the apoplast, as their membrane contains a band impregnated with suberin, which possesses hydrophobic properties. Thus, the only path for further movement of substances through the endodermis is transport via the symplast, which ensures metabolic control of uptake.

Vacuoles are cavities in the cell cytoplasm filled with cell sap containing waste substances, as well as the cell's reserves of nutrients.

substances. The existence in the endodermis of passage cells, in which Casparian strips are underdeveloped or absent, allows a small portion of nutrients to bypass metabolic control. The development of lateral roots above the root hair zone also creates areas where ions can move from the cortex to the stem along the apoplast, which is why a portion of ions and water can reach the xylem without entering the symplast.

The final destination of radial symplastic translocation is the xylem vessels, which constitute an important part of the stele* apoplast (Fig. 48; Danilova M.F., 1974). Ions enter the xylem from xylem parenchyma cells, and there is a special term — "xylem loading." The driving force of trans-root ion transport is the gradient of their electrochemical potential between the boundaries of the symplast: at its entrance (plasmalemma of rhizodermis cells) and at its exit (plasmalemma of xylem parenchyma cells). The entry of ions and molecules into xylem vessels can occur not only passively but also actively, with the help of transport proteins.

Fig. 48. Possible pathways and mechanisms of substance movement in the root. Dashed line – presumed continuous free space to the xylem vessels; simple arrow – diffusive movement of substances along the apoplast; double arrow – movement of substances along the symplast; dotted line – sites of substance metabolism in cells; double dashed line – passive leakage of the solution into the vessels;  – position of the pump ensuring active secretion of substances into the xylem conducting elements; O – membrane transport.

Stele – the axial cylinder of the conducting system of the stem and root in higher plants).