Info

Na+

Plasma membrane

Apoplast

H+ (antiport)

Na+

Tonoplast

Vacuole

H+ (antiport)

Ca2+

Tonoplast

Vacuole

H+ (antiport)

CI" (anion)

Plasma membrane

Cytosol

H+ (symport)

CI" (anion)

Tonoplast

Vacuole

H+ (antiport)

export channels for K+ and anions which are regulated by the membrane potential (so-called gated channels) have been described, as well as a Ca2+-ATPase for Ca2+. The activity of these pumps and channels results in a membrane potential of 0.2 V at the plasmalemma (the cytosol-ic side is negatively charged) and a tonoplast potential of between 0 and +20 mV, with the cy-tosolic side again being negatively charged. When the volumes of the participating apoplast, cytosol and vacuole compartments are taken into account, the membrane potentials give rise to pH gradients, whereby the normal pH value for the cytosol is always 7 and that of the apoplast and the vacuole is 1-2 units more acidic. An overview of the transport systems at present known to participate in the ion homeostasis of higher plants is given in Table 1.6.3.

High salinity places this homeostasis under considerable stress, because particularly Na+, but also Ca2+, enter passively into the cell along the concentration gradient and form large pools there. The accumulation of positive charges in the cytosol breaks down the natural barrier of the membrane potential for Cl~ and, consequently, leads to a massive influx of this anion through the anion channels.

In particular the potassium relations of the cell are threatened by high sodium concentrations. On the one hand, K+ and Na+ compete for the not particularly selective, but very efficient,

K+ uptake systems. These are channels and K+/ Na+-symporters: low affinity cation transporters, or LCTs. At high apoplastic Na+ concentrations the potassium uptake by the cell is accordingly strongly reduced, and the cytosolic potassium pool shrinks (Fig, 1.6.2 B).

On the other hand, the flooding of the cytosol with Na+ results in increased activity of proton pumps, especially of the plasma membrane AT-Pases but also of the tonoplastic Na+/H+ antiport system, and thus in an increase in ATP consumption.

The situation becomes further aggravated in that increased proton transport also results in changes in the intracellular pH relations. External application of NaCl at seawater concentrations results in an alkalisation of the cytosol of barley roots of between 0.5 and 1 pH unit. This alkalisation detrimentally affects the activity of various cytosolic enzymes, particularly of those of catabolic energy metabolism (Katsuhara et al. 1997).

Uptake of Na+ into the vacuole, which effects salt removal from the cytosol, requires a Na+/H+ antiporter (sodium-H+ exchange) which has been found in barley roots and beet in addition to the above-mentioned H+-transporting system. This antiporter couples the increased proton charge of the vacuole with sodium uptake. The synthesis of this transport system can be induced by salinity stress (Fig. 1.6.2 C). The so dium and chloride concentrations in the vacuoles of salt-stressed tobacco cell cultures were eight-fold greater than those in the cytosol (Bin-zel et al. 1988). A consequence of this is a further alkalisation - now of the vacuole - due to the proton-sodium antiport.

Flooding of the cell with Na+ leads finally to an increased uptake of calcium by the cell and release of Ca2+ from cellular compartments, and thus to an increase in the size of the cytosolic Ca2+ pool. Since the cytosolic pool of free Ca2+ has signal function, the augmentation of this pool triggers regulatory processes in the cell, which can in the main be interpreted as constituting repair or adaptation reactions. The increase in the cytosolic Ca2+ concentration could take place due to water potential-induced calcium uptake similar to that occurring in the sto-mata (see also Chap. 1.5.2.4), or upon the opening of Ca2+ channels in the ER or tonoplast in connection with the inositol-triphosphate system.

0 0

Post a comment