| Fig. 1.8.10. A Inhibition of root growth of a sensitive and tolerant bean variety grown for 3 days in an AI3+-containing nutrient solution. B Dependence of inhibition on the Al3+ concentration is the same in both cultivars. However, the tolerant variety recovers quickly from moderate stress (10 |.iM Al3+), whereas root growth of the sensitive variety remains inhibited. (After Cumming et al. 1992)
cultivar Jianxi accumulates up to 0.5 g Al/kg leaf dry weight) which sequesters Al3+ from the cy-tosol into the vacuole where it forms complexes with oxalate (Al3+/oxalate = 1:3; Table 1.8.4). In addition, Al-stressed, buckwheat roots excrete considerable amounts of oxalic acid, which complexes Al ions immediately in the rhizosphere. Exogenous Al oxalate (1:3 and 1:2) does not inhibit root growth, and thus is not toxic. Apparently, such complexes are so stable that under physiological conditions Al is not released.
In hydrangea the counter ion is citrate instead of oxalate. The corresponding complex contains aluminium and citrate at a ratio of 1:1 and accumulates in the vacuole. In contrast, in tea leaves, the bulk of Al ions seems to be bound to tannins.
The assumption that Al tolerance is inducible arises from experiments in which the initially inhibited root growth recovered completely after a few days (Fig. 1.8.10), indicating induction and achievement of Al tolerance.
Upon stress by Al, resistant wheat cultivars produced two additional proteins (51 kDa) in the plasmalemma fraction, which occurred only in the root tips and disappeared again when the stress was alleviated. Formation of these proteins was induced most effectively by Al3+, to a lesser extent also by Cd2+ and Ni2+, but not by Cu2+, Zn2+ or Mn2+. In the corresponding Al-sensitive cultivar only traces of these proteins were found under Al stress. One of these proteins could be a metallothionein. Another protein could be related to the production of "defence mucilage" by the outer root cap cells. Strong mucilage formation was observed upon incubation of root tips in solutions of Al salts. After excretion of the mucigel, root growth (of peas) recovered so fast that it was assumed that the mucilage had adsorbed the aluminium ions and thus detoxified them (Hawes et al. 2000).
These examples show that Al stress may induce a more or less specific protein response, while the physiological role of such proteins still remains to be explained.
1. Aluminium is the most frequent metallic element in the earth's crust, where it appears in many, usually insoluble compounds and complexes. Three classes of aluminium ions available to plants are distinguished: Mononuclear Al3+, polynuclear Al and complexed Al. At an acid pH (< 5) the mononuclear A1(H20)7+ occurs, at neutral pH and at higher Al concentrations polynuclear forms develop, in particular the soluble [Al13]3+. All soluble Al ions are phytotoxic.
2. Aluminium is quickly taken up by adsorption to the cell wall and in the mucigel of the root tip. From there it enters into the cytosol via cation carriers (probably through a Mg2+ channel). Endocytotic uptake from the apoplast and, in grasses, uptake by the siderophore system are also discussed. Al3+ is almost immobile in the plant; aluminium damage occurs, therefore, predominantly in the roots.
3. Inhibition of root elongation growth by Al3+ is particularly striking; upon long-term stress, cell division is also affected. Because of similar ionic radii, Al3+ particularly affects Mg2"1" (e.g. replacing Mg in the Mg-ATP complex) and Fe3+ metabolisms, and interferes with the regulation of the cytosolic level of free Ca2+. By these effects and by the inhibition of signal transduction via an increase in inositol-trisphosphate, aluminium causes major changes in the regulation of cellular reactions. In addition, Al3+ leads to a reorientation and fixation of the cytoskeleton, resulting in anomalous growth in thickness of root tips and the incidence of numerous lesions.
4. (Relative) aluminium resistance may be based on avoidance or tolerance. Exclusion of aluminium ions (stress avoidance) is usually achieved by excretion of chelating acids (e.g. malic acid) which form aluminium complexes outside the root or the protoplast, thus rendering aluminium ions unavailable for uptake systems. Excretion of such chelators appears to be species-specific and is a typical reaction to Al3+, but not to other trivalent cations. A pectin-rich cell wall, carrying numerous negatively charged carboxyl groups, may act as aluminium trap, normally displacing the cross-linking cations Mg2+ and Ca2+. Al3+ in the rhizosphere may also be immobilised or made unavailable for plants by alkalinisation (via proton uptake or reduced proton release).
5. Aluminium-tolerant species are able to accumulate Al3+ in high concentrations. The most important mechanisms for this are export into the apoplast or sequestration into the vacuole. At the sites of deposition the aluminium ions must be complexed (e.g. by Al oxalate or with tannins) or fixed to Al-binding proteins to become inactivated.
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