, Volume 205, Issue 3, pp 389-396

Probing the “malate hypothesis” of differential aluminum tolerance in wheat by using other rhizotoxic ions as proxies for Al

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An Al-stimulated efflux of malate from the root apex has been proposed as the primary mechanism whereby some wheat (Triticum aestivum L.) genotypes demonstrate marked resistance to the rhixotoxic metal Al. Appealing in its simplicity, the model has not been unequivocally validated, and suffers from some significant discrepancies between estimated, steady-state concentrations of malate at the root surface and concentrations that are necessary to explain the resistance of the superior genotypes. Using two other rhizotoxic ions that are also chelated by malate, Cu(II) and La(III), we specifically probed whether the quantities of malate released by tolerant genotypes could effectively detoxify Al. Experiments with exogenous additions of malate to solution showed that ≥200 μM malate is required to account for the difference between Scout 66 (Al-sensitive) and Atlas 66 (Al-tolerant) wheats, and that this level of malate can also partially alleviate the toxicities of Cu and La. When simultaneously exposed to a mildly rhizotoxic level of Al (25 μM) to induce malate efflux, Atlas exhibited a pronounced reduction in sensitivity to Cu. When, La was used as the proxy ion, however, no such Al-induced tolerance to La was observed, a result that refutes the significance of malate efflux to Al tolerance. Additional experiments using Al, Cu, and La in combination suggested that a trivalent ion can alleviate Cu toxicity directly (i.e. via competition for apoplastic binding), providing an alternative explanation for the ability of Al to detoxify Cu in Atlas. Using a weight-of-evidence approach, we argue that malate efflux plays at most a minor role in the differential Al tolerance of wheat, and that a more integrative, multifaceted model of tolerance is needed.

Received: 14 August 1997 / Accepted: 26 November 1997