Organic acid secretion as a mechanism of aluminium resistance: a model incorporating the root cortex, epidermis, and the external unstirred layer

The resistance of some plants to Al (aluminium or aluminum) has been attributed to the secretion of Al³⁺-binding organic acid (OA) anions from the Al-sensitive root tips. Evidence for the 'OA secretion hypothesis' of resistance is substantial, but the mode of action remains unknown because the OA secretion appears to be too small to reduce adequately the activity of Al³⁺ at the root surface. In this study a mechanism for the reduction of Al³⁺ at the root surface and just beneath the epidermis by complexation with secreted OA²⁻ is considered. According to our computations, Al³⁺ activity is insufficiently reduced at the surface of the root tips to account for the Al resistance of Triticum aestivum L. cv. Atlas 66, a malate-secreting wheat. Experimental treatments to decrease the thickness of the unstirred layer (increased aeration and removal of root-tip mucilage) failed to enhance sensitivity to Al³⁺. On the basis of additional modelling, the observed spatial distribution of Al in roots, and the anatomical responses to Al, it is proposed that the epidermis is an essential component of the diffusion pathway for both OA and Al. We suggest that Al³⁺ in the cortex must be reduced to small concentrations in order substantially to alleviate the inhibition of root elongation and so that the outer surface of the epidermis can tolerate relatively large concentrations of Al³⁺. If OA secretion is required for reducing Al³⁺ mainly beneath the root surface, rather than in the rhizosphere, then the metabolic cost to plants will be greatly reduced.