Oxalate exudation into the root-tip water free space confers protection from aluminum toxicity and allows aluminum accumulation in the symplast in buckwheat (Fagopyrum esculentum).

A better understanding of aluminum (Al) uptake and transport is expected to contribute to unravel the apparent contradiction between Al exclusion and Al accumulation in buckwheat. *We studied the effect of Al supply on the root-tip Al and oxalate concentrations of the apoplastic water free space fluid (WFSF) and the symplast as affected by temperature, oxalate supply and the anion-channel blocker phenylglyoxal (PG). *Aluminum supply rapidly activated the release of oxalate to the WFSF to establish a 1 : 1 Al to oxalate ratio. In the symplast, the Al concentration was 100 times higher than in the external solution, and the Al to oxalate ratio was 1 : 2. Loading and unloading of Al, but not of oxalate, into and from the symplast were reduced at low temperature and are thus under metabolic control. Application of PG reduced the constitutive and the Al-enhanced WFSF oxalate concentrations and enhanced Al-induced root-growth inhibition. Unlike a 1 : 3 Al to oxalate ratio, a 1 : 1 ratio ameliorated only partly Al-induced root-growth inhibition without affecting root-tip Al contents or WFSF Al concentrations. *We present a hypothesis with an Al oxalate (Ox)(+) plasma-membrane transporter in the root cortex and a xylem-loading Al citrate (Cit)(n-) transporter in the xylem parenchyma cells as key elements of Al accumulation in buckwheat.