Aluminum Resistance in Triticum aestivum Associated with Enhanced Exudation of Malate

Abstract

Two aluminum (Al)-resistant (Atlas 66, Maringa) and two Al-sensitive (Roblin, Katepwa) cultivars of Triticum aestivum (wheat) were grown under aseptic conditions in the presence and absence of Al to evaluate the potential role of organic anion exudates in conferring resistance to Al. Five organic anions, α-ketoglutarate, citrate, malate, succinate and fumarate, were commonly detected in the root exudates, but only malate and succinate were consistently exuded in all cultivars under all treatments. Under control conditions, malate was exuded in higher quantities from roots of Al-resistant cultivars (Atlas 66 and Maringa), compared with the Al-sensitive cultivars. Exposure to 100 μM Al increased exudation of malate from roots of Al-resistant cultivars by 100–120 %, while in the Al-sensitive cultivars, exudation of malate was reduced. A decrease in exudation of succinate was observed in Atlas 66 and Maringa with 100 μM Al, while no effect was observed in Roblin and Katepwa. Differences between cultivars in the effect of Al on malate accumulation were detected as early as 24 h after exposure. Addition of exogenous malate (250 μM to 500 μM) to nutrient media containing 100 μM Al restored root elongation in Al-sensitive cultivars, Roblin and Katepwa, to control levels. To determine whether exudation of malate from roots reflected de novo synthesis arising from activity of the TCA cycle, plants were labeled with 14C-acetate. With the exception of acetate itself, malate was the only organic anion in which 14C was detected. In Al-resistant cultivars, treatment with Al increased exudation of 14C into malate by 48 to 54 % when expressed as a percent of total label in root exudates. In Al-sensitive cultivars, incorporation of 14C into malate declined by 22 to 29 % with exposure to Al. The unique pattern of 14C labeling and enhanced exudation of malate in the Al-resistant cultivars, Atlas 66 and Marïnga, provides strong although indirect evidence for a role of malate in Al-resistance.