ALUMINUM RESISTANCE IN TRITICUM-AESTIVUM ASSOCIATED WITH ENHANCED EXUDATION OF MALATE

Two aluminum (Al)-resistant (Atlas 66, Maringa) and two Al-sensitive ( Roblin, Katepwa) cultivars of Triticum aestivum (wheat) were grown und er aseptic conditions in the presence and absence of Al to evaluate th e potential role of organic anion exudates in conferring resistance to Al. Five organic anions, or-ketoglutarate, citrate, malate, succinate and fumarate, were commonly detected in the root exudates, but only m alate and succinate were consistently exuded in all cultivars under al l treatments. Under control conditions, malate was exuded in higher qu antities from roots of Al-resistant cultivars (Atlas 66 and Maringa), compared with the Al-sensitive cultivars. Exposure to 100 mu M Al incr eased exudation of malate from roots of Al-resistant cultivars by 100- 120%, while in the Al-sensitive cultivars, exudation of malate was red uced. A decrease in exudation of succinate was observed in Atlas 66 an d Maringa with 100 mu M 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 o f exogenous malate (250 mu M to 500 mu M) to nutrient media containing 100 mu M Al restored root elongation in Al-sensitive cultivars, Robli n and Katepwa, to control levels. To determine whether exudation of ma late from roots reflected de novo synthesis arising from activity of t he TCA cycle, plants were labeled with C-14-acetate. With the exceptio n of acetate itself, malate was the only organic anion in which C-14 w as detected. In Al-resistant cultivars, treatment with Al increased ex udation of C-14 into malate by 48 to 54% when expressed as a percent o f total label in root exudates. In Al-sensitive cultivars, incorporati on of C-14 into malate declined by 22 to 29% with exposure to Al. The unique pattern of C-14 labeling and enhanced exudation of malate in th e Al-resistant cultivars, Atlas 66 and Maringa, provides strong althou gh indirect evidence for a role of malate in Al-resistance.