Protecting cell walls from binding aluminum by organic acids contributes to Al resistance

Aluminum-induced secretion of organic acids from the root apex has been demonstrated to be one major Al resistance mechanism in plants. However, whether the organic acid concentration is high enough to detoxify Al in the growth medium is frequently questioned. The genotypes of Al-resistant wheat, Cassia tora L. and buckwheat secrete malate, citrate and oxalate respectively. Here we found that at a 35% inhibition of root elongation, the Al activities in the solution were 10, 20, and 50 μM with the corresponding malate, citrate, and oxalate exudation at the rates of 15, 20 and 21 nmol cm 12 h respectively for the above three plant species. When exogenous organic acids were added to ameliorate Al toxicity, 2 fold and 8 fold higher oxalate and malate concentrations were required to produce the equal effect by citrate. After the root apical cell walls were isolated and preincubated in 1 mM malate, oxalate or citrate solution overnight, the total amount of Al adsorbed to the cell walls was all decreased significantly to a similar level, implying that these organic acids own an equal ability to protect cell walls from binding Al. These findings suggest that protection of the cell walls from binding Al by organic acids may contribute significantly to Al resistance.