Aluminum-induced citric acid secretion is not the sole mechanism of Al-resistance in maize

Abstract

Aluminum-induced citric acid (CA) root secretion is a widely accepted mechanism to explain Al-resistance in maize. Nonetheless, several aspects of this mechanism remain controversial. In this study, we used paclobutrazol (PBZ), a plant growth retardant, to gain new insights into the relationship between Δ5-sterol composition, membrane permeability, (PM) H+-ATPase activity and CA secretion in an Al-sensitive (UFVM-100) and Al-resistant (UFVM-200) maize genotypes challenged with Al. The Al-sensitive genotype displayed greater concentrations of Al in the root tips and greater inhibition of root elongation (RE), which was accompanied by greater electrolyte leakage and greater reduction in the Δ5-sterols content after Al treatment. CA secretion by roots increased in both genotypes after Al treatment but to a greater extent in the Al-resistant genotype. The (PM) H+-ATPase activity was down-regulated in the sensitive cultivar and up-regulated in its resistant counterpart upon Al treatment. A significant correlation between (PM) H+-ATPase activity and CA secretion was observed, but only in the Al-resistant genotype. Upon adding PBZ to the Al-treated plants, differences in the RE and Δ5-sterol composition between the maize genotypes were fully abolished, whereas genotypic differences in CA secretion and (PM) H+-ATPase activity were reduced but not completely eliminated. Taken together, this information suggests the existence of other processes or mechanisms operating in the Al resistance in these two maize genotypes.