Aluminum-induced high IAA concentration may explain the Al susceptibility in Citrus limonia

Abstract

In acidic soils (pH < 5.0), aluminum (Al) occurs as Al3+, which is phytotoxic and reduces the root growth by hormonal imbalance and/or cell wall rigidity. However, the explanations for the decrease in root growth are not clear. A 60-day study was held with ‘Rangpur’ lime (Citrus limonia) plants grown in nutrient solution containing 0 and 1480 µM Al. We measured plant biometric data and used root apices to analyze auxin (IAA), the expression of some Al-responsive genes that had been differentially expressed in a transcriptome analysis and anatomical profiles. We found up-regulated expression of multidrug and toxic compound exudation (Cl-MATE channel), citrate synthase (Cl-CS) and pectin methylesterase inhibitor (Cl-PMEI) genes, but while Cl-PMEI expression was increased after 7 days, Cl-CS and Cl-MATE were up-regulated only after 60 days, suggesting that the timing of these events was ineffective against Al. These results could be associated with the Al-induced decrease in root growth and anatomical root damage. In addition, genes related to IAA cell transport were not differentially expressed in the transcriptome analysis. Therefore, high IAA and up-regulation of auxin-related small RNAs suggest Al-induced high IAA concentration rather than Al-induced disruption in IAA distribution in root cells of this species.