Aluminum-dependent dynamics of ion transport in Arabidopsis: specificity of low pH and aluminum responses

Low-pH and Al 3+ stresses are the major causes of poor plant growth in acidic soils. However, there is still a poor understanding of plant responses to low-pH and Al 3+ toxicity. Low-pH or combined low-pH and Al 3+ stress was imposed in order to measure rhizosphere pH, ion fluxes, plasma membrane potential and intracellular H + concentration in distal elongation and mature zones (MZs) along the longitudinal axis of Arabidopsis thaliana roots. Low-pH stress facilitated H + influx into root tissues and caused cytoplasmic acidification; by contrast, combined low-pH/Al 3+ treatment either decreased H + influx in the distal elongation zone (DEZ) or induced H + efflux in the MZ, leading to cytoplasmic alkalinization in both zones. Low-pH stress induced an increase in rhizosphere pH in the DEZ, whereas combined low-pH/Al 3+ stress resulted in lower rhizosphere pH in both root zones compared with the low-pH treatment alone. Low-pH stress facilitated K + efflux; the presence of Al 3+ diminished K + efflux or favored K + influx into root tissues. In both zones, low-pH treatment induced plasma membrane (PM) depolarization, which was significantly diminished (P ≤ 0.05) when combined stresses (low-pH/100 μM Al 3+) were imposed. After 60 min of exposure, low pH caused PM depolarization, whereas low pH/100 μM Al 3+ caused PM hyperpolarization. Thus, low pH and Al 3+ toxicity differentially affect root tissues and, consequently, the rhizosphere, which might underpin the differential mechanisms of plant adaptation to these abiotic stresses. Copyright © Physiologia Plantarum 2010.