Aluminium-induced alteration of ion homeostasis in root tip vacuoles of two maize varieties differing in Al tolerance

Root elongation is a primary target of Al toxicity in plants. The objective of this study was to see whether Al-induced disturbance of ion homeostasis is related to the inhibition of root elongation. For this purpose, root growth rate, free cytoplasmic calcium (Ca2+) and vacuolar content of phosphate (Pi), potassium (K+), nitrate (NO3−) and malate, as well as malate and citrate exudation and nitrate reductase activity were analysed in tips of two Zea mays L. varieties differing in Al resistance. Aluminium treatment affected root growth and cytoplasmic Ca2+ in the Al sensitive variety Bakero, but not in the Al tolerant variety Sikuani. However, both varieties suffered Al-induced decrease of vacuolar K+, and phosphate concentrations. Vacuolar malate concentrations were more affected by Al in Bakero than in Sikuani. Vacuolar nitrate concentrations increased upon Al exposure in both varieties. Only in Sikuani rhizosphere, pH slightly increased upon Al exposure. Our data are consistent with the hypothesis that disturbance of Ca2+ homeostasis is an early event in the Al toxicity syndrome. However, Al-induced alterations of the root tip homeostasis of major ions seem unrelated to Al-induced inhibition of root elongation.

Research highlights
► Ion homeostasis in root tip cells of maize differing in Al tolerance was analysed.
► Al distinctively affected root growth and intracellular Ca in Al sensitive maize.
► Vacuolar K+ and Pi decreased in both Al sensitive and tolerant maize upon Al supply.
► Vacuolar malate was more reduced by Al in sensitive than in tolerant maize.
► Alteration of ion homeostasis by Al in tip cells is unrelated to root growth inhibition.