Aluminium induces an increase in cytoplasmic calcium in intact wheat root apical cells

Disturbance of cytoplasmic Ca2+ homeostasis, leading to breakdown in Ca2+-mediated signal transduction processes, has been suggested as a primary mechanism of aluminium (Al) rhizotoxicity in plants. To test this hypothesis, Al-related changes in cytoplasmic free activity of Ca2+ ions ([Ca2+](c)) in root apical cells of near-isogenic wheat (Triticum aestivum L.) lines differing in Al tolerance at a single locus were examined by visualising the Ca2+-sensitive probe Fluo-3 with confocal laser scanning microscopy. Exposure of roots to 50 mu M AlCl3 (pH 4.2) led to an increase in [Ca2+](c) of root apical cells in both Al-sensitive (ES8) and Al-tolerant (ET8) wheat lines. An increase in [Ca2+](c) was greater in ES8 than in ET8; after 1-h treatment with 50 mu M AlCl3 an increase in [Ca2+](c) was 48 and 27% in ET8 and ES8, respectively. An increase in [Ca2+](c) of ES8 roots, but not ET8 roots, was observed upon treatment with 2.6 mu M AlCl3 (pH 4.5). Al-related increases in [Ca2+](c) were correlated with inhibition of root growth. The Al-induced increase in the [Ca2+](c) was reversible upon removing AlCl3. These findings provide direct evidence to support the hypothesis that Al interactions with cytoplasmic Ca2+ are involved in the Al toxicity syndrome in plants.