OSMOTIC-STRESS AND ABSCISIC-ACID REDUCE CYTOSOLIC CALCIUM ACTIVITIES IN ROOTS OF ARABIDOPSIS-THALIANA

Cytosolic Ca2+, ([Ca2+](i)), and elongation growth were measured in th e roots of Arabidopsis thaliana. Exposure of plant tissues to high NaC l and abscisic acid (ABA) concentrations results in a reduction in the rate of growth, but the mechanism by which growth is inhibited is not understood. Both NaCl and ABA treatments are known to influence [Ca2](i), and in this study we measured the effects of salinity and ABA on [Ca2+](i) in cells from the meristematic region of Arabidopsis roots. The Ca2+-sensitive dye Fura-2 and ratiometric techniques were used to measure [Ca2+](i) in cells of the root meristem region. Resting [Ca2](i) was found to be between 100 and 200 mu mol m(-3) in roots of untr eated plants. Resting [Ca2+](i) changed in response to changes in the [Ca2+] surrounding growing roots, An increase of external [Ca2+] incre ased [Ca2+](i); conversely, a decrease of external [Ca2+] decreased [C a2+](i). Exposure of roots to NaCl caused a rapid reduction of [Ca2+]( i), a response that was proportional to the external NaCl concentratio n. Thus, as the NaCl concentration was increased, [Ca2+](i) in root me ristematic cells decreased. Root elongation was also inhibited in prop ortion to the external NaCl concentration, with maximal inhibition occ urring at 120 mol m(-3) NaCl. The [Ca2+](i) of root meristem cells als o changed in response to ABA, and the magnitude of the effect of ABA w as dependent upon ABA concentration. Treatment with 0-2 mmol m(-3) ABA caused a momentary increase in [Ca2+](i) followed by a decrease after 15 min, but 10 mmol m(-3) ABA caused an immediate decline in [Ca2+](i ). There was a strong positive correlation between [Ca2+](i) and root elongation rates. Experiments with the ABA-deficient Arabidopsis mutan t aba-3 indicated that the reduction in [Ca2+](i) brought about by NaC l was unlikely to be mediated via changes in endogenous ABA. Experimen ts with solutes such as sorbitol, KCl and NaNO3 indicated that the eff ects of NaCl could be mimicked by other solutes and was not specific f or NaCl.