Ca²⁺-Dependent and -Independent Abscisic Acid Activation of Plasma Membrane Anion Channels in Guard Cells of Nicotiana tabacum

Drought induces stomatal closure, a response that is associated with the activation of plasma membrane anion channels in guard cells, by the phytohormone abscisic acid (ABA). In several species, this response is associated with changes in the cytoplasmic free Ca²⁺ concentration. In Vicia faba, however, guard cell anion channels activate in a Ca²⁺-independent manner. Because of potential differences between species, Nicotiana tabacum guard cells were studied in intact plants, with simultaneous recordings of the plasma membrane conductance and the cytoplasmic free Ca²⁺ concentration. ABA triggered transient rises in cytoplasmic Ca²⁺ in the majority of the guard cells (14 out of 19). In seven out of 14 guard cells, the change in cytoplasmic free Ca²⁺ closely matched the activation of anion channels, while the Ca²⁺ rise was delayed in seven other cells. In the remaining five cells, ABA stimulated anion channels without a change in the cytoplasmic Ca²⁺ level. Even though ABA could activate anion channels in N. tabacum guard cells independent of a rise in the cytoplasmic Ca²⁺ concentration, patch clamp experiments showed that anion channels in these cells are stimulated by elevated Ca²⁺ in an ATP-dependent manner. Guard cells thus seem to have evolved both Ca²⁺-independent and -dependent ABA signaling pathways. Guard cells of N. tabacum apparently utilize both pathways, while ABA signaling in V. faba seems to be restricted to the Ca²⁺-independent pathway.