Peptides derived from the auxin binding protein elevate Ca2+ and pH in stomatal guard cells of Vicia faba: a confocal fluorescence ratio imaging study.

Dual-excitation confocal laser scanning microscopy (CLSM) was used to image the pH-indicator, BCECF, iontophoretically microinjected into stomatal guard cells of Vicia faba during challenge with peptides derived from hydrophilic domains of the maize auxin-binding protein. Only the peptide corresponding to the C-terminal end (Pz151-163) caused significant changes in cytosolic pH, stimulating rapid alkalinisation of 0.4 +/- 0.1 pH units. Cytosolic pH was clamped using the permeant weak acid, butyrate, and this treatment buffered the peptide evoked alkalinisation. In concert with the electrical events monitored at the plasma membrane using whole-cell voltage clamp, this provides strong evidence for a role of [H+] as a signal intermediate in the guard cell transduction network. In preliminary experiments using single-wavelength imaging of the calcium-indicator, Fluo-3, Pz151-163 also stimulated rapid, reversible increases in cytosolic calcium, whilst two other peptides tested had no effect.