ANIONS MODIFY THE RESPONSE OF GUARD-CELL ANION CHANNELS TO AUXIN

The anion channel in the guard-cell plasma membrane of Vicia faba, GCAC1, possesses recognition sites for the plant growth hormone auxin at the extracellular mouth of the channel (Marten et al. 1991, Nature 353:759-762). Using the patch-clamp technique we could demonstrate that auxins induced a shift of the voltage dependence of the anion channel to hyperpolarized potentials; the shift was attenuated during an increase in the extracellular chloride concentration, indicating that chloride shields the hormone-binding site. The auxin-induced shift was concentration-dependent, characterized by a Michaelis-Menten type of behaviour with a half-saturation constant (K-m) of about 10 mu M naphthalene-1-acetic acid (1-NAA) in the presence of 2 mM Cl- and 12 mu M in 80 mM Cl-. In the presence of malate, another gating modulator of GCAC1, auxins were less effective, indicating that both ligands compete for common sites. Inactive auxins with respect to stomatal opening or stimulation of the plasma membrane H+-ATPase, such as 2-NAA, modulated the activation threshold and kinetics of GCAC1 similar to the active form 1-NAA. At a concentration of 100 mu M 2-NAA the peak-current potential shifted by about 30 mV more negative.