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

The anion channel in the guard-cell plasma membrane of Vicia faba, GCA C1, 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 a uxins induced a shift of the voltage dependence of the anion channel t o hyperpolarized potentials; the shift was attenuated during an increa se in the extracellular chloride concentration, indicating that chlori de shields the hormone-binding site. The auxin-induced shift was conce ntration-dependent, characterized by a Michaelis-Menten type of behavi our 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 m M Cl-. In the presence of malate, another gating modulator of GCAC1, a uxins were less effective, indicating that both ligands compete for co mmon sites. Inactive auxins with respect to stomatal opening or stimul ation of the plasma membrane H+-ATPase, such as 2-NAA, modulated the a ctivation 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.