A NEW SCHEME OF SYMBIOSIS - LIGAND-GATED AND VOLTAGE-GATED ANION CHANNELS IN PLANTS AND ANIMALS

Anion channels in the plasma membrane of both plant and animal cells participate in a number of important cellular functions such as volume regulation, trans-epithelial transport, stabilization of the membrane potential and excitability. Only very recently attention has turned to the presence of anion channels in higher plant cells. A dominant theme among recent discoveries is the role of Ca2+ in activating or modulating channel current involved in signal transduction. The major anion channel of stomatal guard cell protoplasts is a 32-40 pS channel which is highly selective for anions, in particular NO3-, Cl- and malate. These channels are characterized by a steep voltage dependence. Anion release is elicited upon depolarization and restricted to a narrow voltage span of - 100 mV to the reversal potential of anions. During prolonged activation the current slowly inactivates. A rise in cytoplasmic calcium in the presence of nucleotides evokes activation of the anion channels. Following activation they catalyse anion currents 10-20 times higher than in the inactivated state thereby shifting the resting potential of the guard cell from a K+-conducting to an anion-conducting state. Patch-clamp studies have also revealed that growth hormones directly affect voltage-dependent activity of the anion channel in a dose-dependent manner. Auxin binding resulted in a shift of the activation potential towards the resting potential. Auxin-dependent gating of the anion channel is side- and hormone-specific. Its action is also channel-specific as K+ channels coexisting in the same membrane patch were insensitive to this ligand. It remains to be established whether the anion channel possesses an auxin binding site or consists of a hetero-complex with a member of the emerging family of auxin binding proteins. Known inhibitors of anion channels from animal epithelial cells reversibly blocked the anion channel from the extracellular side. At the single-channel level, channel block is caused by a reduction of the long open transitions into flickering bursts. This plant anion channel as characterized by its steep voltage-dependence and interaction with intracellular and extracellular ligands shares interesting similarities with members of gene families of animal anion channels.