NONSELECTIVE BLOCK BY LA3-TRANSDUCTION( OF ARABIDOPSIS ION CHANNELS INVOLVED IN SIGNAL)

Lanthanide ions such as La3+ are frequently used as blockers to test t he involvement of calcium channels in plant and animal signal transduc tion pathways. For example, the large rise in cytoplasmic Ca2+ concent ration triggered by cold shock in Arabidopsis seedlings is effectively blocked by 10 mM La3+ and we show here that the simultaneous large me mbrane depolarization is similarly blocked. However, a pharmacological tool is only as useful as it is selective and the specificity of La3 for calcium channels was brought into question by our finding that it also blocked a blue light (BL)-induced depolarization that results fr om anion channel activation and believed not to involve calcium channe ls. This unexpected inhibitory effect of La3+ on the BL-induced depola rization is explained by our finding that 10 mM La3+ directly and comp letely blocked the BL-activated anion channel when applied to excised patches. We have investigated the ability of La3+ to block noncalcium channels in Arabidopsis. In addition to the BL-activated anion channel , 10 mM La3+ blocked a cation channel and a stretch-activated channel in patches of plasma membrane excised from hypocotyl cells. In root ce lls, 10 mM La3+ inhibited the activity of an outward-rectifying potass ium channel at the whole cell and single-channel level by 47% and 58%, respectively. We conclude that La3+ is a nonspecific blocker of multi ple ionic conductances in Arabidopsis and may disrupt signal transduct ion processes independently of any effect on Ca2+ channels.