Heavy metal toxicity: cadmium permeates through calcium channels and disturbs the plant water status

Abstract

Summary: Because plant wilting has been described as a consequence of cadmium (Cd 2+) toxicity, we investigate Cd 2+ effects on plant water losses, gas exchanges and stomatal behaviour in Arabidopsis thaliana L. Effects of 1‐week Cd 2+ application in hydroponic condition (CdCl 2 10–100 µ m) were analyzed. A 10‐µ m Cd 2+ concentration had no significant effect on the plant–water relationship and carbon assimilation. At higher Cd 2+ concentrations, a Cd 2+ ‐dependent decrease in leaf conductance and CO 2 uptake was observed despite the photosynthetic apparatus appeared not to be affected as probed by fluorescence measurements. In epidermal strip bioassays, nanomolar Cd 2+ concentrations reduced stomatal opening under light in A. thaliana, Vicia faba and Commelina communis. Application of 5 µ m ABA limited the root‐to‐shoot translocation of cadmium. However, the Cd 2+‐induced stomatal closure was likely ABA‐independent, since a 5‐day treatment with 50 µ m Cd 2+ did not affect the plant relative water content. Additionally, a similar Cd 2+‐induced stomatal closure was observed in the ABA insensitive mutant abi1‐1. Interestingly, this mutant displayed a higher transpiration rate than the wild type but did not accumulate more Cd 2+, arguing that Cd 2+ uptake is not dependent only on the transpiration flow. Application of putative calcium channels inhibitors suppressed the inhibitory effect of Cd 2+ in epidermal strip experiments, suggesting that Cd 2+ could enter the guard cell through calcium channels. Patch‐clamp studies with V. faba guard cell protoplasts showed that plasma membrane K+ channels were insensitive to external Cd 2+ application whereas Ca 2+ channels were found permeable to Cd 2+. In conclusion, we propose that Cd 2+ affects guard cell regulation in an ABA‐independent manner by entering the cytosol via Ca 2+ channels.