<b>Cytoplasmic polyamines block the fast‐activating vacuolar cation channel</b>

Abstract

Summary The fast‐activating vacuolar (FV) channel dominates the electrical characteristics of the tonoplast at physiological free Ca 2+ concentrations. Since polyamines are known to increase in plant cells in response to stress, the regulation of FV channels by polyamines was investigated. Patch‐clamp measurements were performed on whole barley ( Hordeum vulgare ) mesophyll vacuoles and on excised tonoplast patches. The trivalent polyamine spermidine and the tetravalent polyamine spermine blocked FV channels with K d ≈ 100 μM and K d ≈ 5 μM, respectively. Increasing cytosolic and vacuolar Ca 2+ had no effect on putrescine and spermidine binding to FV channels but slightly decreased the affinity for spermine. The inhibition of FV channels by all three polyamines was not voltage‐dependent. This points to a different mode of binding compared to inward rectifier K + channels and Ca 2+ ‐permeable glutamate receptor channels from animal cells, which show rectification due to a voltage‐dependent block by polyamines. In plant cells, the common polyamines (putrescine, spermidine and spermine) are likely to mediate a salt stress‐induced decrease of ion flux across the vacuolar membrane by blocking FV channels.