Cation channels are involved in brassinosteroid signalling in higher plants

Brassinosteroids (BRs) are an important class of plant hormones with a multitude of functions. They have been intensively investigated for their biosynthesis, distribution and physiological functions. The aim of this study was to examine possible effects of {BRs} on the plant plasma membrane cation conductances and Ca2+ signalling. The wheat root protoplasts (tested by patch-clamping) and excised arabidopsis roots (analysed by Ca2+-aequorin chemiluminometry), were used. In the whole-cell plasma membrane patches, 24-epibrassinolide, 28-homobrassionolide or 24-epicastasterone (1 ?M) were applied exogenously. 24-Epicastasterone increased the activity of the K+ efflux conductance in 50 of tested protoplasts while 24-epibrassonolide and 28-homobrassionolide did not modify the plasma membrane currents. Addition of 24-epicastasterone at the cytosolic side (to the pipette solution) resulted in dramatic stimulation of a time-dependent K+ efflux current (in 30 of protoplasts) and an activation of Ca2+ influx currents (in 30 of protoplasts). Gadolinium ions, which are blockers of cation channels, inhibited the 24-epicastasterone-induced cation channel activities. In Arabidopsis thaliana plants constitutively expressing aequorin, exogenous 24-epibrassonolide, 28-homobrassionolide and 24-epicastasterone induced a transient elevation of the cytosolic free Ca2+, which was inhibited by Gd3+ and mediated by Ca2+ influx from the bathing solution. In Ca2+-aequorin tests, 10 ?M of exogenous {BRs} was the minimal concentration at which statistically significant changes of the cytosolic Ca2+ were observed. In conclusion, the obtained results suggest that the plasma membrane of root cells contains the brassinosteroid-activated cation-permeable channels, which can probably be involved in rapid regulation of the K+ homeostasis and Ca2+ signalling.