Cation channels are involved in brassinosteroid signalling in higher plants

• 24-Epicastasterone activated K+- and Ca2+-permeable conductances in wheat roots.
• 24-Epibrassinolide and 28-homobrassionolide did not activate ion conductances in wheat.
• Brassinosteroids induced elevation of cytosolic free Ca2+ in Arabidopsis roots.
• Cation channel blocker Gd3+ inhibited brassinosteroid-induced Ca2+ signalling.

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 μM) 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 μM 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.