Engineered silver nanoparticles are sensed at the plasma membrane and dramatically modify the physiology of Arabidopsis thaliana plants.

Silver nanoparticles (Ag NPs) are the world's most important nanomaterial and nanotoxicant. The aim of this study was to determine the early stages of interactions between Ag NPs and plant cells, and to investigate their physiological roles. We have shown that the addition of Ag NPs to cultivation medium, at levels above 300 mg L(-1) , inhibited Arabidopsis thaliana root elongation and leaf expansion. This also resulted in decreased photosynthetic efficiency and the extreme accumulation of Ag in tissues. Acute application of Ag NPs induced a transient elevation of [Ca(2+) ]cyt and the accumulation of reactive oxygen species (ROS; partially generated by NADPH oxidase). Whole-cell patch-clamp measurements on root cell protoplasts demonstrated that Ag NPs slightly inhibited plasma membrane K(+) efflux and Ca(2+) influx currents, or caused membrane breakdown; however, in excised outside-out patches, Ag NPs activated Gd(3+) -sensitive Ca(2+) influx channels with unitary conductance of approximately 56 pS. Bulk particles did not modify the plasma membrane currents. Tests with electron paramagnetic resonance spectroscopy showed that Ag NPs were not able to catalyse hydroxyl radical generation, but that they directly oxidized the major plant antioxidant, l-ascorbic acid. Overall, the data presented shed light on mechanisms of the impact of nanosilver on plant cells, and show that these include the induction of classical stress signalling reactions (mediated by [Ca(2+) ]cyt and ROS) and a specific effect on the plasma membrane conductance and the reduced ascorbate.

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 Ca(2+) signalling. The wheat root protoplasts (tested by patch-clamping) and excised arabidopsis roots (analysed by Ca(2+)-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 Ca(2+) 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 Ca(2+), which was inhibited by Gd(3+) and mediated by Ca(2+) influx from the bathing solution. In Ca(2+)-aequorin tests, 10 µM of exogenous BRs was the minimal concentration at which statistically significant changes of the cytosolic Ca(2+) 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 Ca(2+) signalling.