Involvement of Signalling Molecules NO, H2O2 and H2S in Modification of Plasma Membrane Proton Pump in Cucumber Roots Subjected to Salt or Low Temperature Stress

In the present study we demonstrate that the signalling molecules NO, H2O2 and H2S are important for understanding the mechanisms of modification of plasma membrane H+-ATPase (EC 3.6.3.14) activity in conditions of both salt (50 mM NaCl) and low temperature (10°C, LT) stress. Plants were subjected to stress conditions for 1 or 6 days. After 3 days of exposure to stress some of the plants were transferred to control conditions for another 3 days: post-stressed plants (3 + 3). We measured the endogenous levels of signalling molecules in stressed plants. To determine the physiological significance of NO, H2O2 and H2S induced activity of plasma membrane H+-ATPase (PM H+-ATPase) in salt and LT stresses, we investigated the activity of the plasma membrane proton pump in stress conditions, and plants were additionally supplemented with PTIO (a scavenger of NO), ascorbic acid (a scavenger of H2O2) or hypotaurine (a scavenger of H2S). H2S contributed to increased activity of PM H+-ATPase in short-term salt stress (1 day) and in low temperature treated plants (both 6 days and post-stressed plants), by stimulation of expression of several genes encoding isoforms of the plasma membrane proton pump (CsHA2, CsH4, CsH8, CsH9 and CsHA10). In contrast, NO and H2O2 play a minor role in the regulation of ATPase activity at the genetic level, because they significantly increased the expression of only one isoform, CsHA1, the expression level of which was very low in the tissues of the control plants, and additionally they slightly increased the expression of the gene encoding the isoform CsHA2. However, NO plays an important role in stimulation of the plasma membrane proton pumps under salt stress and low temperature. NO participates in post-translational modifications because it leads to increased enzyme phosphorylation and an increased H+/ATP coupling ratio.