Modification of plasma membrane proton pumps in cucumber roots as an adaptation mechanism to salt stress

The effect of salt stress (50 mM NaCl) on modification of plasma membrane (PM) H+-ATPase (EC 3.6.3.14) activity in cucumber roots was studied. Plants were grown under salt stress for 1, 3 or 6 days. In salt-stressed plants, weak stimulation of ATP hydrolytic activity of PM H+-ATPase and significant stimulation of proton transport through the plasma membrane were observed. The H+/ATP coupling ratio in the plasma membrane of plants subjected to salt stress significantly increased. The greatest stimulation of PM H+-ATPase was in 6-day stressed plants. Increased H2O2 accumulation under salt stress conditions in cucumber roots was also observed, with the greatest accumulation observed in 6-day stressed plants. Additionally, during the sixth day of salinity, there appeared heat shock proteins (HSPs) 17.7 and 101, suggesting that repair processes and adaptation to stress occurred in plants. Under salt stress conditions, fast post-translational modifications took place. Protein blot analysis with antibody against phosphothreonine and 14-3-3 proteins showed that, under salinity, the level of those elements increased. Additionally, under salt stress, activity changes of PM H+-ATPase can partly result from changes in the pattern of expression of PM H+-ATPase genes. In cucumber seedlings, there was increased expression of CsHA10 under salt stress and the transcript of a new PM H+-ATPase gene isoform, CsHA1, also appeared. Accumulation of the CsHA1 transcript was induced by NaCl exposure, and was not expressed at detectable levels in roots of control plants. The appearance of a new PM H+-ATPase transcript, in addition to the increase in enzyme activity, indicates the important role of the enzyme in maintaining ion homeostasis in plants under salt stress.