Modification of vacuolar proton pumps in cucumber roots under salt stress.

The time-dependent effect of 50mM NaCl on the activities of two tonoplast proton pumps was investigated in Cucumis sativus L. var. Krak root cells. Distinct activity profiles for vacuolar proton transporting ATPase (V-ATPase) (EC 3.6.3.14) and vacuolar proton transporting pyrophosphatase (V-PPase) (EC 3.6.1.1) under salinity are presented. ATP-dependent proton transport and ATP hydrolysis increased after 24h of NaCl exposure, and then decreased in roots stressed with NaCl for 4 and 8d. Both PP(i)-driven H(+) transport and PP(i) hydrolysis were clearly inhibited by NaCl at all times examined. It was demonstrated that changes in enzyme activities were not due to the salt action on the expression of encoding genes. The levels of specific transcripts for subunit A of V-ATPase (CsVHA-A), subunit c of V-ATPase (CsVHA-c) and V-PPase (CsVP) were similar in cucumber roots untreated (control) and treated with salt. Such results suggest that alterations of proton pump activities under salinity are rather due to the post-translational alterations induced by NaCl.