Effects Of NaCl And KCl Stress On The Growth And Tonoplast Proton Pump Activities Of Suaeda Salsa

Significant differences were observed in the responses of growth, ion accumulation, tonoplast proton pump activities, expression of V-H+-ATPase of shoots and roots of Suaeda salsa to salt stress, suggesting the cooperation and different functions of shoots and roots in salt resistance. There are significant differences in the responses of Suaeda salsa to NaCI and KC1 in the above aspects. The aim of the present study is to investigate the effects of NaCI and KC1 on growth parameters and vacuolar proton pumps of the halophyte Suaeda salsa. The Suaeda salsa seedlings were cultured under different NaCI and KC1 concentrations, and the tonoplast-enriched vesicles were isolated by sucrose density gradient from shoots and roots of Suaeda salsa plants. In addition to the determination of growth, V-H+-ATPase and V-H+-PPase activities, the subunit composition of V-H+-ATPase was studied by western blot analysis in the tonoplast preparations from plants grown under different conditions in order to correlate changes in enzyme activity with structural modifications. The main results are shown as follows.1 Effects of NaCI on the growth and tonoplast proton pumps of shoots and roots of Suaeda salsa.1.1 Growth and ion accumulationThe fresh weight and dry weight of Suaeda salsa shoots were markedly stimulated by low salinity while reduced by high salinity. The growth of roots was not affected so much as shoots indicating that the growth of shoots were changed drastically under salt stress while roots remained steady in the same conditions. The succulent degree of roots enhanced by salt treatment is much lower than that of shoots.In normal environment, the roots of Suaeda salsa uptaked Na+ and transported it to shoots where NaT act as osmotica which lowered water potential and enhanced water-absorption ability of plants. Cl~ and Na+ mainly accumulated in shoots under salt stress. About 97% of NaTwas stored in shoots and only 3% Na+in roots. Therefore, osmolarity of the cell sap was much lower in the shoots than in the roots at a given salt concentration. The shoots of Suaeda salsa were the main parts of NaT accumulation and the roots did not play a major role in ion accumulation. The shoots of Suaeda salsa used both Na+ and K+ as osmotica while roots prefer to use K+ as osmotica.1.2 Activities and subunits of tonoplast proton pumpsThe subunit composition of V-H+-ATPase in shoots and roots were different for that the antiserum immunostained four polypeptides from the membrane fractions of shoots with apparent molecular masses of 66, 55, 36 and 18 kDa which most probably represent V-H+-ATPase subunits A, B, D and c respectively while the three high molecular mass polypeptides were absent in the roots. The three undetected V-H"-ATPase subunits might not cross-react with the antiserum used in this study. Therefore, the constitutive composition of V-H+-ATPase subunits was different in roots and shoots, and there may be V-H+-ATPase isoforms in Suaeda salsa or three subunits protein amount was too low to be detected in roots. The patterns of V-H+-ATPase subunits in the shoots were not affected by NaCI stress thoughV-H+-ATPase protein amount and activity were significantly increased. A 18 kDa subunit of V-H+-ATPase in root was enhanced by salt stress and an additional polypeptide (16 kDa)was detected in the fractions of roots of NaCl-treated plants. It might be suspected that the 16 kDa polypeptide found in roots under NaCl stress was a hydrolyzed fragment of 18 kDa subunit. Under NaCl stress, Protein amounts of V-H+-ATPase subunits increased significantly and the activities of V-I-T-ATPase also increased. The increase of V-H+-ATPase activity was not due to structural changes of the enzyme, but to a quantitative up-regulation of V-H+-ATPase protein for there were no new subunits and the subunits displayed in the same pattern. A 18 kDa subunit of V-H+-ATPase in the roots was enhanced by salt stress, but the subunit was possibly degraded by salt stress so that the activity of V-H+-ATPase in the roots did not increase d...