Abstract:

Plasma membrane (PM) vesicles of the leaves of two ecotypes of reed (Phragmites communis Trin.), swamp reed (SR) and heavy salt meadow reed (HSMR) growing in the desert region of Northwest China, were purified by two-phase partitioning and the properties of their PM H⁺-ATPases (EC 3.6.1.35) were compared. The specific activity of this enzyme was greater in HSMR than in SR and the K_m lower (1.27 mmol/L in SR and 0.30 mmol/L in HSMR), and the V_max of ATP hydrolysis activity showed no significant difference between the two ecotypes. The PM H⁺-ATPase was more sensitive to denaturing temperatures in HSMR than in SR, and the pH profile also showed a slight difference, suggesting that the catalytic mechanism of this enzyme was different in HSMR compared with that in SR. The p -nitrophenyl phosphate (PNPP) hydrolysis activity of H⁺-ATPase was higher in HSMR than in SR at low concentrations of PNPP, but showed no difference at high PNPP concentration. The K_m for PNPP hydrolysis was 3.61 mmol/L and 1.92 mmol/L in SR and HSMR, respectively. And the V_max of PNPP hydrolysis showed no significant difference in the two reed ecotypes. An experiment with the inhibitor vanadate showed that the catalytic mechanisms of the phosphatase domain of the ATPase were different in the two ecotypes. The data obtained following trypsin treatment showed a difference in the enzyme activity pattern, suggesting that there existed a possible change in the C-terminus of the ATPase, either in the structure or in the property or both of them. In addition, compared with SR, the ATP-dependent H⁺ pumping activity of ATPase and the coupling between proton transport and ATP hydrolysis in HSMR were increased. These results indicated that the properties of PM H⁺-ATPase were changed in HSMR with compared to those in SR, which might include enzyme modifications and different isoforms expressed. The alterations of the properties of this enzyme might be an adaptive response to the habitat.

Key words: plasma membrane (PM), H⁺-ATPase, reed (Phragmites communis) ecotype, saline habitat