Inhibitory Effect of Sucrose on the Plasma Membrane H+ Pump Activity and Possible Involvement of the C-terminal Region in the Sucrose Effect

Abstract

The effect of sucrose on the plant plasma membrane H+ pump activity is unclear. In the plasma membrane vesicles from rye leaves high concentrations of sucrose had significant inhibitory effects on the plasma membrane Hf pump activity. Fusicoccin and trypsin treatment experiments suggest that the C-terminal region of the H+ pump may partially but significantly be involved in the sucrose effect.Most higher plants produce sucrose as a result of photosynthesis. In plant cells, sucrose is not only used as an energy source but also used for making various cellular components. Sucrose is also a main substance translocated via the phloem from source to sink organs.We previously suggested that sucrose might have an inhibitory effect on the plant plasma membrane H+ pump activity, because Mg2+-ATPase activity of isolated plasma membrane vesicles from maize leaves was lowered by the presence of sucrose (200 mM) in assay medium (Kasai and Sawada 1994). There are also reports of reductions of Mg2+-ATPase activity in the plasma membrane vesicles from wheat roots and soybean hypocotyls by the presence of sucrose (-IM) in assay medium (Berczi and M@ller 1987, 1993). To confirm the inhibitory effect of sucrose on the plant plasma membrane H+ pump activity, it is necessary to investigate the effect of sucrose on the Hf pumping activity.Although Grouzis et a/. (1987) showed a small decrease (16%) in ATP-dependent Hf transport activity of maize rootplasma membrane vesicles caused by the presence of 0.25M sucrose in assay medium, it seems problematic because in their experiments control activity was measured without a corresponding osmoticum.In the present study, we found that glycerol had very small inhibitory effects on Mg2+-ATPase activity of isolated plasma membrane vesicles from rye leaves. Therefore, this material was used for H+ pumping assay with the isolated plasma membrane vesicles to examine the inhibitory effect of sucrose on the plant plasma membrane H+ pump activity and to investigate the mechanism responsible for the effect of sucrose. For the investigation of the mechanism responsible for the effect of sucrose, we focused on the C-terminal region in the plasma membrane H+ pump, because the region has been established to be an important part involved in the activity regulation of the H+ pump (see Palmgren 199l).We present here the data that high concentrations of sucrose significantly decrease the H+ pumping activity of isolated plasma membrane vesicles from rye leaves and that the decrease in the H+ pumping activity is significantly diminished by the pre-treatment of leaves with a fungal toxic compound, fusicoccin and the treatment of plasma membrane vesicles with trypsin, each of which has been shown to weaken an inhibitory effect that C-terminal region has on the plasma membrane H+ pump (see Palmgren 1991 and Jahn et a/. 1997). The plasma membrane vesicles were isolated from leaves of rye (Secale cereale L. cv. Elbon) plants hydroponically grown for 14 or 15 days [I4 hr-light (500 pmol photons m2 s-I, 22C) and 10 hr...