Characterisation of the water expulsion vacuole in Phytophthora nicotianae zoospores

The water expulsion vacuole (WEV) in zoospores of Phytophthora nicotianae and other members of the Oomycetes is believed to function in cell osmoregulation. We have used videomicroscopy to analyse the behaviour of the WEV during zoospore development, motility and encystment in P. nicotianae. After cleavage of multinucleate sporangia, the WEV begins to pulse slowly but soon attains a rate similar to that seen in motile zoospores. In zoospores, the WEV has a mean cycle time of 5.7 +/- 0.71 s. The WEV continues to pulse at this rate until approximately 4 min after the onset of encystment. At this stage, pulsing slows progressively until it becomes undetectable. The commencement of WEV operation in sporangia coincides with the reduction of zoospore volume prior to release from the sporangium. Disappearance of the WEV during encystment occurs as formation of a cell wall allows the generation of turgor pressure in the cyst. As in other organisms, the WEV in P. nicotianae zoospores consists of a central bladder surrounded by a vesicular and tubular spongiome. Immunolabelling with a monoclonal antibody directed towards vacuolar H+-ATPase reveals that this enzyme is confined to membranes of the spongiome and is absent from the bladder membrane or zoospore plasma membrane. An antibody directed towards plasma membrane H+-ATPase shows the presence of this ATPase in both the bladder membrane and the plasma membrane over the cell body but not the flagella. Analysis of ATPase activity in microsomal fractions from P. nicotianae zoospores has provided information on the biochemical properties of the ATPases in these: cells and has shown that they are similar to those in true fungi. Inhibition of the vacuolar H+-ATPase by potassium nitrate causes a reduction in the pulse rate of the WEV in zoospores and leads to premature encystment. These results give support to the idea that the vacuolar H+-ATPase plays an important role in water accumulation by the spongiome in oomycete zoospores, as it does in other protists.