CHARACTERIZATION OF THE WATER EXPULSION VACUOLE OF PHYTOPHTHORA ZOOSPORES
HJ MITCHELL and AR HARDHAM
Plant Cell Biology Group, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia
Background and objectives
Disease dissemination is achieved by the production of large numbers of motile zoospores which are able to swim chemotactically towards a host plant. The zoospores are wall-less cells and their outer surface is the plasma membrane. They are therefore unable to build up turgor pressure, and so water that enters the cell down the osmotic gradient is expelled through the action of a water expulsion vacuole. The water expulsion vacuole complex forms during cleavage of multinucleate sporangia to form the uninucleate zoospores, a process that takes about 30 min. The main central vacuole contracts approximately once every 6 s throughout the life of the zoospore. On contact with a host plant, the zoospores encyst, secreting adhesive and forming a cellulosic cell wall. Wall formation allows turgor pressure to build up and the water expulsion vacuole disappears. Expulsion of water from the central vacuole is thought to occur by an actin-based contraction; however, the molecular basis for water accumulation in the vacuole is completely unknown. As part of a study of the properties of Phytophthora zoospore biology, we have analysed H+-ATPase activity in zoospore membranes. The work aims to provide information which will aid development of effective control strategies for Phytophthora diseases.
Results and conclusions
Further studies are focusing on the location of the vacuolar type H+-ATPase in other stages of the Phytophthora life cycle.