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Journal of Cell Science, Vol 90, 225-235, Copyright © 1988 by Company of Biologists
1 Plant Cell Biology Group, Research School of Biological Sciences, The Australian National University Canberra, ACT 2601, Australia
2 Plant Cell Biology Group, Research School of Biological Sciences, The Australian National UniversityCanberra, ACT 2601, Australia
During encystment, zoospores of the dieback fungus Phytophthora cinnamomi attach themselves to the plant root surface by secretion of adhesive material. Previous workers have proposed, on the basis of ultrastructural studies, that the adhesive material is released by rapid exocytosis of large vesicles that underlie the zoospore plasma membrane. In the present study a monoclonal antibody (Cpa-2) that binds to the adhesive material has been used to investigate the sites of storage of this material in the zoospores, and the timing and pattern of its secretion during encystment. Immunolabelling of thin sections of zoospores with Cpa-2 followed by gold-labelled secondary antibodies has revealed that the adhesive material is not stored in the large peripheral vesicles but in a population of small vesicles in the cell periphery. In studies of other Oomycete fungi, these small vesicles have usually been described as microbodies. Another monoclonal antibody (Lpv-1) does bind to the contents of the large peripheral vesicles. Double immunolabelling of zoospores and encysting cells with Cpa-2- Au10 and Lpv-1-Au18, coupled with morphometric analysis, has shown that 85% of the adhesive material is secreted within the first minute following induction of encystment and that the large peripheral vesicles are not exocytosed during encystment but instead tend to migrate towards the centre of the cell. Immunoblot analysis with Cpa-2 shows the presence of several high molecular weight bands (Mr > 300 000) which also react with the lectin, soybean agglutinin. These high molecular weight components are susceptible to digestion with Pronase. Lpv-1 also reacts with high molecular weight bands but of different mobility from those recognized by Cpa-2. Concanavalin A binds to bands corresponding to those labelled by Lpv-1. We conclude that adhesion of cysts to plant roots involves the secretion of high molecular weight glycoproteins by rapid exocytosis of small peripheral vesicles and that the material contained in the large peripheral vesicles is also glycoprotein in nature but is not secreted during encystment.
Key words: adhesion, cell surface, immunocytochemistry, Phytophthora cinnamomi, secretion, zoospore encystment
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