The cell biology behind Phytophthora pathogenicity

Species of Phytophthora cause serious soilborne diseases of important crop plants and threaten natural ecosystems on a vast scale. Successful infection is usually initiated by motile, biflagellate zoospores that are chemotactically attracted to nearby roots. The zoospores home in at sites on the root surface that are favourable for subsequent penetration. There they encyst, secreting adhesive that glues them to the root surface. The germ tube that emerges from the cysts penetrates the root epidermis, usually growing intercellularly along the anticlinal cell walls. Intracellular growth also occurs and haustoria can develop in cortical cells. Within 2-3 days, the pathogen sporulates with the production of chlamydospores in cortical cells and multinucleate sporangia on the root surface. The sporangia undergo cytokinesis and release motile zoospores into the soil. A number of features of the infection cycle contribute to the rapid dissemination of infective propagules and successful disease establishment by Phytophthora pathogens. While basic aspects of the infection cycle were described 30-40 years ago, in the last decade immunocytochemical studies have helped uncover new details of cellular and molecular mechanisms involved in processes such as zoospore osmoregulation, motility and adhesion, and asexual sporulation. The increase in our understanding of Phytophthora pathogenesis that has resulted has already pointed to potential targets for novel inhibitors of Phytophthora diseases. Further investigations of the cell and molecular biology of Phytophthora pathogenicity promise to be an integral part of our development of highly specific and sustainable control measures in the future.