OBLIGATE PARASITISM BY PHYTOPATHOGENIC FUNGI

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Abstract
Summary: 1. This review is concerned with the obligate parasitism of the downy mildew, powdery mildew and rust fungi.2. Mechanisms which determine resistance and susceptibility are incompletely understood. The three possibilities so far advanced, namely, hypersensitivity, phyto‐alexins and nutrition are all inadequate to explain specificity. It is postulated that compounds carrying specificity are released by the invading pathogens and the interaction of the compounds with specific sites in the host cell determines whether a successful relationship is established. This hypothesis is based on the observation that host and pathogen protoplasts do not come in direct contact with each other and cells removed from the invading pathogen are affected. It should be possible to investigate this question now that some rust fungi have been cultured axenically.3. Invasion of host tissue by these pathogens leads to alteration in host metabolism. Respiration rates and the activity of the pentose phosphate pathway are enhanced. Possible mechanisms for the reoxidation of NADPH by molecular oxygen are discussed. In the later stages of infection, photosynthetic activity is decreased and this decrease is coupled to the enhanced activity of the pentose phosphate pathway through NADPH availability.It is still not clear whether photosynthetic activity is increased or decreased during the early stages of infection. Nutrients from other parts of the plant accumulate at sites of infection, resulting in retarded growth of the host. Possible roles of hormones in nutrient mobilization and ‘green island’ formation are discussed.Changes in intermediary metabolism can be correlated with changes in amounts of nucleic acids in infected host cells. Chloroplast polysomes and ribosomes decrease during the early stages of infection but cytoplasmic ribosomes increase during patho‐genesis. This increase more than compensates for the decrease in chloroplast ribosomes resulting in an overall increase in cellular RNA.4. Some members of the Pucciniaceae and one member of the family Melamp‐soraceae have now been cultured axenically. Nutritional studies indicate that these fungi are relatively non‐specific with respect to their carbohydrate and amino‐acid nitrogen sources but do have a specific requirement for either cysteine, methionine or homocysteine. Essential metabolites such as cysteine and cysteine‐containing peptides move from hyphae into the medium and these may be important in amino‐acid transport.Two stages in the development of axenic rust cultures are still imperfectly understood ‐ the transition from germ tube to saprophytic mycelium and the fate of the colonies once formed. Further knowledge of the factors which control these stages may well suggest approaches to the axenic culture of other groups of obligately parasitic fungi.