Cyclical systemic colonization in Verticillium-infected tomato

It is generally believed that Verticillium moves slowly up a plant through spore trapping sites in the vascular tissue in a stepwise fashion. In a resistant plant, rapid defence responses around these sites slow pathogen growth and often restrict the fungus to the root and stem base. In a susceptible plant, the fungus escapes and eventually proliferates logarithmically in the upper stem and leaves. In the present study Craigella tomatoes were infected with Verticillium albo-atrum. Amounts of pathogen were monitored in the upper stems by a PCR-based quantitative assay and S-1 nuclease protection was used to study expression of a fungal induced phenylalanine ammonia lyase gene (tPAL5) during a 2-21 day time course. Even in resistant plants fungal colonization and PAL gene expression were found to be fully systemic from the earliest sporulation events (i.e. 2-4 days post-inoculation) and to occur in cycles, the PAL gene cycle following the colonization cycle. Peaks of pathogen growth occurred at 2-4 days and 12-15 days, succeeded by intermittent periods of fungal elimination. The results indicated that wilt is more systemic and also more dynamic in terms of fungal population fluctuations than previously thought (C) 1998 Academic Press.