CYCLICAL SYSTEMIC COLONIZATION IN VERTICILLIUM-INFECTED TOMATO

It is generally believed that Verticillium moves slowly up a plant thr ough spore trapping sites in the vascular tissue in a stepwise fashion . In a resistant plant, rapid defence responses around these sites slo w pathogen growth and often restrict the fungus to the root and stem b ase. In a susceptible plant, the fungus escapes and eventually prolife rates logarithmically in the upper stem and leaves. In the present stu dy Craigella tomatoes were infected with Verticillium albo-atrum. Amou nts of pathogen were monitored in the upper stems by a PCR-based quant itative 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 an d PAL gene expression were found to be fully systemic from the earlies t 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 inte rmittent periods of fungal elimination. The results indicated that wil t is more systemic and also more dynamic in terms of fungal population fluctuations than previously thought (C) 1998 Academic Press.