Pathogenicity studies of different Verticillium dahliae isolates on tomato: Comparing disease development and phenylalanine ammonia-lyase (PAL) gene expression
Resistance, commonly understood as the lack of disease symptoms in Verticillium wilt, may result from tolerance to 'Verticillium dahliae'. Studies of tolerance, when plants do not show disease symptoms although substantial pathogen growth occurs, require the quantification of fungal biomass. A quantitative PCR-based assay was used to study amounts of pathogen in resistant and susceptible tomatoes infected with four 'V. dahliae' isolates. Despite a large variation in levels of infection between experiments, two isolates behaved consistently but the other two were more variable in terms of causing resistance, tolerance and susceptibility. In some interactions significant fungal colonization was not accompanied by disease development. Verticillium wilt symptoms may be a result of plant defense responses dependent on phenylalanine ammonia-lyase (PAL). Analysis of the two transcripts of the PAL5 gene indicated that both were responsive to fungal infection and one of them (shorter) may be suppressed by 'V. dahliae'. Preliminary comparison of the effects of live vs. killed spores of 'V. dahliae' suggests that suppression may require fungal metabolism and could affect both resistance to colonization and symptom expression.