Gene suppression in a tolerant tomato-vascular pathogen interaction.

A plant can respond to the threat of a pathogen through resistance defenses or through tolerance. Resistance has been widely studied in many host pathogen systems but little is known about genetic changes which underlie a tolerant interaction. A recently developed model system for a tolerant tomato (Lycopersicon esculentum Mill) interaction with a fungal wilt pathogen, Verticillium dahliae Kleb, is examined with respect to changes in gene expression and compared to a susceptible infection. The results indicate that genetic changes can be dramatically different and some genes that are strongly elevated in the susceptible interaction are actually down-regulated in tolerance. Similar levels of fungal DNA and an up-regulation of many pathogenesis related genes indicate that in both types of interaction the presence of fungus is clearly recognized by the plant but other changes correlate with the absence of symptoms in the tolerant interaction. For example, a gene encoding a known 14-3-3 regulatory protein and a number of genes normally affected by this protein are down-regulated. Furthermore, genes which may contribute to foliar necrosis and cell death in the susceptible interaction also appear to be suppressed in the tolerant interaction, raising the possibility that the wilt symptoms, chlorosis and necrosis which are observed in the susceptible interaction, are actually programmed to further limit the growth of the fungal pathogen, and protect the general tomato population.