Tomato Ve resistance locus; defense or growth

Verticillium colonization does induce a cascade of defense/stress proteins but the Ve1 gene also promotes enhanced root growth, which appears to allow the plant to outgrow the pathogen and avoid symptoms associated with an exaggerated defense response. In tomato, the Ve1 gene provides resistance to the vascular pathogen, Verticillium dahliae, race 1; ve1 plants are susceptible. However, the physiological basis of the resistance is unknown. While developing alternative lines of mutant Ve1 gene transformants to address this question a striking difference was observed in transformation frequency resulting from the inefficient rooting of plantlets from ve1 callus relative to Ve1 callus. Subsequent experiments with resistant and susceptible near-isolines of the cultivar Craigella, as well as Ve1 transformants, showed that in both artificial medium and soil, root growth was significantly enhanced in the resistant cultivar. Parallel studies of Verticillium colonization indicated a significantly lower overall concentration in the resistant plant characteristic of the resistant phenotype, but an almost equal total fungal biomass in both resistant and susceptible roots. Proteomic analyses of the roots of Verticillium-infected plants revealed elevated levels of defense/stress proteins, which correlated with the fungal concentration rather than resistance. Hormone analyses demonstrated a higher cis-ABA level in the resistant isoline consistent with enhanced root growth. Taken together these studies indicate a similar fungal biomass in the roots of both isolines where the Ve1 gene also promotes root production. In the case of the Craigella/Vd1 pathosystem, this appears to allow the host to resist better by outgrowing the pathogen with less wilt rather than reliance only on partial immunity.