IMPAIRED FUNGICIDE ACTIVITY IN PLANTS BLOCKED IN DISEASE RESISTANCE SIGNAL-TRANSDUCTION

Fungicide action is generally assumed to be dependent on an antibiotic effect on a target pathogen, although a role for plant defense mechan isms as mediators of fungicide action has not been excluded. Here, we demonstrate that in Arabidopsis, the innate plant defense mechanism co ntributes to the effectiveness of fungicides. In NahG and nim1 (for no ninducible immunity) Arabidopsis plants, which normally exhibit increa sed susceptibility to pathogens, the fungicides metalaxyl, fosetyl, an d Cu(OH)(2) are much less active and fail to control Peronospora paras itica. In contrast, the effectiveness of these fungicides is not alter ed in Arabidopsis mutants defective in the ethylene or jasmonic acid s ignal transduction pathways. Application of the systemic acquired resi stance activator benzothiadiazole (BTH) in combination with these fung icides results in a synergistic effect on pathogen resistance in wild- type plants and an additive effect in NahG and BTH-unresponsive nim1 p lants. Interestingly, BTH treatment normally induces long-lasting path ogen protection; however, in NahG plants, the protection is transient. These observations suggest that BTH treatment can compensate only par tially for an impaired signal transduction pathway and support the ide a that pathogen defense mechanisms are under positive feedback control . These observations are strikingly reminiscent of the reduced efficac y of antifungal agents in immunocompromised animals.