A comparison of the effects of DNA-damaging agents and biotic elicitors onthe induction of plant defense genes, nuclear distortion, and cell death

(Pisum sativum L. cv Alcan) endocarp tissue challenged with an incompatible fungal pathogen, Fusarium solani f, sp. phaseoli or fungal elicitors resul ts in the induction of pathogenesis-related (PR) genes and the accumulation of pisatin, a phytoalexin. Essentially the same response occurs in pea tis sue exposed to DNA-specific agents that crosslink or intercalate DNA. In th is study, the effects of DNA-damaging agents were assessed relative to the inducible expression of several pea PR genes: phenylalanine ammonia lyase, chalcone synthase, and DRR206. Mitomycin C and actinomycin D mimicked the b iotic elicitors in enhancing the expression of all three PR genes. The acti vities of these PR gene promoters, isolated from different plants, were eva luated heterologously in transgenic tobacco. It is remarkable that P-glucur onidase expression was induced when plants containing the heterologous phen ylalanine ammonia lyase, chalcone synthase, and DRR206 promoter-beta -glucu ronidase chimeric reporter genes were treated by DNA-damaging agents. Final ly, cytological analyses indicated that many of these agents caused nuclear distortion and collapse of the treated pea cells. Yet we observed that cel l death is not necessary for the induction of the PR gene promoters assesse d in this study. Based on these observations and previously published resul ts, we propose that DNA damage or the associated alteration of chromatin ca n signal the transcriptional activation of plant defense genes.