SALICYLIC-ACID INDUCED RESISTANCE TO ERWINIA-CAROTOVORA SUBSP CAROTOVORA IN TOBACCO

Salicylic acid (SA) has been inferred to be an endogenous signal in the systemic acquired resistance response of plants. In this study, we demonstrated that exogenously added SA can enhance plant resistance to the phytopathogenic enterobacterium Erwinia carotovora. Addition of SA to the growth medium of axenically growing tobacco seedlings made them almost fully resistant to subsequent infection by the soft rot pathogen E. c. subsp. carotovora. Both the development of soft rot symptoms (tissue maceration) and the in planta proliferation of E. c. subsp. carotovora were inhibited in SA-treated plants. The observed effect was not caused by direct action of SA on growth, nor on extracellular enzyme production or activity of the pathogen at the physiological SA concentrations, but was rather a consequence of induction of plant defense response. This was suggested by the development of hypersensitive-like reactions in SA-treated Erwinia-infected plants, by the temporal pattern of resistance development, and by the parallel increase in pathogenesis-related proteins. The plants reacting hypersensitively to E. c. subsp. carotovora showed a further increase in endogenous SA levels, indicating that SA and SA-controlled processes such as systemic acquired resistance are involved in Erwinia-plant interaction. The molecular mechanism of the SA-induced resistance to E. c. subsp. carotovora is not clear but appears to involve inhibition of plant cell wall-degrading enzymes secreted by this pathogen.