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

Salicylic acid (SA) has been inferred to be an endogenous signal in th e systemic acquired resistance response of plants. In this study, we d emonstrated 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 the m almost fully resistant to subsequent infection by the soft rot patho gen E. c. subsp. carotovora. Both the development of soft rot symptoms (tissue maceration) and the in planta proliferation of E. c. subsp. c arotovora were inhibited in SA-treated plants. The observed effect was not caused by direct action of SA on growth, nor on extracellular enz yme production or activity of the pathogen at the physiological SA con centrations, but was rather a consequence of induction of plant defens e response. This was suggested by the development of hypersensitive-li ke reactions in SA-treated Erwinia-infected plants, by the temporal pa ttern of resistance development, and by the parallel increase in patho genesis-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 acqui red resistance are involved in Erwinia-plant interaction. The molecula r mechanism of the SA-induced resistance to E. c. subsp. carotovora is not clear but appears to involve inhibition of plant cell wall-degrad ing enzymes secreted by this pathogen.