The SA-dependent defense pathway is active against different pathogens in tomato and tobacco.
The importance of salicylic acid (SA) in plant defense against pathogen attack has been elaborately documented. Benzothiadiazole (BTH, BION), a chemical analogue of SA, also induces resistance through the SA-dependent pathway. We investigated the role of SA in both basal defense and induced resistance of tomato (Lycopersicon esculentum) and tobacco (Nicotiana tabacum) against Oidium neolycopersici (a biotrophic pathogen) and Botrytis cinerea (a necrotrophic pathogen). A comparison of NahG transgenic tomato and tobacco (unable to accumulate SA) to their respective wild types revealed that in both crops, SA was not involved in basal defense against O. neolycopersici. SA also played no role in the basal defense of tobacco against B. cinerea but NahG tomato plants were significantly more sensitive to B. cinerea than wild type plants. Activation of the SA-dependent defense pathway via BTH resulted in induced resistance against B. cinerea in tomato but not in tobacco. In contrast, BTH induced resistance against O. neolycopersci in tobacco but not in tomato. Microscopic analysis revealed that BTH treatment could prevent penetration of the odium germ tube through the epidermal cell wall of tobacco leaves whereas penetration was successful on tomato leaves, irrespective of BTH treatment. We conclude that even in two related plant species such as tomato and tobacco, the SA-dependent defense pathway does not trigger the same defense responses. It also means that the outcome of a BTH treatment cannot be predicted and has to be tested for each plant-pathogen combination.