Fusicoccin activates pathogen-responsive gene expression independently of common resistance signalling pathways, but increases disease symptoms in Pseudomonas syringae-infected tomato plants

Fusicoccin (FC), an activator of the plant plasma membrane H+-ATPase, induces several components of plant pathogen resistance responses, including defence hormone biosynthesis and pathogenesis-related (PR) gene expression. The mechanism by which these responses occur, and the effect they have on plant–pathogen interactions is unknown. Here, we show that PR gene expression in response to FC in tomato (Lycopersicon esculentum Mill.) plants does not strictly require the common defence hormones, salicylic acid, jasmonic acid and ethylene. We also show that FC-induced PR gene expression requires neither Ca2+ nor reactive oxygen species, typical early pathogen-resistance response signals. The possibility that PR gene expression is related to FC-induced dehydration stress is also discounted. Finally, we show that the defence responses elicited by FC in tomato are not sufficient to confer resistance to the bacterial pathogen Pseudomonas syringae. Rather, FC increases the rate and severity of disease symptom formation in an ethylene-dependent manner.