Modification of defense responses in tobacco and capsicum following treatment with Fosetyl-Al [Aluminium tris (o-ethyl phosphonate)

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The systemic anti-oomycete compound, Fosetyl-Al, protected capsicum fruit segments from decay caused by Phytophthora nicotianae var. parasitica. The appearance of the phytoalexin, capsidiol, was advanced by 18–24 h, and accumulated to higher levels in infection droplets on Fosetyl-Al-treated fruit segments. As well, an intensified necrotic response was seen in the epithelial tissue of treated segments. Pretreatment of genetically susceptible tobacco plants with Fosetyl-Al protected them from the black shank disease, caused by Phytophthora nicotianae var. nicotianae. Microscopic examination of zoospore-infected tobacco seedling leaves revealed a hypersensitive-like response in Fosetyl-Al-treated susceptible seedlings similar to that observed in genetically resistant seedlings. In stems of treated and untreated resistant and susceptible tobacco plants inoculated with mycelium of P. nicotianae var. nicotianae, fungal growth proceeded at similar rates for the first 24 h. In resistant and Fosetyl-Al-treated susceptible plants there was no subsequent growth, whereas in untreated, susceptible plants mycelium progressed down the stem at a constant rate. The cessation of growth in resistant and treated, susceptible plants coincided with the maintenance of high levels of the phytoalexin, capsidiol, and the appearance of intense necrosis near the inoculation site. Except in the very early stages of infection, Fosetyl-Al treatment induced significantly greater levels of capsidiol accumulation in susceptible plants. Even higher capsidiol concentrations were recorded in untreated, genetically resistant plants in the first 48 h. Fosetyl-Al treatment of genetically resistant plants induced high capsidiol concentrations in the first 48 h, but a rapid decline followed. Despite this, fungal growth failed to resume. The data is consistent with the hypothesis that enhanced hypersensitivity and phytoalexin accumulation play a role in the mode of action of Fosetyl-Al. Further studies are needed to determine whether this is a primary role or a supplementary role to some, as yet undiscovered, activity of this compound.