Inhibitors of Antioxidant Enzymes Systemically Protect Cucumber Plants from Scab

The study is aimed at the induction of systemic disease resistance by a local oxidative burst caused by inhibition of plant antioxidant enzymes. A possible involvement of ROS was ascertained. Inhibitors of superoxide dismutase and catalase, respectively, diethyldithiocarbamate (DDC) and aminotriazole (AT) were applied to the fist (local) true leaf of cucumber seedlings ( Cucumis sativus L.). When the second and third (systemic) leaves developed, they were inoculated with spores of the virulent fungus Cladosporium cucumerinum Ell. et Arth. causing cucurbit scab. The inhibitors at concentrations nontoxic to leaves or spores greatly reduced the disease symptoms on the systemic leaves. The inhibition of both enzymes was confirmed, and increased superoxide production was found in the chemically treated local leaf. In case of a treatment with water, diffusates of the healthy systemic leaves stimulated spore germination, and those of infected systemic leaves were ineffective. Treatment of the local leaf with any compound systemically suppressed the aforementioned stimulation in the healthy counterpart and provided fungitoxicity in the infected one. Both antifungal effects were abolished by diffusate boiling, suggesting protein involvement. Meanwhile, the effects were insensitive to antioxidants and, apparently, independent of reactive oxygen. DDC and AT did not promote salicylic acid accumulation in infected systemic leaves; presumably, the disease control did not represent systemic acquired resistance. It is suggested that both inhibitors induce some kind of systemic resistance through the local oxidative burst caused by inhibition of antioxidant enzymes. The systemic implementation of the resistance may include antifungal effects.