Mechanisms of phosphate-induced disease resistance in cucumber

Abstract

Certain phosphate salts are known inducers of systemic acquired resistance (SAR). In the present study, a local spray application of dipotassium hydrogenphosphate (K2HPO4) was effective in inducing a high level of systemic protection in cucumber plants against anthracnose caused by Colletotrichum lagenarium. Resistance induction by K2HPO4 was associated with localized cell death in cucumber leaves treated with the phosphate salt. The cell death observed, subsequently resulted in the appearance of macroscopically visible, necrotic spots. Appearing lesions resembled those provoked by tobacco necrosis virus (TNV) during a hypersensitive response (HR) that leads to pathogen-induced activation of SAR. Phosphate-mediated cell death was preceeded by a rapid generation of superoxide and hydrogen peroxide. As a further consequence of phosphate application, a local and systemic increase in free and conjugated salicylic acid (SA) levels was detected. The phosphate-induced responses were also identified with a similar time range in cucumber leaves that had been pre-inoculated with TNV. In contrast, none of these responses was triggered by application of the commercial plant activator benzo[1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester (BTH), which nevertheless was highly effective in inducing SAR in cucumber against anthracnose. In conclusion, the chemical SAR inducer K2HPO4 and the biological inducer TNV share some common early steps in signal transduction leading to SAR in cucumber, which differ from those involved in BTH-mediated SAR.