Induced Resistance against Pseudomonas syringae pv. lachrymans in Cucumber by Spraying Cell-Free Microalgae Supernatant.

Chlorella is a genus of aquatic photosynthetic microalgae used in the production of dietary supplements, cosmetics, and biofuels, and also recently utilized as biological control agents or biofertilizers in agricultural supplements. Chlorella supernatant elicits induced resistance in Arabidopsis thaliana but its effects on crop plants remain largely unknown. This study tested whether application of Chlorella supernatant elicited induced resistance in cucumber (Cucumis sativus L.). Foliar application of supernatants from six microalgal strains revealed that supernatants from the high biofuel-producing strains HS2 and ABC001 elicited induced resistance against Pseudomonas syringae pv. lachrymans, which causes angular leaf spot in cucumber. In addition, spraying plants with D-lactic acid, a previously known determinant of induced resistance in the Chlorella supernatant, reduced the severity of disease caused by P. syringae pv. lachrymans in cucumber leaves by activating the salicylic acid and jasmonic acid signaling pathways. The application of Chlorella supernatant thus protects a crop plant against disease while offering a cost-effective method of recycling waste supernatant.