Beyond the two compartments Petri-dish: optimising growth promotion and induced resistance in cucumber exposed to gaseous bacterial volatiles in a miniature greenhouse system

Abstract

Background: Bacterial volatiles promote plant growth and elicit immunity responses in plants grown in two-compartment Petri dishes. Due to the limitations of bacterial volatile compound (BVC) treatments such as their high evaporation rates, it is convenient to apply BVCs in closed systems such as greenhouses. However, the concentrations of BVCs must be optimised. We therefore attempted to optimise BVC emissions from bacteria grown on solid medium and synthetic BVC treatment in order to maximise plant growth and induced resistance in a miniature greenhouse system. Results: We cultivated the model BVC emitter Bacillus subtilis GB03 on complex medium for continuous treatment, which we placed near 1-week-old cucumber seedlings in a miniature greenhouse. Aboveground and belowground plant growth parameters were significantly increased at 1 and 2 weeks after treatment with BVCs released by B. subtilis GB03. Moreover, this treatment protected cucumber seedlings against the angular leaf spot pathogen Pseudomonas syringae pv. lachrymans. In addition, cucumber shoot growth was promoted in response to the slow release of BVCs from filter paper that had absorbed 1000 and 10 μM synthetic 2,3-butanediol, a key BVC from B. subtilis strain GB03. However, induced resistance was only elicited when 10 plates containing 10 μM 2,3-butanediol were utilised in the miniature greenhouse. The mechanism of induced resistance appears to involve the activation of the jasmonic acid signalling pathway. Conclusions: To overcome the difficulties associated with treatment using a single application of BVC in the greenhouse, we optimised conditions for BVC application via consistent exposure in a slow-release system.