Pseudomonas sp. ST4 produces variety of active compounds to interfere fungal sexual mating and hyphal growth.

Sexual mating of compatible sporida is essential for Sporisorium scitamineum to form dikaryotic mycelia and then cause infection on sugarcane. Our previous work identified a Pseudomonas sp. ST4 from a soil sample, which showed a promising biocontrol potential by inhibiting the mating of S. scitamineum sporida and hyphal growth. In this study, we set to isolate the active compounds from Pseudomonas sp. ST4 through solid fermentation. High-performance liquid chromatography (HPLC) separation coupling with bioassay showed that Pseudomonas sp. ST4 produced a range of antimicrobial compounds. Two of the major components were purified following acetate extraction, silica gel and HPLC separation. Nuclear magnetic resonance (NMR) and liquid chromatography-mass spectrometry (LC-MS) analysis identified these active compounds are 4-hydroxybenzaldehyde and indole-3-carbaldehyde respectively. Further analysis showed that the former compound only inhibited the hyphal growth of the fungus at a concentration of 3 mM, while the latter interfered the fungal sexual mating at a concentration of 0.6 mM and affected hyphal growth at a concentration of 2 mM. Treatment of corn plants with 3 mM indole-3-carbaldehyde significantly inhibited corn smut infection, with a control rate up to 94%. Further analysis of the structure and activity relationship revealed that indole has a much stronger inhibitory activity against the fungal sexual mating than indole-3-carbaldehyde. The results from this study provide new agents for control and prevention of the sugarcane smut disease, and the active compounds could also be used to probe the molecular mechanisms of fungal sexual mating.