Antifungal activity and potential inhibition mechanism of 4-(diethylamino)salicylaldehyde against Rhizoctonia solani

Rhizoctonia solani is a significant soil-borne pathogenic fungus that poses a significant threat to the economically important agricultural crops. 4-(Diethylamino)salicylaldehyde (DSA) is a secondary metabolite produced by Streptomyces sp. KN37, which has antifungal activity, meanwhile its inhibitory mechanism is still unclear. In this study, we explored the antifungal efficacy of DSA and its potential mechanism of inhibiting R. solani. It was found that DSA exhibited significant antifungal activity against six tested plant pathogenic fungi, with R. solani being the most sensitive (EC₅₀ = 26.904 μg/mL). Notably, DSA effectively reduced the mycelial mass and inhibited sclerotia germination, demonstrating a good control efficacy of cucumber damping-off disease. Morphological observation showed that DSA significantly disrupted the shape and ultrastructure of the mycelium. Transcriptomic and metabolomic analyses revealed that DSA impacted the integrity of the cell membrane, redox processes, and energy metabolism in R. solani. The results of fluorescence staining, relative conductivity, H₂O₂ content, and antioxidant enzyme activity showed that the accumulation of ROS in hypha cells after DSA treatment possibly resulted in damage to cell membrane integrity. Furthermore, the reduction in ATP content, along with decreased ATPase and citrate synthase activity, indicates that energy production may be inhibited. Molecular docking analysis further showed that DSA may competitively inhibit citrate synthase, thereby inhibiting cell energy production and ultimately inducing apoptosis. Our study provides new insights into the potential mechanism by which DSA inhibits the mycelial growth of R. solani.