Plant Coumarin Metabolism–Microbe Interactions: An Effective Strategy for Reducing Imidacloprid Residues and Enhancing the Nutritional Quality of Pepper

Imidacloprid (IMI) stress positively correlates with the potential of coumarins to alleviate abiotic stress. However, little is known about the pathways and mechanisms by which coumarin reduces the IMI residue by regulating plant secondary metabolism and plant–microbe interactions. This study examined the impact of coumarin on the uptake, translocation, and metabolism of IMI in pepper plants by modulating the signal molecule levels and microbial communities in the rhizosphere and phyllosphere. Analysis of 2 h─28 d pesticide residue dynamics revealed that coumarin dramatically reduced IMI concentration in pepper fruits. Coumarin upregulated the phenylpropane pathway genes, which increased the levels of flavonoids, phenolic acids, phytohormones, and capsaicinoids. Importantly, phyllosphere and rhizosphere microbial diversity results showed that coumarin improved the abundance of beneficial microorganisms and positively correlated with secondary metabolite secretion. Therefore, coumarin exploited the interaction between the phenylpropane and coumarin synthesis pathways and beneficial microbes to enhance the nutritional quality and IMI degradation.