Antimicrobial mechanisms of g-C3N4 nanosheets against the oomycetes Phytophthora capsici: Disrupting metabolism and membrane structures and inhibiting vegetative and reproductive growth

To understand the potential of urea-synthesized g-C3N4 nanosheets (0.125–1 mg/mL) as antimicrobial agents against oomycetes, an investigation of the interaction mechanism between g-C3N4 nanosheets and Phytophthora capsici was conducted. Transcription analysis showed that after being exposed to g-C3N4 nanosheets for 1 h, P. capsici triggered a sharp upregulation of antioxidant activities and structural constituents and a downregulation of metabolic pathways, including ATP generation, autophagy disruption, membrane system disorders and other complex adaptive processes. All the life stages of P. capsici, including mycelial growth, sporangium formation, zoospore numbers and zoospore germination were remarkably inhibited and even injured. A mutual mechanism is proposed in this work: ROS stress upon exposure to visible irradiation and, combined with their sharp nanosheet structure, cause perturbations of the cell membrane and induce damage to the ultrastructure of mycelial growth, sporangium and zoospores. Given that the antimicrobial action of g-C3N4 nanosheets were derived from the damage throughout the duration of treatment and was not limited to a single target, these complex mechanisms could favor the avoidance of drug resistance and benefit other oomycetes management. More importantly, in addition to restraining P. capsici infection in host plants, g-C3N4 nanosheets promoted pepper plant growth. Hence, g-C3N4 nanosheets have potential as a new non-metal antimicrobial agent to control oomycotal disease in crops.