Melatonin-mediated dual regulation of physiological-biochemical traits and rhizosphere microbial communities alleviates drought stress in Bougainvillea

Background Drought conditions limit the distribution of Bougainvillea varieties, reduce their ornamental value, and increase maintenance costs. Although the rhizospheric melatonin-mediated mechanisms of plant drought resistance have been extensively documented. its role in the drought stress response of Bougainvillea , especially the regulatory effect on rhizosphere microorganisms, remains unclear. Using Bougainvillea glabra ‘Cypheri’ as plant material, this study employed a pot-based natural drought experiment combined with physiological indicator measurement, rhizosphere microbiome high-throughput sequencing, microbial network analysis, PICRUSt2 functional prediction, and redundancy analysis (RDA) to systematically investigate the regulatory mechanisms of melatonin on aboveground-belowground synergistic drought resistance. Results Compared with the control group, drought stress significantly enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as the proline (Pro) content in Bougainvillea , while increasing malondialdehyde (MDA) accumulation. Conversely, leaf relative water content (RWC) and chlorophyll (Chl) content were markedly reduced. Simultaneously, the rhizosphere microbial community balance was disrupted, bacterial network robustness diminished, and carbon metabolic pathways were suppressed. Exogenous melatonin application further significantly enhanced antioxidant enzyme activities and Pro accumulation under drought stress, reduced MDA content, and improved the physiological status of plants. Simultaneously, it promoted the enrichment of Proteobacteria, modulated the abundance balance of Ascomycota, enhanced microbial network stability, and optimized carbon, nitrogen, and energy metabolic pathways. Collectively, these results demonstrate that melatonin enhances drought tolerance in Bougainvillea by regulating the structure and network stability of rhizosphere microbial communities, synergistically improving aboveground drought resistance physiology, and mediating aboveground–belowground synergistic interactions. Conclusion This study provides a theoretical foundation for the application of melatonin in drought-resistant cultivation practices for Bougainvillea species.