Bacillus cereus ZW21-derived VOCs, hydroxyacetone and 3-methyl-3-buten-1-ol, control postharvest papaya anthracnose through direct antifungal activity and host transcriptional reprogramming

The search for natural and safe alternatives to synthetic fungicides is crucial for controlling postharvest anthracnose in papaya, a disease primarily caused by Colletotrichum species. This study identifies Bacillus cereus ZW21 as a potent biocontrol agent and delineates the mechanism of its volatile organic compounds (VOCs) directly in a fruit system. In vivo assays on papaya fruit demonstrated that ZW21 VOCs significantly reduced lesion area and disease incidence by up to 80.95% and 70.46%, respectively. Bioactivity-guided identification revealed hydroxyacetone (HA) and 3-methyl-3-buten-1-ol (ME) as the key antifungal VOCs. These compounds exhibited direct antifungal activity by disrupting fungal membrane integrity, inducing mitochondrial dysfunction, and elevating reactive oxygen species. Beyond direct inhibition, transcriptomic analysis of treated fruit revealed that HA and ME activated host defense pathways, including phenylpropanoid biosynthesis and hormone signaling, while suppressing ripening-related metabolism. A critical distinction was observed: ME enhanced glycolysis and pyruvate metabolism, providing a mechanistic explanation for potential phenolic overflow and fruit blackening, whereas HA avoided this metabolic overdrive. Our findings not only validate the efficacy of a novel biocontrol strain and its defined VOCs but also provide a comprehensive mechanistic framework supporting their application for sustainable postharvest protection.