Abstract
Anthracnose of papaya (Carica papaya L.) caused by the fungus Colletotrichum spp. is one of the most economically important postharvest diseases. Coating with chitosan (CS) and Ruta graveolens essential oil (REO) might represent a novel eco-friendly method to prevent postharvest anthracnose infection. These compounds show both antimicrobial and eliciting activities, although the molecular mechanisms in papaya have not been investigated to date. In this study, the effectiveness of CS and REO alone and combined (CS-REO) on postharvest anthracnose of papaya fruit during storage were investigated, along with the expression of selected genes involved in plant defense mechanisms. Anthracnose incidence was reduced with CS, REO, and CS-REO emulsions after 9 days storage at 25°C, by 8, 21, and 37%, respectively, with disease severity reduced by 22, 29, and 44%, respectively. Thus, McKinney’s decay index was reduced by 22, 30, and 44%, respectively. A protocol based on reverse transcription quantitative real-time PCR (RT-qPCR) was validated for 17 papaya target genes linked to signaling pathways that regulate plant defense, pathogenesis-related protein, cell wall-degrading enzymes, oxidative stress, abiotic stress, and the phenylpropanoid pathway. CS induced gene upregulation mainly at 6 h posttreatment (hpt) and 48 hpt, while REO induced the highest upregulation at 0.5 hpt, which then decreased over time. Furthermore, CS-REO treatment delayed gene upregulation by REO alone, from 0.5 to 6 hpt, and kept that longer over time. This study suggests that CS stabilizes the volatile and/or hydrophobic substances of highly reactive essential oils. The additive effects of CS and REO were able to reduce postharvest decay and affect gene expression in papaya fruit.
Highlights
Papaya (Carica papaya) is a fruit cultivated in tropical and subtropical regions but appreciated worldwide
The AI for this combined treatment showed an additive effect between these 0.5% CS and 0.5% Ruta graveolens essential oil (REO) emulsions when applied together to the papaya fruits
This study initially confirms that the incorporation of REO into the edible CS coating improves the control of postharvest decay of papaya fruit compared to the use of these treatments individually
Summary
Papaya (Carica papaya) is a fruit cultivated in tropical and subtropical regions but appreciated worldwide. It is known for its high nutritional and economic potential (Parven et al, 2020). Being a climacteric fruit, it is subject to intense metabolic activity, fast maturation, high susceptibility to fungal diseases, and short shelf life (Batista et al, 2020). The most common fungal disease of papaya fruit is anthracnose, which is caused by Colletotrichum spp. and can result in 30–50% postharvest losses (Gunathilake et al, 2018). Several technologies have been used to extend the postharvest shelf life of fruit, including fungicides, low-temperature storage, thermal processing, diverse packaging conditions, and preserving compounds obtained from natural sources that are “generally recognized as safe” (Droby and Wisniewski, 2018).
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