Hot water treatment enhances mandarin fruit resistance to Penicillium italicum via histone H3K9 and H3K36 methylation modification

Citrus blue mold, caused by Penicillium italicum, is a significant postharvest disease that severely impacts fruit quality and storage life. This study investigated the effects of hot water (HW) treatment at 50 °C for 2.5 min on mandarin (Citrus reticulata Blanco 'Shatangju') fruit resistance to blue mold, focusing on transcriptional and epigenetic changes of disease-related genes. The results showed that HW treatment effectively reduced lesion diameter and disease incidence, delaying both disease onset and progression. RT-qPCR analysis identified six CrPG (polygalacturonase), whose expression was downregulated following HW treatment, thereby likely contributing to enhanced cell wall integrity and reduced fungal invasion. Additionally, three CrPR (pathogenesis-related proteins) exhibited elevated expression levels in HW-treated fruit, suggesting an enhanced defense responses. ChIP-qPCR analysis revealed that HW treatment increased H3K9me2 levels at the loci of CrPG2, CrPG29, CrPG30, CrPG31, CrPG34, and CrPG37, associated with transcriptional repression, while H3K36me2 levels were elevated at the loci of CrPR5c, CrPG5d, and CrPR5f, correlating with transcriptional activation. These findings suggest that HW treatment enhances mandarin fruit resistance to blue mold by modulating gene expression through histone methylation. This study provides insights into the molecular mechanisms underlying postharvest disease resistance and supports the potential of HW treatment as a sustainable postharvest strategy.