Comparative metabolomics and transcriptomic profiling reveal the mechanism of fruit quality deterioration and the resistance of citrus fruit against Penicillium digitatum

Decay, caused by Penicillium infection, is one of the main problems during postharvest storage in citrus fruit. A deeper understanding of the mechanism of fruit quality deterioration and the fruit's defense responses may lead us to develop rational approaches for disease control. To identify the metabolic and regulatory mechanisms of citrus against Penicillium digitatum, Powell orange (Citrus sinensis) fruit pulps were collected after 40 h and 60 h infected with P. digitatum for RNA-Seq analysis complemented with metabolic profiling of polar metabolites and volatile emissions. Results demonstrated that several primary metabolites and volatile organic compounds (VOCs) were significantly regulated in response to P. digitatum infection. The contents of sugars (e.g. glucose and sucrose), organic acids (e.g. citric acid, malic acid, and oxalic acid), vitamin C and D-limonene were decreased, while others such as ethanol and a-terpineol were in outstanding increases, resulting in fruit quality deterioration. Rhamnose, inositol, serine, threonine and γ-aminobutyric acid (GABA) also accumulated, thereby enhancing the resistance to P. digitatum infection. RNA-Seq data revealed a series of significantly enriched pathways. P. digitatum infection induced G-protein and RLK signal pathways and enhanced the transcription of stress-related genes including peroxidase and NBS-LRR. Furthermore, P. digitatum infection triggered a defense response via both the jasmonic acid and ethylene pathways, and increased the transcript abundance of several transcription factors such as ERFs, WRKYs, and MYB. Phenylpropanoids pathways were also activated at the transcriptomic level. In summary, our results unravel the significantly regulated metabolites and biological pathways, which provide new insights into the mechanism of fruit quality deterioration and the induction of resistance against P. digitatum in Powell fruit.