Metabolic And Regulatory Mechanism Underlying Postharvest Fruit Senescence And Penicillium Infection In Powell(Citrus Sinensis(L.) Osbeck)

Decay or quality deterioration, one of the main problems in citrus industry, is caused by physiological disorders and diseases during postharvest storage of citrus fruit. Fruit senescence is an inevitable developmental process that involves dynamic alterations in many metabolic and regulatory pathways. Hence, the knowledge of metabolic and regulatory mechanisms during postharvest fruit senescence and fungal infection has significant implications for developing new handling technology for delaying senescence and maintaining fruit quality. Powell(Citrus sinensis(L.) Osbeck), a later-ripening sweet orange in Chong Qing, was used as materials in this study. Metabolic and transcriptomic profiling analysis were performed during postharvest storage and penicillium infection. The main results were as follows:1. The information of fruit quality components such as amino acid, sugar, organic acid, and VC in Powell was obtained. 48 volatile organic compounds(VOCs) were identified using HS-SPME-GC/MS, which including 10 monoterpenes, 13 sesquiterpenes, 8 aldehydes, 7 esters, 5 alcohols, 2 ketones, 2 acids, and 1 hydrocarbon.2. The dynamic profiles of primary and volatile metabolites were analyzed using GC-MS analysis during postharvest storage in Powell orange. PCA analysis showed that there was an obvious difference in metabolic profiles at 60 and 90 days after storage under room temperature(RT), and accordingly, no significant changes were observed in the duration of low temperature(LT)-storage. Under RT storage, the sugars including D-fructose, D-glucose, D-galactose and Myo-inositol were remarkably increased, whereas sucrose displayed an outstanding decrease. The contents of malic acid, citric acid and α-ketoglutaric acid were reduced conspicuously. We also found an amino acid, γ- aminobutyrate(GABA) showing extremely higher levels after lengthy storage. There was a remarkable difference in volatile metabolites after storage both under RT and LT. Under RT storage, the contents of some VOCs including D-limonene and sesquiterpens such as valencene and α-selinene were increased, while others including alcohols, aldehydes, and esters were decreased, such as citral, linalool, and citronellol acetate. The contents of some monoterpens and aldehydes such as pinene, citral, decanal, and lauraldehyde were in decline under LT storage.3. RNA-sequencing(RNA-Seq) was employed to characterize the transcript profiles during postharvest storage under RT and LT in Powell fruits. Results showed that RT storage accelerated primary metabolic pathways, which involved in sucrose degradation, glycolysis, and organic acid utilization. Furthermore, RT-storage enhanced Ca, ABA and ethylene signaling pathways via up-regulation of CMLs, CBL-CIPKs and CPKs PYLs, ABI5 and ERFs. Transcription factors including WRKY, AP2/ERF, and NAC were significantly up-regulated. LT-storage inhibited primary metabolic pathways and AUX/IAAs, up-regulated chlcone metabolism. Hence, we hypothesis that RT storage accelerates calcium, ABA and ethylene signals, which could positively regulate fruit senescence and result in induced primary metabolism and fruit quality deterioration. LT storage might accelerate the auxin signal, thus delaying senescence and maintaining fruit quality.4. Significantly changed metabolites and differentially regulated pathways were identified by transcriptomic and metabolomic analysis during Penicillium infection. During this process, the contents of sugars, organic acids and D-limonene were decreased, while ethanol and α-terpineol were increased, resulting in fruit quality deterioration. Accumulation of GABA, rhamnose, inositol, serine and threonine were observed. Penicillium infection induced G-protein and RLK signal pathways and enhanced transcription of stress-related genes including POD and PR(LRR). Penicillium infection triggered a defence response via both the jasmonic acid and ethylene pathways and increased the transcript abundance of several transcription factors such as AP2/ERFs, WRKYs, and MYBs. Activation of phenylpropanoids pathway was also observed at the transcriptomic level. The above significantly regulated pathways revealed the bases of the induction of resistance in Powell fruit against Penicillium digitatum.5. The regulatory patterns of ethylene on climacteric tomato and non-climacteric Powell during postharvest fruit senescence were described based on transcriptome data.The results showed that different ACS and ACO members such as ACS2, ACS4, ACO, ACO1, ACO4 and ACO5, play crucial regulatory roles during the transition from postharvest fruit ripening to senescence in tomato. We also observed that there was a positive correlation between ABA and ethylene biosynthesis during postharvest senescence in tomato, which was not presented in Powell fruits.In summary, the overall information database of fruit quality components was established and a working hypothesis of metabolic and regulatory pathway during postharvest fruit senescence was constucted in this study. Moreover, transcriptomic and metabolomic analysis revealed the mechanism of fruit quality deterioration as well as the resistance of Powell aganist Penicillium infection.