Physiological and transcriptome analysis of the restorative mutant from the early-ripening ‘Gannanzao' navel orange

Abstract: 【Objective】Selection of early or late ripening varieties is an important target of citrus breeding. The regulation mechanism of citrus fruit ripening is of great significance for breeding early or late ripening varieties. Previous studies regarding ripening mechanism of citrus fruits mainly focused on the flesh, little attention was paid on the pericarp. Herein, both flesh and pericarp of‘Gannanzao'navel orange (Wild type, WT) , and its restorative mutant (Mutant type, MT) were investigatedin order to explore the regulation mechanism of the ripening of navel orange through comprehensive comparison of the physiological and transcriptional differences between the WT and the MT.【Methods】The fruit qualities and physiological properties, including the contents of soluble sugar, organic acid and phytohormones, in both flesh and pericarp of the WT and the MT were determined. Transcriptome data of both flesh and pericarp of the WT and the MT were obtained and analyzed by high-throughput sequencing.【Results】MT featured an obvious and stable late-maturing character. The soluble sugar content was significantly different only in the pericarp of the WT and the MT, while the organic acid content was significantly different in both of the flesh and the pericarp. The MT had much lower contents of both malic acid and citric acid, and higher content of quinic acid compared with those of the WT. At200 DAF, the contents of GA, IAA and JA in the pericarp of the MT were significantly higher than those of the WT, whereas the content of ABA in the pericarp of the MT was obviously lower. than that of the WT. Meanwhile, the comparison of transcriptome sequencing between the MT and the WT showed that the number of differentially expressed genes (DEGs) were 980 and 289 in the pericarp and the flesh respectively, and 94 DEGs were the common DEGs of the pericarp and the flesh. Interestingly, the significant enrichment (p ≤ 0.05) of GO terms and KEGG pathways were only found in the pericarp. A total of 38 GO terms were significantly enriched in the pericarp. Furthermore, a total of 6 KEGG pathways being involved in photosynthesis, photosynthesis-antenna proteins, peroxisome, cutin, suberine and wax biosynthesis, protein digestion and absorption, and ubiquinone and other terpenoid-quinone biosynthesis were significantly enriched (p ≤ 0.05) in the pericarp. In ABA synthesis signal transduction pathway, the key limiting gene CsNCED1 of ABA synthesis was downregulated in both of the pericarp and the flesh in the MT, and the decomposition gene CsCYP707 A1 was up-regulated in the MT pericarp.【Conclusion】The ripening date of the mutant (MT) was 30 days later than that of the WT. The delay of both chlorisis and color transition might be related to the increase of GA accumulation and the up-regulated expression of CsCPS1 and CsKAO. The down-regulated and up-regulated expression of CsNCED1 and CsCYC707 A1 in the MT pericarp might lead to the inhibition of ABA accumulation. The differences of the physiological and transcriptome levels in the pericarp between the WT and the MT were greater than those in the flesh, indicating that the pericarp may play an important role in citrus fruit ripening. It seems that the investigation of the physiology and transcriptome both in the flesh and the pericarp are essentially necessary for studying the mechanism of the citrus fruit ripening.