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Transcriptome Analysis Unravels Metabolic and Molecular Pathways Related to Fruit Sac Granulation in a Late-Ripening Navel Orange (Citrus sinensis Osbeck)

1
Key Laboratory of Horticultural Plant Biology, Ministry of Education, College of Horticulture and Forestry Sciences, Huazhong Agricultural University, Wuhan 430070, China
2
Research Institute of Fruit and Tea, Hubei Academy of Agricultural Science, Wuhan 430064, China
*
Authors to whom correspondence should be addressed.
Plants 2020, 9(1), 95; https://doi.org/10.3390/plants9010095
Received: 24 November 2019 / Revised: 31 December 2019 / Accepted: 8 January 2020 / Published: 11 January 2020
(This article belongs to the Section Plant Physiology and Metabolism)
Lanelate navel orange (Citrus sinensis Osbeck) is a late-ripening citrus cultivar increasingly planted in China. The physiological disorder juice sac granulation often occurs in the fruit before harvest, but the physiological and molecular mechanisms underlying this disorder remain elusive. In this study, we found that fruit granulation of the late-ripening navel orange in the Three Gorges area is mainly caused by the low winter temperature in high altitude areas. Besides, dynamic changes of water content in the fruit after freezing were clarified. The granulation of fruit juice sacs resulted in increases in cell wall cellulose and decreases in soluble solid content, and the cells gradually became shrivelled and hollow. Meanwhile, the contents of pectin, cellulose, and lignin in juice sac increased with increasing degrees of fruit granulation. The activities of pectin methylesterase (PME) and the antioxidant enzymes peroxidase (POD), superoxide dismutase, and catalase increased, while those of polygalacturonase (PG) and cellulose (CL) decreased. Furthermore, a total of 903 differentially expressed genes were identified in the granulated fruit as compared with non-disordered fruit using RNA-sequencing, most of which were enriched in nine metabolic pathways, and qRT-PCR results suggested that the juice sac granulation is closely related to cell wall metabolism. In addition, the expression of PME involved in pectin decomposition was up-regulated, while that of PG was down-regulated. Phenylalanine ammonia lyase (PAL), cinnamol dehydrogenase (CAD), and POD related to lignin synthesis were up-regulated, while CL involved in cellulose decomposition was down-regulated. The expression patterns of these genes were in line with those observed in low-temperature treatment as revealed by qRT-PCR, further confirming that low winter temperature is associated with the fruit granulation of late-ripening citrus. Accordingly, low temperature would aggravate the granulation by affecting cell wall metabolism of late-ripening citrus fruit. View Full-Text
Keywords: late-ripening navel orange; low temperature; juice sac granulation; physiological biochemistry; transcript late-ripening navel orange; low temperature; juice sac granulation; physiological biochemistry; transcript
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Wu, L.-M.; Wang, C.; He, L.-G.; Wang, Z.-J.; Tong, Z.; Song, F.; Tu, J.-F.; Qiu, W.-M.; Liu, J.-H.; Jiang, Y.-C.; Peng, S.-A. Transcriptome Analysis Unravels Metabolic and Molecular Pathways Related to Fruit Sac Granulation in a Late-Ripening Navel Orange (Citrus sinensis Osbeck). Plants 2020, 9, 95.

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