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

Li-Ming Wu,Ce Wang,Ying-Chun Jiang,Shu-Ang Peng,Ji-Hong Liu,Li-Gang He,Jun-Fan Tu,Zhu Tong,Zhi-Jing Wang,Wen-Ming Qiu,Fang Song

doi:10.3390/plants9010095

Li-Ming Wu, Ce Wang + Show 9 more

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https://doi.org/10.3390/plants9010095

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Abstract

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.

Highlights

Late-ripening citrus (Citrus sinensis Osbeck) needs overwintering cultivation due to its late natural maturation
At the altitude of 520 m, the average monthly temperature in the orchards dropped to 3.8 ◦ C in February, and the extremely low temperature was part of fruit peel showing freezing damage spots and oil gland collapse, and fruit falling was observed in late February (300 days after flowering (DAF))
No fruit granulation was detected at 235 DAF in late-ripening citrus orchards at 520 m altitude in the Three Gorges area (Figure 1B)

Summary

Introduction

Late-ripening citrus (Citrus sinensis Osbeck) needs overwintering cultivation due to its late natural maturation. Plants 2020, 9, 95 structure of citrus varieties, as well as to prolong the maturation and achieve a year-round fresh fruit supply. The Three Gorges Reservoir is the most suitable area for the planting of late-ripening citrus in. Freezing injury should be avoided for late-ripening varieties so that the fruit can survive the winter safely. In commercial production, the late-ripening variety is vulnerable to the influence of low temperature in winter and the fruits are prone to granulation. Overwintering fruits in nature are affected by low temperature in winter [2], and may show severe granulation symptoms at the flowering and shoot growth stages of the following year, but the role of temperature in fruit granulation mechanism is still unknown

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