Comparative Transcriptome Analyses between a Spontaneous Late-Ripening Sweet Orange Mutant and Its Wild Type Suggest the Functions of ABA, Sucrose and JA during Citrus Fruit Ripening

Abstract

A spontaneous late-ripening mutant of ‘Jincheng’ (C. sinensis L. Osbeck) sweet orange exhibited a delay of fruit pigmentation and harvesting. In this work, we studied the processes of orange fruit ripening through the comparative analysis between the Jincheng mutant and its wild type. This study revealed that the fruit quality began to differ on 166th days after anthesis. At this stage, fruits were subjected to transcriptome analysis by RNA sequencing. 13,412 differentially expressed unigenes (DEGs) were found. Of these unigenes, 75.8% were down-regulated in the wild type, suggesting that the transcription level of wild type was lower than that of the mutant during this stage. These DEGs were mainly clustered into five pathways: metabolic pathways, plant-pathogen interaction, spliceosome, biosynthesis of plant hormones and biosynthesis of phenylpropanoids. Therefore, the expression profiles of the genes that are involved in abscisic acid, sucrose, and jasmonic acid metabolism and signal transduction pathways were analyzed during the six fruit ripening stages. The results revealed the regulation mechanism of sweet orange fruit ripening metabolism in the following four aspects: First, the more mature orange fruits were, the lower the transcription levels were. Second, the expression level of PME boosted with the maturity of the citrus fruit. Therefore, the expression level of PME might represent the degree of the orange fruit ripeness. Third, the interaction of PP2C, PYR/PYL, and SnRK2 was peculiar to the orange fruit ripening process. Fourth, abscisic acid, sucrose, and jasmonic acid all took part in orange fruit ripening process and might interact with each other. These findings provide an insight into the intricate process of sweet orange fruit ripening.

Exogenous Application of Sucrose Promotes Postharvest Ripening of Kiwifruit

“…Exogenous sucrose can accelerate the ripening of strawberry fruit [14]. In mutant sweet orange, which has a lower sucrose content, fruit ripening is delayed [40]. In the climacteric fruit tomato, exogenous sucrose treatment can promote postharvest fruit ripening by affecting its metabolism and enhancing ethylene synthesis and signal transduction [15].…”

Low temperature transcriptionally modulates natural peel degreening in lemon (Citrus limon L.) fruit independently of endogenous ethylene

“…JAs have thus far been studied in the context of plant adaptive responses to various biotic and abiotic stresses (Zhang et al ., 2019). However, JA has also been shown to promote fruit ripening in citrus (Zhang et al ., 2014), strawberry (Concha et al ., 2013) and tomato (Liu et al ., 2012). This is consistent with the present findings as lemon fruit degreening was accompanied by an increase in the levels of JA-Ile (Fig.…”

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“…The NCED1 were expressed only at the onset stage of ripening in peach and grape, when ABA content became high (Zhang et al, 2009a). Zhang et al (2014) studied the mechanism of a spontaneous late-maturing mutant of 'Jincheng' sweet orange and its wild type through the comparative analysis. The highest expression of CsNCED1 was at 215 DAA in WT.…”

“…ABA treatment can rapidly induced flavonol and anthocyanin accumulation in berry skins of the Cabernet Sauvignon grape suggesting that ABA could stimulate berry ripening and ripening-related gene expression (Koyama et al, 2010). ABA also participated in the regulation of fruit development and ripening of tomato (Zhang et al, 2009b;Sun et al, 2011), cucumber (Wang et al, 2013), strawberry (Jia et al, 2011), bilberry (Karppinen et al, 2013), citrus (Zhang et al, 2014) and grape (Nicolas et al, 2014). Recent studies showed that ABA is a positive regulator of ripening and exogenous ABA application could effectively regulate citrus fruit maturation (Wang et al, 2016).…”

“…However, there is little information available about the role of NCED1 genes in citrus fruit maturation (Zhang et al, 2014).…”