Abstract
Peel degreening is an important aspect of fruit ripening in many citrus fruit, and previous studies have shown that it can be advanced by ethylene treatment or by low-temperature storage. However, the important regulators and pathways involved in natural peel degreening remain largely unknown. To determine how natural peel degreening is regulated in lemon fruit (Citrus limon), we studied transcriptome and physiochemical changes in the flavedo in response to ethylene treatment and low temperatures. Treatment with ethylene induced rapid peel degreening, which was strongly inhibited by the ethylene antagonist, 1-methylcyclopropene (1-MCP). Compared with 25 ºC, moderately low storage temperatures of 5-20 °C also triggered peel degreening. Surprisingly, repeated 1-MCP treatments failed to inhibit the peel degreening induced by low temperature. Transcriptome analysis revealed that low temperature and ethylene independently regulated genes associated with chlorophyll degradation, carotenoid metabolism, photosystem proteins, phytohormone biosynthesis and signalling, and transcription factors. Peel degreening of fruit on trees occurred in association with drops in ambient temperature, and it coincided with the differential expression of low temperature-regulated genes. In contrast, genes that were uniquely regulated by ethylene showed no significant expression changes during on-tree peel degreening. Based on these findings, we hypothesize that low temperature plays a prominent role in regulating natural peel degreening independently of ethylene in citrus fruit.
Highlights
Fruit ripening is a multifaceted process comprising various physiochemical and structural changes such as softening, degradation of starch to sugars, development of colour, and production of aroma volatiles (Cherian et al, 2014; Seymour and Granell, 2014)
All the three identified carotenoids displayed a substantial decrease after 4 d of ethylene treatment, and the ethylene-induced reduction was inhibited by pretreatment with 1-MCP (Fig. 1C).Taken together, these results demonstrated that ethylene played a key role in regulating peel degreening, and that 1-MCP treatment effectively rendered the fruit insensitive to ethylene
In agreement with these findings, our results showed that moderately low storage temperatures (5–20 °C) promoted peel degreening in lemon fruit, and induced substantial changes in chlorophyll and carotenoid contents (Fig. 2)
Summary
Fruit ripening is a multifaceted process comprising various physiochemical and structural changes such as softening, degradation of starch to sugars, development of colour, and production of aroma volatiles (Cherian et al, 2014; Seymour and Granell, 2014). Colour development, commonly known as peel degreening, is a critical part of ripening and is characterized by a colour change in the peel from green to yellow/red/orange (Iglesias et al, 2007). Peel degreening is an important aspect for marketability of citrus fruit (Porat, 2008), and there is wide interest. The second pathway is carotenoid biosynthesis, which starts with the condensation of two geranylgeranyl pyrophosphate (GGPP) molecules by phytoene synthase (PSY) to form phytoene. Genes encoding various enzymes for the main steps of chlorophyll degradation and carotenoid metabolism have been isolated and functionally characterized (Rodrigo et al, 2013)
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