Accumulation of soluble sugars in peel at high temperature leads to stay-green ripe banana fruit

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Abstract
Bananas (Musa acuminata, AAA group) fail to develop a yellow peel and stay green when ripening at temperatures >24 °C. The identification of the mechanisms leading to the development of stay-green ripe bananas has practical value and is helpful in revealing pathways involved in the regulation of chlorophyll (Chl) degradation. In the present study, the Chl degradation pathway was characterized and the progress of ripening and senescence was assessed in banana peel at 30 °C versus 20 °C, by monitoring relevant gene expression and ripening and senescence parameters. A marked reduction in the expression levels of the genes for Chl b reductase, SGR (Stay-green protein), and pheophorbide a oxygenase was detected for the fruit ripening at 30 °C, when compared with fruit at 20 °C, indicating that Chl degradation was repressed at 30 °C at various steps along the Chl catabolic pathway. The repressed Chl degradation was not due to delayed ripening and senescence, since the fruit at 30 °C displayed faster onset of various ripening and senescence symptoms, suggesting that the stay-green ripe bananas are of similar phenotype to type C stay-green mutants. Faster accumulation of high levels of fructose and glucose in the peel at 30 °C prompted investigation of the roles of soluble sugars in Chl degradation. In vitro incubation of detached pieces of banana peel showed that the pieces of peel stayed green when incubated with 150 mM glucose or fructose, but turned completely yellow in the absence of sugars or with 150 mM mannitol, at either 20 °C or 30 °C. The results suggest that accumulation of sugars in the peel induced by a temperature of 30 °C may be a major factor regulating Chl degradation independently of fruit senescence.

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