Characterization of the stage dependency of high temperature on green ripening reveals a distinct chlorophyll degradation regulation in banana fruit

Bananas (Musa, MA group) develop a bright yellow appearance when ripen at temperatures between 18 degrees C and 24 degrees C. At high ambient temperatures, the fruit display green-ripe and become un-acceptable in the market. To understand the mechanisms that govern the color changing during banana green-ripening, the influence of high temperature on chlorophyll degradation at various ripening stages were investigated. After ripening initiation, the fruit were subjected to 8 different temperature changing regimes at both 20 degrees C and 30 degrees C in 5 days. The ripening and senescence parameters were monitored and relevant gene expression was analyzed by RNA gel blot hybridization. Temperature during the first 3 days, or precisely the stage around the on-set of the respiration peak, was found to be critical for color formation in bananas. When the fruit were held at 30 degrees C at this stage, the fruit developed green-ripe with >30% of the original chlorophyll remaining, regardless the other stages were at 20 degrees C. The repressed chlorophyll degradation by 30 degrees C was positively correlated to the reduced expression of several key genes functioning in chlorophyll degradation, MaSGR, MaNYC and MaPaO, which behaved in a closely temperature responding manner. Conversely to chlorophyll degradation patterns, moving to 30 degrees C induced fruit respiration rate, the expression of MaACO, an ethylene synthesis related gene, and accelerated the decrease in fruit firmness and chlorophyll florescence (F-v/F-m), indicating that the ripening of the fruit was accelerated at 30 degrees C. Accordingly, in the context of fruit ripening but under not agreeable temperature (30 degrees C), the expression of chlorophyll degradation genes in bananas was predominantly regulated by temperature, not responding to the fruit ripening proceeding. Interestingly, by using the alternating temperature system, we here the first time reported the sensitivity of chlorophyll degradation genes, MaSGR, MaPAO and MaNYC, to high temperature during banana fruit ripening. (C) 2014 Elsevier B.V. All rights reserved.