Expansin gene expression in cherimoya fruit is correlated with flesh firmness during fruit ripening and softening

Abstract

SummaryExpansins are proteins that have been shown to contribute to fruit softening. However, whether expansins are involved in the loss of flesh firmness in harvested cherimoya fruit has not yet been confirmed. In this study, three different full-length expansin gene (AcExp1, AcExp2, and AcExp3) cDNAs from ripe cherimoya fruit were isolated and characterised, and their patterns of mRNA expression were investigated in relation to flesh firmness in fruit stored at different temperatures (10°C, 15°C, or 20°C) and in fruit stored at 20°C and pretreated with propylene. The results showed that flesh firmness began to decline on day-2 after storage and declined rapidly by day-4. Meanwhile, ethylene production increased significantly by day-4 after storage at 20°C. Flesh firmness declined slowly and fruit exhibited a low level of ethylene production throughout storage at 10°C. RNA hybridisation revealed that AcExp1 mRNA was detectable throughout 8 d of storage at 10°C, 15°C, or 20°C, and that AcExp2 mRNA was detectable only after the decline in flesh firmness in fruit stored at 15°C or 20°C. Moreover, the accumulation of AcExp3 mRNA coincided with the decline in flesh firmness in fruit stored at 15°C or 20°C. Propylene treatment increased ethylene production, accelerated the decline in flesh firmness in fruit stored at 20°C, and increased the accumulation of AcExp2 and AcExp3 mRNAs. These findings therefore show that AcExp3, together with AcExp2, may be involved in cherimoya fruit softening.