Anthocyanins accumulation and related gene expression in red orange fruit induced by low temperature storage

The aim of this work was to study the impact of moderately long storage periods at 4°C upon red orange [Citrus sinensis (L.) Osbeck] anthocyanins production and the expression of structural genes involved in their biosynthesis such as phehylalanine ammonia lyase (PAL), chalcone synthase (CHS), dihydroflavonol 4-reductase (DFR), and UDP-glucose flavonoid glucosyl transferase (UFGT). Our results showed that low temperature-induced anthocyanins accumulation in red orange juice vesicles after 75 days reached values eight times higher than those kept at 25°C. Furthermore, real-time polymerase chain reaction showed that expression of PAL, CHS, DFR, and UFGT was strongly induced during low temperature exposure since levels of all transcripts increased at least 40-fold with respect to control samples. Interestingly, in orange fruits subjected to a brief exposure at low temperature (45 days) and subsequently kept at 25°C, the anthocyanins content dropped although samples still maintained higher levels of these pigments than those registered in control oranges. Concordantly, the expression of chs, dfr, and ufgt declined upon return to control conditions, but it was always much higher in samples subjected to brief cold induction than in the control samples. On the contrary, the amount of PAL transcripts became negligible immediately after the temperature change from 4 to 25°C, thus indicating that "early" and "late" genes, respectively, implicated in the first and in the last steps leading to the anthocyanins, might be affected by different regulation mechanisms.