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Summary: Carotenoids contribute to fruit coloration and are valuable sources of provitamin A in the human diet. Abscisic acid (ABA) plays an essential role in fruit coloration during citrus fruit ripening, but little is known about the underlying mechanisms. Here, we identified a novel bZIP transcription activator called CsbZIP44, which serves as a central regulator of ABA‐mediated citrus carotenoid biosynthesis. CsbZIP44 directly binds to the promoters of four carotenoid metabolism‐related genes (CsDXR, CsGGPPs, CsBCH1 and CsNCED2) and activates their expression. Furthermore, our research indicates that CsHB5, a positive regulator of ABA and carotenoid‐driven processes, activates the expression of CsbZIP44 by binding to its promoter. Additionally, CsHB5 interacts with CsbZIP44 to form a transcriptional regulatory module CsHB5‐CsbZIP44, which is responsive to ABA induction and promotes carotenoid accumulation in citrus. Interestingly, we also discover a positive feedback regulation loop between the ABA signal and carotenoid biosynthesis mediated by the CsHB5‐CsbZIP44 transcriptional regulatory module. Our findings show that CsHB5‐CsbZIP44 precisely modulates ABA signal‐mediated carotenoid metabolism, providing an effective strategy for quality improvement of citrus fruit and other crops. [ABSTRACT FROM AUTHOR]

• A novel biological postharvest treatment by staying with apple (SWA) is proposed. • SWA treatment promotes peel coloration and maintain fruit freshness. • SWA treatment has similar effects to ethylene treatment on citrus peel coloration. • SWA treatment significantly alleviates calyx senescence caused by ethylene treatment. A novel biological treatment of staying with commercially mature apple fruits was proposed to promote the peel coloration of 'Newhall' navel orange fruit (Citrus sinensis Osbeck), which could conduce to the formation of uniform orange red peel color. At 15 days after treatment (DAT), the staying with apple (SWA) treatment led to a nearly 4-fold increase in the total carotenoid content compared with the control. Specifically, the red color apocarotenoid β-citraurin showed the highest degree of enhancement (~6 fold) under SWA treatment. Simultaneously, the expression level of β-citraurin biosynthetic gene carotenoid cleavage dioxygenase 4b (CCD4b) was dramatically induced (about 35 folds). Furthermore, SWA had the same effect of promoting coloration and obviously alleviate calyx senescence of citrus fruit compared with ethylene treatment. In conclusion, SWA treatment is a potential environment-friendly and recyclable postharvest technology to improve peel color and maintain fruit freshness for citrus. [ABSTRACT FROM AUTHOR]