Abstract
The ripening of fleshy fruits is coupled with the degradation of both chlorophyll and cell walls, as well as changes in the metabolism of phenylpropanoids, flavonoids, starch/sucrose, and carotenoids. These processes are controlled by phytohormones and other factors, including abscisic acid (ABA), ethylene, auxin, polyamines, sugar, and reactive oxygen species. The ripening of climacteric fruits is controlled by ethylene and non-climacteric fruit ripening is regulated mainly by ABA. Also, ABA and ethylene may interact in both types of fruit ripening. ABA concentrations in fleshy fruits are regulated in response to developmental and environmental cues and are controlled by the relative rates of ABA biosynthesis and catabolism, the former mainly via 9-cis-epoxycarotenoid dioxygenases (NCEDs) and β-glucosidases and the latter via ABA 8'-hydroxylases (CYP707As) and β-glycosyltransferases. In strawberry fruit ripening, ABA is perceived via at least two receptors, Pyrabactin resistance (PYR)/PYR-like (PYL) and putative abscisic acid receptor (ABAR), which are linked separately to the conserved signaling pathway ABA-FaPYR1-FaABIl-FaSnRK2 and the novel signaling pathway ABA-FaABAR-FaRIPK1-FaABI4. Downstream signaling components include important transcription factors, such as AREB (ABA responsive element binding protein)/ABF (ABRE binding factors ABA responsive factor), ethylene response factor (ERF), and V-myb Myeloblastosis viral oncogene homolog (MYB), as well as ripening-related genes. Finally, a comprehensive model of ABA linked to ethylene, sugar, polyamines, auxin and reactive oxygen species in the regulation of strawberry fruit ripening is proposed. Next, new integrated mechanisms, including two ABA signaling pathways, ABA and ethylene signaling pathways, and ABA/ethylene to other phytohormones are interesting and important research topics in ripening, especially in non-climacteric fruits.
Highlights
Angiosperm fruits can generally be divided into dry or fleshy types
The molecular mechanisms of dry fruit development and maturation have been revealed through studies of the model plant Arabidopsis (Gazzarrini and Tsai, 2015), while studies of various non-ripening tomato fruit mutants have defined the mechanisms of fleshy fruit ripening (Liu et al, 2015)
In climacteric fruits, such as tomato and banana, peaks in both respiration and ethylene level occur during ripening; in contrast, no such peaks occur in non-climacteric fruits, such as grape and strawberry, the ripening of which is controlled by abscisic acid (ABA) in an ethylene-independent manner (Kumar et al, 2014; Shen and Rose, 2014)
Summary
Angiosperm fruits can generally be divided into dry or fleshy types. Flowering plants have evolved both conserved and divergent mechanisms for development and maturation of both fleshy and dry fruits (Hershkovitz et al, 2011; Garceau et al, 2017). In non-climacteric strawberry fruit, ABA and auxin (IAA) are important regulators, functioning together or independently in response to developmental and environmental cues; auxin is involved in receptacle development and ABA participates in fruit ripening. Thereby, the interaction of ABA with ethylene plays a vital role in non-climacteric fruit ripening.
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