Abstract
Abscisic acid (ABA) is a key signaling molecule promoting ripening of non-climacteric fruits such as sweet cherry (Prunus avium L.). To shed light on the role of other hormones on fruit development, ripening and anthocyanin production, the synthetic auxin 1-naphthaleneacetic acid (NAA) was applied to sweet cherry trees during the straw-color stage of fruit development. NAA-treated fruits exhibited higher concentrations of 1-aminocyclopropane-1-carboxylic acid (ACC) and ABA-glucose ester (ABA-GE), which are a precursor of ethylene and a primary storage form of ABA, respectively. Consistent with these observations, transcript levels of genes encoding ACC synthase and ACC oxidase, both involved in ethylene biosynthesis, were increased after 6 days of NAA treatment, and both ABA concentration and expression of the regulator gene of ABA biosynthesis (NCED1 encoding 9-cis-epoxycarotenoid dioxygenase) were highest during early fruit ripening. In addition, transcript levels of key anthocyanin regulatory, biosynthetic and transport genes were significantly upregulated upon fruit exposure to NAA. This was accompanied by an increased anthocyanin concentration and fruit weight whilst fruit firmness and cracking index decreased. Altogether our data suggest that NAA treatment alters ethylene production, which in turn induces ripening in sweet cherry and enhanced anthocyanin production, possibly through ABA metabolism. The results from our study highlight the potential to use a single NAA treatment for manipulation of cherry ripening.
Highlights
Fruit species can be classified into two main groups depending on their pattern of ripening
The synthetic hormone naphthaleneacetic acid (NAA) is the compound of choice in applications involving auxins due to its low toxicity and superior metabolic stability compared with naturally occurring auxins or their precursors, such as indole 3-butyric acid (IBA) [40,41,42]
In our study the application of NAA to sweet cherry trees during the straw-color stage of fruit ripening resulted in increased fruit weight and anthocyanin concentration, whilst reducing cracking index and fruit firmness (Table 1, Figure 1)
Summary
Fruit species can be classified into two main groups depending on their pattern of ripening. Past research has established abscisic acid (ABA) as a central signaling molecule for non-climacteric fruit ripening This hormone regulates fruit softening, color change through anthocyanin accumulation, and other compositional modifications during ripening [11,12,13]. In addition to ABA, other phytohormones such as auxins, jasmonic acid (JA), gibberellins (GAs) and cytokinins are involved in the ripening of non-climacteric fruit Regulatory effects of these hormones include increasing fruit size through initiation of cell division and expansion, and the exogenous application of synthetic auxins in particular has resulted in increased fruit weight and size in a variety of fruit species [14,15,16]. In grapevine, which is a non-climacteric fruit species with similar ripening characteristics to sweet cherry, a delay in sugar and anthocyanin accumulation and an increase in fruit size have been observed in different cultivars following the application of natural or synthetic auxins [18,19,20]
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