In plants, ascorbate (vitamin C) is an essential antioxidant and cofactor in redox reactions. Ascorbate is at the forefront of reactive oxygen species detoxification during pathogen infection and other stress conditions. It is also strongly associated with numerous important cellular processes and general plant development. Ascorbate is predominantly synthesized in the Smirnoff-Wheeler pathway and the amount of ascorbate is tightly regulated at the transcriptional and translational levels
[1].
Humans have lost the ability to synthesize ascorbate and rely on vegetables and fruit for their dietary intake of this key molecule
[2]. Thus breeding high-ascorbate plant cultivars, which will not only serve to enhance people’s dietary intake, but also may help to protect the plant during growth and maintain fruit quality during postharvest storage.
In the paper by An et al.
[3], the authors describe the analysis of progeny of a cross between the white-fleshed apple cv. Orin and the red-fleshed apple cv. Guanghui. There are a number of red-fleshed apple breeding programs around the world aimed at producing novel high-quality apple cultivars
[4]. In that study, the authors show how the segregating red-fleshed fruit contain relatively higher concentrations of ascorbate. In apple, ascorbate concentrations are often relatively low
[5], so efforts to increase this through selective breeding would be highly desirable. A number of QTLs have been identified, collocating with potential ascorbate candidates, including one of the key biosynthetic enzymes, GGP, and the dehydroascorbate reductase gene
DHAR[2]. While some studies have linked the simultaneous production of anthocyanin and ascorbate, this is generally as a result of some stress condition, such as high light
[6]. Red flesh in apple is derived from a mutation in the promoter sequence of the anthocyanin MYB regulator MdMYB10 (MYB1/A), creating a self-activating transcriptional loop
[7]. It is known that MdMYB10 activates the anthocyanin pathway, but until now MdMYB10 has not been linked with either ascorbate biosynthesis or ascorbate recycling.
An and colleagues
[3] show that not only do the red-fleshed fruit have a higher ascorbate concentration, but also a lower concentration of DHA (involved in ascorbate oxidation) as well as enhanced DHAR enzyme expression levels than the white-fleshed progeny. Furthermore, the transcript abundance of
MdMYB1 was positively correlated with the expression level of
MdDHAR. These authors used overexpression of
MdMYB1 in apple callus to verify these findings experimentally. The callus showed increased ascorbate concentrations, again accompanied by lower DHA content, higher DHAR enzyme and higher expression levels of
MdDHAR. Also, the overexpression of
MdMYB1 in apple callus promoted the expression of the ascorbate oxidase gene
MdAO, the ascorbate peroxidase gene
MdAPX1 and the monodehydroascorbate reductase gene
MdMDHAR, indicating that MdMYB1 may contribute to ascorbate homeostasis. To validate the mode of function the authors used both ChIP-PCR assays and EMSA to determine the binding affinity of MdMYB1 to the promoter region of
MdDHAR and validated these results using the dual luciferase assay to show enhanced transcriptional activity of the
MdDHAR promoter in the presence of MYB1.
These findings provide evidence for a new mechanism for increased ascorbate concentrations in apple via the MYB1-induced activation of MdDHAR, leading to increased ascorbate. Combining traits of red flesh and higher vitamin C would seem a useful future breeding target for apple improvement. From a consumer perspective, such an apple would not only be more visually attractive but would also potentially offer enhanced health benefits from both the higher anthocyanin and ascorbate concentrations.
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