Flavonoids are major polyphenol compounds in plant secondary metabolism. Wild red-fleshed apples (Malus sieversii f. niedzwetzkyana) are an excellent resource because of their much high flavonoid content than cultivated apples. In this work, R6R6, R6R1 and R1R1 genotypes were identified in an F segregating population of M. sieversii f. niedzwetzkyana. Significant differences in flavonoid composition and content were detected among the three genotypes by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry analysis. Furthermore, two putative flavonoid-related genes encoding R2R3-MYB transcription factors, designated MYB12 and MYB22, were cloned and characterized. The expression patterns of MYB12 and MYB22 directly correlated with those of leucoanthocyanidin reductase and flavonol synthase, respectively. Their roles in flavonoid biosynthesis were identified by overexpression in apple callus and ectopic expression in Arabidopsis. MYB12 expression in the Arabidopsis TT2 mutant complemented its proanthocyanidin-deficient phenotype. Likewise, MYB22 expression in an Arabidopsis triple mutant complemented its flavonol-deficient phenotype. MYB12 could interact with bHLH3 and bHLH33 and played an essential role in proanthocyanidin synthesis. MYB22 was found to activate flavonol pathways by combining directly with the flavonol synthase promoter. Our findings provide a valuable perspective on flavonoid synthesis and provide a basis for breeding elite functional apples with a high flavonoid content.

In plants, flavonoids play critical roles in resistance to biotic and abiotic stresses, and contribute substantially to the quality, flavor, and nutritional quality of many fruit crops. In apple (Malus × domestica), inbreeding has resulted in severe decreases in the genetic diversity and flavonoid content. Over the last decade, we have focused on the genetic improvement of apple using wild red-fleshed apple resources (Malus sieversii f. niedzwetzkyana). Here, we found that the MYB transcription factors (TFs) involved in the synthesis of proanthocyanidins can be classified into TT2 and PA1 types. We characterized a PA1-type MYB transcription factor, MdMYBPA1, from red-fleshed apple and identified its role in flavonoid biosynthesis using overexpression and knockdown-expression transgenes in apple calli. We explored the relationship between TT2- and PA1-type MYB TFs, and found that MdMYB9/11/12 bind the MdMYBPA1 promoter. In addition, MdMYBPA1 responded to low temperature by redirecting the flavonoid biosynthetic pathway from proanthocyanidin to anthocyanin production. In binding analyses, MdbHLH33 directly bound to the low-temperature-responsive (LTR) cis-element of the MdMYBPA1 promoter and promotes its activity. In addition, the calli expressing both MdMYBPA1 and MdbHLH33, which together form a complex, produced more anthocyanin under low temperature. Our findings shed light on the essential roles of PA1-type TFs in the metabolic network of proanthocyanidin and anthocyanin synthesis in plants. Studies on red-fleshed wild apple are beneficial for breeding new apple varieties with high flavonoid contents.

Ultraviolet‐B (UV‐B) radiation and low temperature promote the accumulation of anthocyanins, which give apple skins their red colour. Although many transcription regulators have been characterized in the UV‐B and low‐temperature pathways, their interregulation and synergistic effects are not well understood. Here, a B‐box transcription factor gene, MdBBX20, was characterized in apple and identified to promote anthocyanin biosynthesis under UV‐B conditions in field experiments and when overexpressed in transgenic apple calli. The transcript level of MdBBX20 was significantly induced by UV‐B. Specific G‐box elements in the promoters of target genes were identified as interaction sites for MdBBX20. Further experimental interrogation of the UV‐B signalling pathways showed that MdBBX20 could interact with MdHY5 in vitro and in vivo and that this interaction was required to significantly enhance the promoter activity of MdMYB1. MdBBX20 also responded to low temperature (14°C) with the participation of MdbHLH3, which directly bound a low temperature‐response cis elements in the MdBBX20 promoter. These findings demonstrate the molecular mechanism by which MdBBX20 integrates low‐temperature‐ and UV‐B‐induced anthocyanin accumulation in apple skin.

This paper aimed to study the time course changes in taste compounds of Dezhou-braised chicken during the entire cooking process mainly consisting of deep-frying, high-temperature boiling, and low-temperature braising steps. For this purpose, meat samples at different processing stages were analyzed for 5'-nucleotides and free amino acids, and were also subjected to electronic tongue measurements. Results showed that IMP, Glu, Lys, and sodium chloride were the main compounds contributing to the taste attributes of the final product. IMP and Glu increased in the boiling step and remained unchanged in the following braising steps. Meanwhile, decrease in Lys content and increase in sodium chloride content were observed over time in both boiling and braising steps. Intensities for bitterness, saltiness, and Aftertaste-B obtained from the electronic tongue analysis were correlated with the concentrations of these above chemical compounds. Therefore, the electronic tongue system could be applied to evaluate the taste development of Dezhou-braised chicken during processing.

Flavonoids play essential roles in human health. Apple (Malus domestica Borkh.), one of the most widely produced and economically important fruit crops in temperate regions, is a significant source of flavonoids in the human diet and is among the top nutritionally rated and most widely consumed fruits worldwide. Epidemiological studies have shown that the consumption of apples, which are rich in a variety of free and easily absorbable flavonoids, is associated with a decreased risk of various diseases. However, apple production is challenged by serious inbreeding problems. The narrowing of the hereditary base has resulted in apples with poor nutritional quality and low flavonoid contents. Recently, there have been advances in our understanding of the roles that Malus sieversii (Ledeb.) M.Roem has played in the process of apple domestication and breeding. In this study, we review the origin of cultivated apples and red-fleshed apples, and discuss the genetic diversity and construction of the core collections of M. sieversii. We also discuss current research progress and breeding programs on red-skinned and red-fleshed apples and summarize the exploitation and utilization of M. sieversii in the breeding of high-flavonoid, and red-fleshed apples. This study highlights a valuable pattern of horticultural crop breeding using wild germplasm resources. The future challenges and directions of research on the molecular mechanisms of flavonoid accumulation and high-flavonoid apple breeding are discussed.