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Sunburn in apple peel significantly affects fruit appearance and reduces its commercial value. Previous research has shown that apple peel reduces sunburn by increasing the accumulation of proanthocyanidins (PAs) and other protective compounds. However, the precise molecular regulatory mechanism remains unclear. In this study, we systematically investigated MdANR, a key gene involved in PAs biosynthesis. We found that MdANR expression in apple peel is responsive to temperature and light fluctuations, with higher expression levels observed under increased temperature and light exposure. Functional analysis revealed that MdANR overexpression in apple peel and callus enhanced resistance to high-temperature and -light-intensity stress, accompanied by a corresponding increase in PAs and chlorogenic acid contents. In addition, we demonstrated that MdMYBR9 can activate MdANR promoter activity and promote its expression through yeast one-hybrid, dual-luciferase, and electrophoretic mobility transfer experiments. The results indicated that MdMYBR9 was an upstream regulator of MdANR. Based on these findings, this study proposes the MdMYBR9-MdANR-PAs regulatory model for apple sunburn resistance, providing a molecular framework for enhancing sunburn tolerance in apple breeding programs. Full article

Flower color is a charming phenotype with very important ornamental and commercial values. Anthocyanins play a critical role in determining flower color pattern formation, and their biosynthesis is typically regulated by R2R3-MYB transcription factors (TFs). Paeonia suffruticosa is a famous ornamental plant with colorful flowers. However, little is known about the R2R3-MYB TFs that regulate anthocyanin accumulation in P. suffruticosa. In the present study, two R2R3-MYB TFs, namely, PsMYB114L and PsMYB12L, were isolated from the petals of P. suffruticosa ‘Shima Nishiki’ and functionally characterized. Sequence analysis suggested that PsMYB114L contained a bHLH-interaction motif, whereas PsMYB12L contained two flavonol-specific motifs (SG7 and SG7-2). Subsequently, the in vivo function of PsMYB114L and PsMYB12L was investigated by their heterologous expression in Arabidopsis thaliana and apple calli. In transgenic Arabidopsis plants, overexpression of PsMYB114L and of PsMYB12L caused a significantly higher accumulation of anthocyanins, resulting in purple-red leaves. Transgenic apple calli overexpressing PsMYB114L and PsMYB12L also significantly enhanced the anthocyanins content and resulted in a change in the callus color to red. Meanwhile, gene expression analysis in A. thaliana and apple calli suggested that the expression levels of the flavonol synthase (MdFLS) and anthocyanidin reductase (MdANR) genes were significantly downregulated and the dihydroflavonol 4-reductase (AtDFR) and anthocyanin synthase (AtANS) genes were significantly upregulated in transgenic lines of PsMYB114L. Moreover, the expression level of the FLS gene (MdFLS) was significantly downregulated and the DFR (AtDFR/MdDFR) and ANS (AtANS/MdANS) genes were all significantly upregulated in transgenic lines plants of PsMYB12L. These results indicate that PsMYB114L and PsMYB12L both enhance anthocyanin accumulation by specifically regulating the expression of some anthocyanin biosynthesis-related genes in different plant species. Together, these results provide a valuable resource with which to further study the regulatory mechanism of anthocyanin biosynthesis in P. suffruticosa and for the breeding of tree peony cultivars with novel and charming flower colors. Full article