Apple (Malus × domestica Borkh.) fruit firmness is a crucial determinant of fruit quality and postharvest storage capacity. The DNA-binding one-finger (Dof) transcription factor family has been demonstrated to play a significant role in regulating fruit ripening and softening; however, the genome-wide role of Dofs in determining apple firmness remains unclear. Here, we examined the expression patterns of 50 MdDofs in two apple varieties, ‘Fuji’ and ‘Cripp's Pink’, known for their distinct fruit firmness and cell wall components during fruit development and ripening. We also clarified the function and regulatory network of the MdDof43 in regulating apple fruit firmness. Functional validation indicated that MdDof43 enhanced the water-soluble pectin content, increased neutral sugar loss, decreased hemicellulose content and fruit firmness in transgenic apple, calli and tomato. RNA sequencing analysis revealed that MdDof43 overexpression induced the expression levels of many genes involved in the cell wall degradation in apple calli. DNA affinity purification and sequencing (DAP seq) demonstrated that MdDof43 directly binds to the promoters of Mdβ-Gal2 and Mdα-AF3, two genes encoding pectin side-chain regulating enzymes. In vitro assays confirmed that MdDof43 positively regulates the expression of Mdβ-Gal2 and Mdα-AF3, facilitating the degradation of cell wall components and promoting fruit softening. These findings provide insights into the regulatory mechanisms of cell wall structures that contribute to apple fruit firmness, which has practical significance for fruit quality in breeding.
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The aim of this study was to explore the effects of different color of fruit bags on the fruit quality of Ruixue apple (Malus × domestica Ruixue), so as to provide the theoretical guidance and technical support for the production of high-quality Ruixue apples.
The experimental materials were five-year-old trees of Ruixue using M26 as rootstock, bagged with red, orange, yellow, green, blue, indigo and purple single-layer fruit bags at 70 days after full bloom, with white bags as control. The fruits were harvested at 190 days after full bloom. The transmitted light spectrum of bags was measured by spectrometer. The indicators of fruit quality were determined, such as peel chlorophyll, ascorbic acid, soluble sugar and aroma compounds contents. To explore the effects of different light qualities on fruit quality, the relationship between light and fruit quality was analyzed by correlation analysis.
The difference in the color of the fruit bags changed the composition of the light quality and had a positive correlations between light quality and fruit bags. Compared with white bag control, the transmittances of purple in purple bag were the highest, which were 1.15 times than that in white bag; meanwhile, the transmittances of red in red bag were the highest (1.13 times). The external quality of fruits was significantly improved after the treatment of different color fruit bags. Among the indicators, the L* and b* values of the peel were higher than white bag control, while the a* value, fruit point index and coloration index were lower. Compared with other color fruit bags, some indicators of purple fruit bags were significantly improved (P < 0.05), such as chlorophyll (0.0548 mg·g-1), total phenolic (1.325 mg·g-1), and total flavonoid content of fruits (1.361 mg·g-1). Under different light quality environments, there were significant differences in fruit firmness, and the fruit firmness of the red fruit bag was significantly lower, which was 16.1% lower than white bag control. In addition, other internal quality measurements showed that the indigo fruit bags significantly improved the flavor of the fruits, which improved mainly soluble solids, sugar components and sweetness value. The detection of aroma compounds showed that the blue fruit bag significantly increased the total amount of fruit aroma compounds, which was significantly increased by 2.89 times compared with white bag control. Based on the above indicators, a correlation analysis between light quality and various quality indicators was conducted, and found that there was a close correlation between fruit external quality and light quality. The correlation coefficients between the purple/indigo, orange, and purple/blue values and the L*, a*, and b* values of the peel were 0.901, 0.810, and 0.854, respectively. The internal quality of fruit, such as chlorophyll, total flavonoid, total phenolic and total aroma compounds, were most closely related to blue light, and the correlation coefficients of them with blue light were 0.947, 0.822, 0.851 and 0.757, respectively.
According to the comprehensive analysis, purple and blue light were beneficial to improve the external and internal quality of Ruixue apple. Therefore, the transmission amount of purple and blue light could be appropriately increased in the fruit bag during Ruixue bagging cultivation to further improve fruit quality.
Apple fruit firmness is a crucial index for measuring the internal quality of apples, which influences palatability, storage capacity and transportability. The primary cause of reduced firmness during fruit development is the hydrolysis of cell wall polysaccharides. Xyloglucan endotransglycosylase/hydrolase (XTH) is a key enzyme involved in the depolymerization of cell wall polysaccharides, but the mechanism of its involvement in the formation of fruit firmness remains unclear. Here, we identified the gene MdXTH2 by integrating metabolomic and transcriptomic data, and analyzed its function and molecular mechanism in the formation of apple fruit firmness. The results showed downward trends in both fruit firmness and cell wall components throughout fruit development. The contents of cell wall material, cellulose, and hemicellulose in various apple varieties exhibited significant positive correlations with firmness, with total correlation coefficients of 0.862, 0.884, and 0.891, respectively. Overexpression of MdXTH2 significantly increased fruit firmness in apple and tomato, inhibited fruit ripening, and significantly suppressed the growth of calli. The upstream transcription factor MdNAC72 of the MdXTH2 gene can promote the expression of fruit ripening-related genes. Furthermore, dual-luciferase, yeast one-hybrid, and electrophoretic mobility shift assay (EMSA) demonstrated that MdNAC72 down-regulates the transcription of MdXTH2 by binding to its promoter. In summary, the results of this study provide a strategy for examining fruit quality regulation and a theoretical basis for breeding apple varieties with moderate firmness through genetic improvement.
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Research Article
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Apples are one of the most important economic crops worldwide. Because of global warming and an aggravation of environmental, abnormally high temperatures occur frequently in fruit-growing season and seriously affect normal fruit growth and reduce fruit quality and yield. We took five-year-old ‘Ruixue’ (Qinfu 1 × Pink Lady; CNA20151469.1) fruits as test materials, and the ambient temperature during fruit development was monitored. The results showed that during the fruit-growing season, especially during the rapid growth stage (July to August), the maximum daily temperature exceeded 30 ℃ and lasted for more than 40 days. To determine the effects of high temperature stress on the apple fruit resistance, we treated expanding, veraison, and maturity-period fruits at different temperatures. It was found that the fruits of the expanding period showed strong resistance to high temperature stress, whereas during veraison and maturity, fruit resistance to high temperature stress decreased, and the fruit peel browning phenotype appeared. Meanwhile, the content of malonaldehyde (MDA), hydrogen peroxide (H2O2), and superoxide anion (O2.-) in the peel gradually increased with increasing temperature. The content of total phenols, flavanol, and flavonoids in the peel decreased substantially at 45 ℃. Moreover, it was found that polyphenol oxidase gene (MdPPO1) was most sensitive to high temperature stress in apple. Furthermore, transient and stable MdPPO1 overexpression significantly promoted peel browning. The transgenic materials were more sensitive to high temperatures, and browning was more severe compared to non-genetically modified organism (WT). Stable MdPPO1 knockout calli obtained via clustered regularly interspersed short palindromic repeats (CRISPR/Cas9) gene knockout technology reduced the browning phenotype, and the resultant fruits were not sensitive to the effects of high temperature stress. Thus, MdPPO1 expression may be a key factor of high temperature–related changes observed in the browning phenotype that provides a scientific theoretical basis for the selection of high temperature–resistant varieties and apple cultivation and management in the future.
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