Association study between endogenous hormones and sugar-acid metabolism during fruit development in Yanzhihong apricot

【Objective】In this study, the molecular mechanisms regulating endogenous hormones and sugar- acid metabolism during fruit development were examined in Yanzhihong apricot.【Methods】 Based on the anatomical characteristics, the growth and development of the fruit were categorized into four stages: rapid growth (S1), core-hardening (S2), expansion (S3), and maturation (S4). At each stage, ten fruits with uniform size, consistent appearance, and non-damage were selected for sampling. Three small pieces were taken from different parts of each fruit, sectioned into uniform pieces, placed into centrifuge tubes, and wrapped in aluminum foil. The samples were immediately frozen in liquid nitrogen and stored at -80 ℃ after returning to the laboratory. Using transcriptomic analysis, hormone content measurements, sugar- acid component analysis, and weighted gene co- expression network analysis (WGCNA), we aimed to elucidate the complex interactions between hormonal regulation and metabolic pathways influencing fruit quality. At different stages of apricot fruit development, we measured the levels of key endogenous hormones: gibberellins (GA), indole-3-acetic acid (IAA), abscisic acid (ABA), cytokinins (CKs), and brassinosteroids (BRs), using enzyme-linked immunosorbent assay (ELISA) kits. 【Results】Significant fluctuations in key hormones were observed as the apricot fruit developed. GA levels decreased gradually from the rapid growth stage to maturity, reflecting its role in cell division and elongation during early development. IAA peaked in mid-development before declining, confirming itsrole in regulating cell division and elongation. ABA levels increased progressively, highlighting its role in maturation and stress response. CKs gradually increased, peaking in mid- development, suggesting their role in promoting fruit enlargement and cell proliferation. We also analyzed changes in sugar and organic acid contents, which directly affected fruit flavor. Glucose, fructose, and sucrose levels were measured using specific assays. Glucose and fructose levels decreased significantly as the fruit matured, with glucose dropping from 32.33 mg·g-1 to 7.50 mg·g-1 and fructose from 8.04 mg·g-1 to 3.34 mg·g-1 . In contrast, sucrose levels increased from 59.43 mg·g-1 during the rapid growth stage to 87.22 mg·g-1 in the mature fruit, suggesting sucrose became the dominant sugar. Organic acid levels, including citric acid and malic acid, decreased during maturation, with citric acid dropping from 20.59 mg·g-1 to 5.50 mg·g-1 and malic acid from 8.91 mg · g-1 to 2.77 mg · g-1 , contributing to increased sweetness. RNA sequencing (RNA- seq) was conducted on apricot fruit from four developmental stages. Total RNA was extracted, and sequencing was generated over 103.98 Gb of high-quality data. Reads were aligned to the Prunus sibirica genome, achieving a high alignment rate (88.62%- 92.76% ). Differentially expressed genes (DEGs) were identified: 1444 DEGs between S1 and S2, 4588 DEGs between S2 and S3, and 177 DEGs between S3 and S4. These DEGs highlighted metabolic shifts, especially in hormone biosynthesis and sugar-acid metabolism. Functional annotation revealed key pathways related to sucrose metabolism, with genes such as SUS1 and GH9C2 upregulated during maturation, correlating with increased sucrose content. Similarly, genes involved in starch synthesis, including F10A12.18, GBSS1, and BMY3, exhibited differential expression, consistent with changes in starch and sugar metabolism. Weighted gene co-expression network analysis (WGCNA) identified highly connected modules related to fruit quality traits. The blue module, consisting of 4154 genes, was correlated with the sucrose content (r = 0.86, P = 0.000 33). The turquoise module, with 11 961 genes, showed strong correlations with glucose (r = 0.98, P = 2.4 × 10- 8 ), fructose (r = 0.81, P = 0.001 4), citric acid (r = 0.72, P = 0.008 3), and malic acid (r = 0.86, P = 0.000 33), indicating their role in regulating the sugar-acid balance. WGCNA identified key hub genes, such as ERF071, CNGC1, and HAT14, with high connectivity, suggesting they regulated fruit quality through both hormonal regulation and metabolic pathways. ERF071 and CNGC1 were likely the central regulators of sugar and organic acid metabolism, while HAT14 influenced cytokinin and auxin signaling pathways, linking hormone regulation to fruit development.【Conclusion】In conclusion, this study thoroughly analyzes changes in endogenous hormones and sugar-acid metabolism in apricot fruits at different developmental stages and identifies key regulatory genes that could be used as targets for genetic improvement. These findings not only offer a new molecular basis for improving apricot fruit quality, but also provide a theoretical foundation for future apricot cultivar breeding and quality enhancement. Specifically, in apricot storage and processing, maintaining fruit flavor and nutritional content while preventing over-ripening and quality decline will be a key focus for future research. However, this study does not explore the precise causal relationship between hormones and metabolism, a question that should be addressed through further experimental designs, such as combining hormone application and gene editing techniques to validate the causal mechanisms. Additionally, there is room for deeper analysis of key time points in apricot fruit development, including the impact of different cultivation conditions and environmental factors on endogenous hormones and metabolites, as well as the environmental adaptability of gene expression. Future research can focus on several aspects: Firstly, exploring the mechanisms by which endogenous hormones influence different metabolic pathways during apricot fruit development, particularly using gene editing techniques to validate the function of key regulatory genes; Secondly, expanding the study on metabolic pathway differencesacross apricot cultivars under various climatic conditions to select those that are highly adaptable to environmental changes and exhibit stable quality; Thirdly, exploring how to apply these molecular regulatory mechanisms to apricot fruit cultivation management and post-harvest technologies to enhance fruit market competitiveness and storability. In summary, this study provides new molecular targets for apricot genetic improvement and offers valuable theoretical guidance and practical advice for fruit tree cultivation management and quality enhancement, with significant academic and practical implications.