Research progress of organic acids in fruits

China has a wide variety of fruits, primarily consumed fresh, where fruit quality plays a crucial role in attracting consumers. Fruit quality encompasses both external appearance and internal traits, with the latter primarily consisting of organic acids, soluble sugars, aroma compounds, carotenoids, flavonoids, amino acids, and other metabolites. These diverse metabolic substances collectively influence the flavor quality of fruits. Among these, organic acids directly determine the sourness of the fruits. In recent years, although fruit production has increased, the quality of flavor has shown a noticeable decline, which hinders the steady development of the fruit industry. Consequently, improving the flavor quality of fruit and enhancing production efficiency have become imperative. Organic acids not only are essential primary metabolites in fruits, with their content and types directly influencing sourness, but also serve important physiological functions, such as promoting digestion, boosting immunity, providing antioxidant effects, and inhibiting bacterial growth. Therefore, research on organic acid accumulation in fruits is of great significance for improving fruit quality. The study of the characteristics of or-ganic acids mainly focuses on the types, content, biosynthesis, transport, and molecular regulatory mechanisms. Malic acid and citric acid are the predominant organic acids in most fruits, while tartaric acid is the major organic acid in certain fruits. The content of different organic acids varies significantly during the growth and development of fruit. Most organic acids gradually increase in concentration with the fruit development, peaking just before maturity, after which their levels decline. The accumulation of organic acids is closely linked to their synthesis, degradation, and transport. Citrate is primarily synthesized in the mitochondria, with key enzymes such as citrate synthase (CS), aconitase (ACO), isocitrate dehydrogenase (IDH), phosphoenolpyruvate carboxylase (PEPC), NAD- malate dehydrogenase (NAD-MDH), and NADP-malic enzyme (NADP-ME) playing crucial roles in the synthesis of citrate and malate in fruits. The γ- aminobutyric acid (GABA) shunt pathway significantly contributes to the degradation of organic acids, with ACO and ATP-citrate lyase (ACL) being particularly important in citrate degradation. Organic acid transport is primarily facilitated by tonoplast dicarboxylate transporters (tDT/TDT), aluminum-activated malate transporters (ALMT), vacuolar citrate transporters (Cit1), and tonoplast sugar transporters (TST). Among these, tDT proteins have been identified as playing a significant role in malate transport in tomato, grape, and plum fruits. ALMT primarily transports malate anions into the vacuole, while the citrate transporter gene CsCit1 encodes a novel vacuolar citrate symporter that maintains vacuolar citrate homeostasis. The tonoplast sugar transporter TST mediates the transmembrane transport of cytoplasmic glucose into the vacuole and also negatively regulates organic acid accumulation. Additionally, proton pumps, including V-type (VHA and VHP) and P-type (PHA) proton pumps, are crucial for organic acid accumulation. Multi-omics analyses have been successfully applied to study the regulation of flavor compound accumulation in fruits. Quantitative trait loci (QTLs) associated with organic acid accumulation have been identified in apple, grape, peach, and jujube, along with several candidate genes, laying a solid foundation for research on organic acid regulation. Through multi- omics analyses and molecular biology techniques, transcription factors such as members of the MYB, bHLH, and WRKY families have been found to be closely associated with organic acid accumulation in apple, pear, strawberry, kiwifruit, and citrus fruits. These transcription factors primarily influence organic acid levels by regulating the expression of genes involved in organic acid synthesis and transport. Moreover, environmental factors also exert significant influences on organic acid accumulation. For instance, salicylic acid (SA) promotes organic acid accumulation, while drought, waterlogging, auxin, abscisic acid (ABA), and ethylene also modulate organic acid levels. However, the molecular mechanisms underlying these effects require further investigation. In summary, fruit acidity is a crucial trait that significantly impacts fruit quality and nutritional value. Therefore, a comprehensive understanding of its genetic regulation and molecular basis is essential. Research on organic acid accumulation and its regulation holds great significance for improving fruit quality. Although recent studies have yielded substantial findings, most research on organic acid regulation has focused on apples. The key genes and regulatory networks governing organic acid accumulation in many other fruits remain to be elucidated. By integrating multi- omics technologies and breeding strategies, effective approaches can be developed for the targeted improvement and breeding of fruit acidity traits.