Dynamics of Citrate Synthase and Malate Dehydrogenase Gene Expression, Enzymatic Activity, and Physicochemical Changes During Postharvest Storage of Soursop (Annona muricata L.) GUANAY-1 Fruit

Soursop (Annona muricata L.) is a climacteric tropical fruit belonging to the Annonaceae family, widely cultivated in tropical and subtropical regions due to its economic importance, nutritional value, and abundance of bioactive compounds. Its pulp is highly appreciated for fresh consumption and food processing; however, its high respiration rate and ethylene production accelerate ripening, resulting in rapid quality deterioration and a short postharvest shelf life. During ripening, soursop fruit undergoes physiological, biochemical, and molecular changes associated with organic acid metabolism, in which citrate synthase (CS) and malate dehydrogenase (MDH) play key roles within the tricarboxylic acid cycle. However, the transcriptional dynamics of genes associated with the tricarboxylic acid cycle during postharvest storage of tropical fruits remains poorly characterized. This study aimed to identify in silico genes encoding CS and MDH and to evaluate changes in physicochemical traits, enzymatic activity, and the expression of selected CS and MDH genes during postharvest storage of soursop (A. muricata) GUANAY-1 fruit. Fruits were stored at 26 ± 1 °C and evaluated on days 1, 2, 3, 4, and 5. Titratable acidity, pH, total soluble solids, CS activity, MDH activity, and the expression of selected CS and MDH genes were analyzed. Physicochemical analysis showed that the major changes in acidity, TSS, and pH occurred on day 3. In silico analysis identified 16 CS genes and 47 MDH genes in soursop, whereas transcriptome analysis identified four CS genes and three MDH genes with differential expression, designated as CS1, CS2, CS3, CS4, MDH1, MDH2, and MDH3, respectively. The Neighbor-Joining analysis grouped the CS sequences into two clusters and the MDH sequences into three clusters. Differential expression analysis revealed that CS1, CS2, CS3, CS4, MDH1, MDH2, and MDH3 were upregulated on day 3 relative to day 0 of postharvest storage. Functional enrichment analysis indicated that these genes were mainly associated with cellular respiration and the tricarboxylic acid cycle. The highest CS activity was recorded on day 3, coinciding with the maximum expression of CS1, CS2, and CS4. In contrast, MDH activity peaked on day 2, whereas MDH1 and MDH2 reached their highest expression on day 3. Correlation analysis showed positive associations between CS2 and CS4 expression and both TSS and titratable acidity. Overall, these results demonstrate temporal changes in physicochemical traits, CS and MDH enzymatic activities, and the expression of selected CS and MDH genes during postharvest storage of GUANAY-1 soursop fruit.