Molecular basis of 1-methylcyclopropene regulating organic acid metabolism in apple fruit during storage

Organic acids play a critical role in flavor and overall organoleptic quality of fruit. Malate has been considered to be the major organic acid determining apple fruit acidity. Here, we found that 1-methylcyclopropene (1-MCP) delayed the loss of malate and citrate during storage. 1-MCP-treated fruit showed higher level of phosphoenol pyruvate carboxylase (PEPC) and cytosolic NAD-dependent malate dehydrogenase (cyNAD-MDH) activities, but lower or similar phosphoenolpyruvate carboxylase kinase (PEPCK) and cytosolic NAD-dependent malate dehydrogenase (cyNADP-ME) activities compared to the control. Moreover, the expression of acid transport genes, including MdVHA-A, MdVHP and Ma1, was up-regulated in 1-MCP-treated fruit, resulting in retention of malate in the vacuole, and eventual higher malate content. Hence, 1-MCP maintained fruit acidity by regulating the balance between malate biosynthesis and degradation. These findings provide molecular evidence for understanding the mode of action of 1-MCP regulating organic acid metabolism in apple fruit during storage.