McACO2 gene promotes ethylene-mediated scopoletin biosynthesis in noni (Morinda citrifolia)

The noni (Morinda citrifolia) fruit undergoes exclusive postharvest changes in metabolites to counteract spoilage. The second day after harvest is the key time point for noni fruit, which would generate high content of scopoletin after that. According to our previous RNA-seq data, the McACO2 genes encode aminocyclopropane carboxylic acid oxidase, a key enzyme involved in ethylene biosynthesis, were identified to be closely related to the postharvest ripening process of noni fruit. Ethylene is a signaling molecule for scopoletin biosynthesis. To reveal the relationship between McACO2 and scopoletin biosynthesis, here we cloned the McACO2 gene and verified its function through noni genetic transformation. In McACO2-OE lines, McACO2 expression, McACO enzyme activity, and scopoletin content were significantly higher than those of wild type. Conversely, in McACO2-RNAi lines, McACO2 expression, McACO enzyme activity, and scopoletin content were significantly lower than those of wild type. The expression levels of genes enriched in the MAPK signaling, phenylpropanoid biosynthesis, ethylene biosynthesis, and plant hormone signal transduction pathways were altered by McACO2. The McACO2 gene promoter responded to ethylene, and the ethylene-responsive element in -1802 to -1619 bp was negatively regulated by exogenous ethylene, suppressing the expression of downstream McACO2 gene, while the ethylene-responsive element in -659 to -467 bp was positively regulated by exogenous ethylene, promoting the expression of downstream McACO2 gene. In summary, the McACO2 gene plays a critical role in scopoletin biosynthesis regulation by altering ethylene levels and the expression of corresponding genes related to the phenylpropanoid pathway, and improves scopoletin contents in the postharvest noni fruit. Our findings suggest an ethylene-mediated scopoletin biosynthesis pathway involving the McACO2 gene, which provides new insights for maintaining or improving the quality of postharvest noni fruit.