Functional characterization of McACO2 reveals coordinated metabolic reprogramming, enhanced antioxidant capacity, and in vitro antifungal activity in Morinda citrifolia

Morinda citrifolia (noni) is a tropical fruit tree rich in bioactive compounds. However, the molecular mechanisms regulating development, metabolic regulation, and stress-related functional traits in noni remain largely unclear. Building on our previous work, this study aimed to determine how altered expression of 1-aminocyclopropane-1-carboxylate oxidase 2 ( McACO2 ) affects plant development, metabolic reprogramming, antioxidant capacity, and in vitro antifungal activity in noni, and to identify the metabolic pathways associated with these phenotypic and functional changes. To this end, functional characterization of McACO2 was performed by integrating phenotypic assessment, transcriptomic and metabolomic analyses, in vitro antifungal assays, reactive oxygen species (ROS) staining, and antioxidant capacity assays. The results showed that the relative transcript level of McACO2 in McACO2 -overexpressing lines ( McACO2 -OE) reached 9.62-fold that of the wild-type (WT), whereas its expression was significantly reduced in McACO2 RNA interference lines ( McACO2- Ri). McACO2 overexpression markedly accelerated fruit ripening and altered multiple developmental traits, whereas McACO2 -Ri lines exhibited generally opposite phenotypic trends. Integrated multi-omics analyses revealed that differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) were significantly enriched in pathways related to phenylpropanoid and flavonoid biosynthesis, galactose metabolism, glutathione metabolism, and amino acid metabolism. McACO2 -OE lines also exhibited enhanced antioxidant capacity, characterized by reduced ROS accumulation, increased antioxidant enzyme activities, elevated glutathione levels, and a significantly higher 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical-scavenging rate of 65.44%, approximately 1.34-fold that of the WT . Moreover, McACO2 -OE lines showed stronger in vitro antifungal activity, with peak inhibition rates of 62.41% against Diaporthe and 80.51% against Erysiphe at 96 h. Collectively, altered McACO2 expression was associated with coordinated changes in plant development, broad metabolic network reprogramming, antioxidant capacity, and in vitro antifungal activity in noni. This study provides a theoretical basis for the molecular improvement of noni and offers new perspectives for the development of plant-derived antioxidants, antifungal agents, and related bioformulations.