The primary metabolites in Fengtang, Kongxin and Cuihong plum fruits were qualitatively and quantitatively analyzed by ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The differential metabolites among varieties were identified by principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA), and the differential metabolic pathways were analyzed through KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis. The results showed that a total of 307 metabolites belonging to 12 categories were detected, including amino acids and derivatives, nucleotides and derivatives, organic acids, sugars, alcohols, and free fatty acids. There were 70 differential metabolites between Fengtang and Cuihong plum, among which, 43 (61.43%) were up-regulated and 27 (38.57%) were down-regulated. Five significantly differential metabolic pathways were recognized, including metabolic pathways, aminoacyl-tRNA biosynthesis, valine, leucine and isoleucine degradation, glucosinolate biosynthesis and lysine degradation. In total, 96 differential metabolites were found between Fengtang and Kongxin plum, of which, 84 (87.50%) were up-regulated and 12 (12.50%) were down-regulated, and only one significantly differential metabolic pathway was identified, namely tryptophan metabolism. A total of 75 differential metabolites were found between Kongxin and Cuihong plum, of which, 19 (25.33%) were up-regulated and 56 (74.67%) were down-regulated, and two significantly differential metabolic pathways were identified, including purine metabolism and glucosinolate biosynthesis.

Ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based metabolomics was used to investigate primary metabolites in ‘Fengtang’ plum fruits from three production regions in Guizhou province. Multivariate statistical analyses such as principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) as well as metabolic pathway analysis were employed to explore the effect of the geographical origin on the primary metabolites of ‘Fengtang’ plum fruits. The results showed that a total of 307 metabolites were identified, and the geographic affected the type and content of metabolites in ‘Fengtang’ plum fruits. Meanwhile, 13 differential metabolites were identified between ‘Fengtang’ plum fruits from Liangtian and Liuma, and 49 differential metabolites were identified in ‘Fengtang’ plum fruits from Huishui versus Liuma, and Huishui versus Liangtian. In addition, the OPLS-DA results showed a clear discrimination between ‘Fengtang’ plum fruits from different geographical origins, and the samples showed an obvious clustering distribution, indicating that the geographical origin had a profound impact on the primary metabolites of ‘Fengtang’ plum fruits. Metabolic pathway analysis showed that the major mechanism for the difference in the primary metabolites of ‘Fengtang’ plum fruits from different production areas lay in the metabolism of amino acids, and it was found that the geographical origin imposed the most significant impact on amino acids. The results from this study can provide a theoretical basis for identifying and tracing the geographical origin of ‘Fengtang’ plum fruits.

Melatonin, a trace efficient natural plant signaling molecule, has been widely reported in plant growth and development, abiotic stress, and postharvest preservation and other aspects. In recent years, melatonin has been found to play a positive role in inducing disease resistance in fruits. This paper describes the effect of melatonin on pathogens in vitro and in infected fruits, with a focus on the mechanism by which melatonin induces fruit resistance to invasive diseases. Furthermore, this paper reveals that melatonin and plant signal transduction pathways constitute a regulation network to mediate disease resistance responses in fruits. We conclude this paper with an outlook on future research directions in order to provide a theoretical reference for further research on the mechanism by which melatonin induces disease resistance.