N-Acetyl-d-glucosamine Inhibition of Hexokinase Results in Downregulation of the Phenylpropanoid Metabolic Pathway and Decreased Resistance to Brown Rot in Peach Fruit

To explore the role of hexokinase (HXK) on disease resistance in peach fruit, peaches were treated with N-acetyl-d-glucosamine (NAG), a known HXK inhibitor, and then inoculated with Monilinia fructicola. We demonstrate that NAG significantly inhibits HXK activity, which in turn results in significantly reduced resistance to M. fructicola infection. In the HXK-inhibited fruit, the sucrose content was higher and the glucose and fructose contents were lower than in the control fruit. By transcriptome analysis, we found 347 differentially expressed genes (DEGs) between NAG-treated and control peaches, most of which were involved in the mitogen-activated protein kinase signaling pathway in plants, plant-pathogen interaction, plant hormone signal transduction, and the phenylpropanoid biosynthesis pathway. In particular, the DEGs related to phenylpropanoid metabolism, such as peroxidase, flavonoid, and isoflavonoid biosynthesis were significantly downregulated. Nontargeted metabolomic analysis revealed 44 differential metabolites, 9 of which were increased and 35 of which were decreased in the NAG-treated fruit. The decreased metabolites were secondary metabolites, including polyphenols, flavonoids, terpenoids, and glycosides. The relationship between HXK and phenylpropanoid metabolism was further investigated, and we found that in HXK-inhibited fruits the activities of phenylalanine ammonia-lyase, 4-coumarate-CoA ligase, and cinnamate 4-hydroxylase were significantly decreased over the control fruit, as well as the total phenol and total flavone contents were also significantly decreased. These results demonstrate that the inhibition of HXK activity decreases the disease resistance of peach fruits by affecting sugar metabolism and the phenylpropanoid pathway.