ABSTRACT: Information on disease process and pathogenicity mechanisms is important for understanding plant disease. Spring black stem and leaf spot caused by the necrotrophic pathogen Phoma medicaginis var. medicaginis Malbr. & Roum causes large losses to alfalfa. However, till now, little is known about alfalfa-P. medicagnis interactions and the pathogenicity mechanisms of the pathogen. Here, alfalfa inoculated with P. medicaginis was subjected to GC-MS based metabolic profiling. The metabolic response in P. medicaginis-inoculated and mock-inoculated alfalfa leaves was assessed at 2, 4, 6, 8, 12, 16, 20, 24, 26 and 28 days post inoculation. In total, 101 peaks were detected in the control and inoculated groups, from which 70 metabolites were tentatively identified. Using multivariate analysis, 16 significantly regulated compounds, including amino acids, nitrogen-containing compounds and organic acids, polyols, fatty acids, and sugars were tentatively identified (Variable importance values, VIP>1.0 and p <0.05). Among these metabolites, the levels of malate, 5-oxoproline, palmitic acid and stearic acid were increased significantly in P. medicaginis-infected alfalfa leaves compared to the controls. In contrast, the levels of?-aminobutyric acid and 2-pyrrolidinone were significantly decreased in infected leaves compared to the controls. Further metabolic pathway analysis of the 16 significantly regulated compounds demonstrated that glycolysis, the tricarboxylic acid cycle, and ?-oxidation of fatty acids were significantly induced in the alfalfa leaves at later stages of P. medicaginis infection. The strong induction of tricarboxylic acid cycle pathways at later infection stages caused by the pathogen may induce senescence in the alfalfa leaves, leading to plant death. However, intermediate metabolites of these metabolic pathways, and inositol phosphate, glutathione, the metabolic pathways of some amino acids accumulated rapidly and strongly at early stages of infection, which may enhance the ability of alfalfa to resist necrotrophic P. medicaginis disease. Understanding metabolic pathways is essential for understanding pathogenesis.
