Abstract: Objective: To reveal the roles of the tricarboxylic acid (TCA) cycle and the pentose phosphate pathway (PPP) in fruits infected by pathogenic fungi. Methods: Muskmelon fruit (cv. ‘Yujinxiang’) were artificially wounded and inoculated with Trichothecium roseum. The change of lesion diameter was observed during storage at ambient temperature (22 ± 2) ℃ and 55%–60% relative humidity. The changes in the activities of the key enzymes and the contents of intermediate products in the TCA cycle and PPP at the junction of diseased and healthy tissues were measured with the aim of evaluating the roles of the TCA cycle and PPP at different stages of infection. Results: Lesion diameter in infected fruit did not obviously change within 24 hours after inoculation, but then increased greatly at 48 h, and was significantly higher than that of the uninoculated control at 72 h (P < 0.05). At the early stage of infection, the activity of isocitrate dehydrogenase was inhibited significantly (P < 0.05) and the activity of malate dehydrogenase (MDH) was promoted significantly (P < 0.05) compared with the control, showing a 2.14-fold increase at 24 h. Meanwhile, the contents of nicotinamide adenine dinucleotide (NAD) and reduced nicotinamide adenine dinucleotide (NADH) were increased by 50.37% and 50.31% compared with those in the control group at 24 h, respectively. The infection also promoted NAD kinase activity. The activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were enhanced by 1.83 and 2.51 times relative to the control group at 48 h, respectively. The synthesis of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and glucose-6-phosphate was accelerated at the late stage of infection (P < 0.05). In addition, the activity of ribose-5-phosphate isomerase and the content of ribulose-5-phosphate in infected fruit was significantly decreased at the early and middle stages. Conclusion: The TCA cycle in muskmelon fruit is induced at the early stage of Trichothecium roseum infection and PPP is stimulated at the late stage, indicating that the TCA cycle participates in defense responses in infected fruit mainly at the early stage, while PPP plays a more important role in defense responses at the late stage.

Key words: muskmelon; Trichothecium roseum; infection; tricarboxylic acid cycle; pentose phosphate pathway