Pathogenic Mechanism of Fusarium Infection in Climacteric and Nonclimacteric Melons during Postharvest Storage

In this study, we compared the differences in pulp disease resistance between two melon cultivars, Karakokouqi (KKQ), a specialty cultivar in southern Xinjiang, and Xizhoumi 25 (XZM) as a control. To this end, melons were artificially wounded and inoculated with Fusarium equiseti before refrigerated storage. The results showed that during the whole storage period, XZM underwent respiratory climacteric, while KKQ did not. The onset of Fusarium rot was 5 day later in KKQ than in XZM, and the diameter and depth of rotten spots on the 25th day of infection were 1.26 and 1.19 times higher in XZM than in KKQ, respectively. During the infection period, the expression of the chitinase (CHT) and β-1,3-glucanase (GLU) genes was always higher in KKQ than in XZM, and the relative expression levels of CmGLU, CmCHT1 and CmCHT2 were high on the 5th day, indicating that the expression of resistance protein genes was rapidly induced in KKQ after pathogen infection. CHT and GLU activities tended to increase and then decrease with storage time, and were higher in KKQ than in XZM, indicating that disease resistance in KKQ was closely related to the activities and gene expression of CHT and GLU. The relative expression levels of the ethylene synthesis-related genes CmACS1, CmACS2, and CmACO1 were higher in XZM than in KKQ, and the relative expression levels of the ethylene receptor genes CmETR1 and CmERS1 were significantly higher in KKQ than in XZM, indicating that the ethylene receptor genes negatively regulated ethylene synthesis. Correlation analysis showed that the diameter of the infection lesions on XZM was highly significantly positively correlated with the ethylene receptor genes, suggesting that elevated ethylene-related enzyme activities and gene expression accelerated Fusarium infection in XZM. Thus, elevated disease resistance-related enzyme activities and gene expression hindered infection of KKQ by pathogenic fungi, whereas elevated ethylene-related enzyme activities and gene expression accelerated infection of XZM by pathogenic fungi.