Differential Responses of Cucurbita pepo to Podosphaera xanthii Reveal the Mechanism of Powdery Mildew Disease Resistance in Pumpkin

Powdery mildew is one of the most destructive diseases and the major cause to the production losses of cucurbit worldwide. A number of strategies have been developed and applied to discover suitable and safer methods to manage the occurrence of powdery mildew disease in pumpkins (Cucurbita pepo L.), but information is limited in screening tolerant germplasms and exploring their mechanisms in preventing the disease occurrence at physiological, biochemical, and molecular levels. Therefore, we investigated the responses of two commercial pumpkin cultivars to Podosphaera xanthii infection. Compared with mock-inoculated seedlings, few small and sparse powdery areas were observed on the leaves of the Sixing F(1) cultivar on the 13(th) day after inoculation with P. xanthii, whereas a large number of diseased powdery areas and a layer of white powdery mildew were observed on the surface of Jin(12) F(1) leaves. The inoculation duration (7, 9, 11, and 13 days) significantly and continuously increased the disease incidence and index of pumpkin seedlings. The contents of H(2)O(2), MDA, lignin, and total phenolics in the leaves of Sixing F(1) and Jin(12) F(1) cultivars were markedly increased after inoculation with P. xanthii. However, the Sixing F(1) cultivar exhibited much less reactive oxygen species (ROS) accumulation, a lower rate of lipid peroxidation, and a higher level of lignin and total phenolics contents after inoculation than the Jin(12) F(1) cultivar. Compared with untreated control pumpkin seedlings, significantly higher activities and gene expressions of the phenylpropanoids pathway enzymes (PAL and PPO), ROS scavenging defense enzymes (SOD, CAT, POD, and APX), and other salicylic acid (SA) signaling pathway marker genes were observed in the leaves of both cultivars after P. xanthii inoculation at different inoculation time points. These enhancements were significantly higher in Sixing F(1) than Jin(12) F(1). Our results indicate that the Sixing F(1) cultivar exhibited a much stronger ability in resistance to P. xanthii infection than the Jin(12) F(1) cultivar. Our results suggest that one possible mechanism of C. pepo cultivars to prevent the pathogen P. xanthii infection is by activating and enhancing the activity and gene expression of the phenylpropanoids pathway to synthesize phenolic substances and lignin, ROS scavenging defense enzymes to eliminate the harmful effects of ROS, and signaling pathway marker gene expression to improve plant disease resistance.