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

Shuwu Zhang; Jia Liu; Bingliang Xu; Jingjiang Zhou

doi:10.3389/fpls.2021.633221

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

Front Plant Sci. 2021 Apr 12:12:633221. doi: 10.3389/fpls.2021.633221. eCollection 2021.

Authors

Shuwu Zhang^{1

2}, Jia Liu¹, Bingliang Xu^{1

2}, Jingjiang Zhou¹

Affiliations

¹ College of Plant Protection, Gansu Agricultural University, Biocontrol Engineering Laboratory of Crop Diseases and Pests of Gansu Province, Lanzhou, China.
² Gansu Provincial Key Laboratory of Arid Land Crop Science, Gansu Agricultural University, Lanzhou, China.

Abstract

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 F₁ 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 Jin₁₂ F₁ 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 H₂O₂, MDA, lignin, and total phenolics in the leaves of Sixing F₁ and Jin₁₂ F₁ cultivars were markedly increased after inoculation with P. xanthii. However, the Sixing F₁ 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 Jin₁₂ F₁ 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 F₁ than Jin₁₂ F₁. Our results indicate that the Sixing F₁ cultivar exhibited a much stronger ability in resistance to P. xanthii infection than the Jin₁₂ F₁ 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.

Keywords: Cucurbita pepo; Podosphaera xanthii; antioxidative defense system; disease incidence and index; gene expression; phenylpropanoids and SA pathways; powdery mildew; reactive oxygen species.