Overexpression of CsGSH2 Alleviates Propamocarb Residues and Phytotoxicity in Cucumber by Enhancing Antioxidant and Glutathione Detoxification Properties

Propamocarb is a pesticide widely used to control cucumber downy mildew. The overuse of propamocarb has resulted in residues and phytotoxicity. However, the detoxification and metabolic process of propamocarb have not been documented well. Our previous work showed differences in the propamocarb residues among the different genotypes of cucumber and their regulation by multiple genes. Based on the already reported data on gene expression profiles under propamocarb treatment, we identified the glutathione pathway, including six different genes (Csa4M303130, Csa3M133380, Csa5M409710, Csa7M395820, Csa3M597320, and Csa1M571280), involved in propamocarb detoxification. The qPCR analysis showed that Csa1M571280 (CsGSH2) was most significantly and differentially expressed at 48 h after propamocarb spray in the cucumber varieties Y3F604 (low propamocarb residues) and M729 (high propamocarb residues). In Y3F604, CsGSH2 expression increased from 6 to 48 h after spraying propamocarb, and the expression was positively correlated with propamocarb residues, whereas M729 showed no significant difference in CsGSH2 expression. Therefore, we presumed CsGSH2 as a key gene in managing propamocarb residues. Gene functional analysis showed that propamocarb residues decreased in CsGSH2-overexpressing plants and increased in CsGSH2-antisense plants. Overexpression of CsGSH2 enhanced glutathione (GSH) accumulation and glutathione S-transferase (GST), glutathione reductase (GR), and glutathione peroxidase (GPX) activities, probably for propamocarb detoxification. The activity of antioxidant enzymes (SOD, POD, CAT, and APX) increased to maintain a high antioxidant capacity in CsGSH2-overexpressing plants. The superoxide (O2&minus:), hydrogen peroxide (H2O2), and malondialdehyde (MDA) levels decreased in CsGSH2-overexpressing plants, promoting the antioxidant system composed of ascorbic acid and glutathione (AsA-GSH). Thus, we conclude that CsGSH2 alleviates propamocarb residues and phytotoxicity by enhancing cucumber&rsquo:s antioxidant and glutathione detoxification potential.