Molecular Pathogens, 2025, Vol.16, No.4, 193-206 http://microbescipublisher.com/index.php/mp 201 7 Case Analysis: Hormone and Genetic Regulation Mechanism of Cucumber to Powdery Mildew 7.1 Process and symptoms of powdery mildew pathogen infection Powdery mildew is one of the most common diseases in cucumber production, and its pathogen is exogenous parasitic fungi that are spread all over the world. Podosphaera xanthii is the main pathogenic species. This bacteria is transmitted through conidia. The suitable temperature is 15 ℃ to 25 ℃, and the humidity is moderate to spread (Meng et al., 2022). After the spores of Powdery Bacteria fall on the surface of cucumber leaves, they germinate and produce attached cells. The attached cells further form a suction device that invades the tissue and absorb nutrients from epidermal cells (Figure 2). Since most of the mycelium of Powdery Bacteria grows on the surface of the leaves, local white powdery mold spots can be seen on the front of the leaves in the early stage of infection, which is a typical symptom of the formation of mycelium and conidia accumulation (Tek and Calis, 2022). As the infection progresses, mold spots gradually expand and fuse into large areas, the photosynthesis of the leaves is blocked, and yellowing patches appear. In severe cases, the entire leaf is covered with white powdery substances and dies early (early fallen leaves). The harm of powdery mildew to cucumber is not only to reduce the photosynthetic area, but also to cause early aging of leaves, affecting nutrient distribution, resulting in a decrease in yield and quality. It is reported that the yield reduction of cucumber varieties in epidemic years can reach 20% to 40%, which is a greater threat to facility cultivation. Figure 2 Process of infection of P. xanthii of the PM-susceptible cucumber strain B21-a-2-2-2 and the PM-resistant cucumber strain B21-a-2-1-2. Cd, conidia; PGT, primary germ tube; App, appressorium; Pp, penetration peg; FC, fungal colony; Cdp, conidiophore. The bar in panels A through D and G through K is 20 μm, and the bar in panels E through F and L is 60 μm (Adopted from Meng et al., 2022) 7.2 The role of SA and JA signaling in cucumber resistance to powdery mildew In the case of a biological pathogen like powdery mildew, cucumbers mainly rely on SA-mediated defense to fight the disease. Studies have shown that in the early stage of infection, the SA content of the cucumber material with powdery mildew rapidly increased and activated the expression of PR genes related to SAR reactions, such as PR1 and PR5, which accumulate in large quantities (Kim and Kang, 2023; Wang et al., 2024). Exogenous administration of SA or its analogues (salicylic acid analogue BTH, etc.) can significantly reduce the incidence of cucumber powdery mildew, confirming the positive effect of SA's anti-epidemic. On the other hand, the role of JA signal in powdery mildew defense is more complicated. It is generally believed that the JA pathway is effective against necrotic pathogens, but its effect on powdery mildew, a living trophic pathogen, is not as significant as SA. To a certain extent, the excessive activation of JA will antagonize SA resistance, making the plants more susceptible to powdery mildew. However, some studies have also pointed out that JA has not made no contribution to the late stage of powdery mildew defense. After the disease-resistant plants have been infected with powdery white bacteria for a period of time, the JA level rises moderately, which can induce the accumulation of some defense-related secondary metabolites (such as cucumberin, coumaric acid, etc.), which helps inhibit the formation and release of pathogenic spores. Especially when environmental conditions are not conducive to SA reactions (such as high temperature conditions), the JA pathway may partially make up for the shortcomings of the SA pathway.
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