Molecular Pathogens, 2025, Vol.16, No.4, 193-206 http://microbescipublisher.com/index.php/mp 198 Traditionally, the disease resistance of cucumber cultivars mostly comes from horizontal resistance (partial resistance controlled by multiple genes), but in recent years, some disease resistance genes with great effects have been discovered and cloned one after another, providing the possibility for precise breeding. For example, CsMLO1, a susceptible gene for powdery mildew, is the first cucumber disease-resistant gene identified using map cloning methods. Studies have shown that the CsMLO1 gene encodes a transmembrane protein that promotes infectious powdery mildew during normal function; and when the gene undergoes deficit mutations, cucumbers show broad-spectrum and long-lasting resistance to powdery mildew. This phenomenon of "loss-functional disease resistance gene" has been verified in crops such as barley and wheat, and is an important source of broad-spectrum resistance. Based on this principle, cucumber materials with high powdery mildew resistance have been successfully cultivated through directed mutagenesis or gene editing to knock out susceptible genes such as CsMLO1/CsMLO8. The Rgenes that resist downy mildew in cucumbers are relatively complex, and most of them are quantitative resistance. However, in recent years, through infiltration system construction, a main-effect anti-downdy QTL on chromosome 5 (pm5.1) was located, and the candidate gene CsLRR1 (leucine-rich repeat protein 1) was identified to contribute to downy mildew resistance (Zhang et al., 2020). 5.2 Regulatory role of transcription factors in pathogen response (such as WRKY, NAC) During the process of plant disease-resistant signaling, various transcription factors are responsible for the task of converting hormone and receptor signals into defense gene expression and are important nodes in the regulatory network. Among cucumbers, many transcription factors from the WRKY family and NAC family have been shown to be involved in pathogen-induced transcriptional reprogramming. WRKY transcription factors usually regulate their expression by identifying W-box elements that defend against relevant gene promoters, playing a role in plants in response to biological and abiotic stresses (Luan et al., 2019; Xia et al., 2025). NAC transcription factors are also a class of regulatory factors closely related to stress response. Some members of the NAC family are involved in the regulation of secondary metabolism and cell death related to disease resistance. In Arabidopsis, AtNAC1 and others participate in the amplification of the SA signal pathway and promote the formation of allergic reactions. In cucumbers, although the disease resistance function of the NAC family is rarely studied, it is speculated that some CsNACgenes are also involved in disease response regulation. Studies have shown that cucumber NAC transcription factor genes have changed expression after pathogen invasion, some of which may enhance resistance by activating lignin synthesis pathways or PRgenes. In addition, members of the CsNAC family in cucumbers associated with fruit development have also been found to have environmental stress response functions, suggesting that they play a role in stress resistance. 5.3 Regulation of non-coding RNA (miRNA, lncRNA) in disease defense In addition to protein-encoded genes, non-coding RNAs also play an important role in plant disease resistance regulation. Among them, small molecule microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) have been shown to be involved in the regulation of disease-resistant gene expression and signaling pathways. In the interaction between cucumber and pathogens, miRNA-mediated gene silencing is one of the important mechanisms for fine regulation of immune responses. When pathogen infestation occurs, the expression levels of many miRNAs in plants are significantly altered, and some miRNAs regulate the expression intensity of these genes by targeting mRNAs of disease-resistant genes to optimize defense responses (Nie et al., 2021). In cucumbers, a similar mechanism is speculated to exist: when the pathogen is not present, these miRNAs maintain low background expression of disease-resistant genes to avoid waste of resources; and when the pathogen invades, miRNAs are downregulated, relieving the inhibition of disease-resistant genes, allowing them to be expressed rapidly and in large quantities to exert disease-resistant function. Research on the function of long non-coding RNA (lncRNA) in plant immunity has just begun, but there is evidence that lncRNA is also involved in the regulation of disease resistance. lncRNA works in a variety of ways, including competitive endogenous RNA (ceRNA) as miRNAs or directly affecting chromatin status (He et al., 2020; Kęska et al., 2021).
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