Molecular Pathogens, 2025, Vol.16, No.4, 193-206 http://microbescipublisher.com/index.php/mp 200 sequence and plays a regulatory role in plant defense response. The degree of methylation of gene promoters or coding regions in plant genomes can affect the transcriptional activity of disease-resistant genes. Similar findings have also been found in the study of cucumber interactions. Although there is currently limited understanding of the whole genome methylation map of cucumber, reports have pointed out that there are differences in the methylation status of some gene promoters between downy mildew plants and disease-resistant plants, which are mostly related to hormone signals or resistance pathways (Zhang et al., 2013; Innark et al., 2020). Especially in disease-resistant plants, some disease-resistant gene promoters have lower levels of CpG sites, which is speculated to help improve the basal expression of these genes and enable the plants to initiate defense faster. In addition, environmental conditions can also affect cucumber disease resistance by changing DNA methylation. High temperature and humidity may induce methylation silencing of certain resistance genes, thus explaining some of the reasons why cucumber downy mildew is more prone to outbreaks under greenhouse conditions. 6.2 Histone modification and chromatin accessibility changes In addition to DNA methylation, post-translational modification of histones is also an important form of epigenetic regulation, which has a significant impact on the expression of cucumber disease-resistant genes. Modifications such as acetylation, methylation, and ubiquitination of histones can change the structure of chromatin and thus affect the transcriptional activity of specific genes. When a plant is infected with pathogens, the modification map on its nucleosome histones will be reprogrammed. For example, the acetylation of histones H3K9 and H3K14 that promote transcription significantly increases in many defense gene promoter regions, making these regions loose chromatin and easier to recruit transcriptional machines, and the defense genes are rapidly activated (Liu et al., 2016). Some studies have pointed out that some pathogenic effector factors can directly or indirectly affect the activity of host histone modification enzymes. For example, Cucumber Grey Bacteria may induce the host to upregulate histone deacetylase, thereby shutting down the expression of some resistance genes. This speculation needs to be verified experimentally, but it is not without basis. In Arabidopsis, pathogenic infection has been found to rapidly recruit histone acetyltransferases to the defense gene promoter, turning on chromatin to promote expression. If similar mechanisms exist in cucumbers, we may enhance plant resistance by regulating these histone modification enzymes (such as inhibiting histone deacetylase, or activating specific acetyltransferases). 6.3 Small RNA-mediated post-transcriptional regulation As mentioned earlier, miRNAs and other roles play in the regulation of disease-resistant gene expression, which is actually part of epigenetic regulation, namely post-transcriptional gene silencing (PTGS). The cucumber genome encodes numerous small RNAs that can complement each other with the target mRNA bases, triggering shear or translational repression, thereby changing gene expression products. This mechanism is particularly important in plant antivirals. When cucumbers are infected with viruses, they mobilize the RNA silencing pathway to produce siRNA to silen viral genes, thereby alleviating virus replication and movement. At the same time, the virus will also produce its own miRNA-like molecules (milRNAs) that interfere with the host gene. For example, some studies have found that after CGMMV infects cucumber, a small amount of milRNA derived from the virus was detected, which can target cucumber defense-related genes to weaken host resistance. This is a "attack and defense battle" between pathogens and hosts around small RNAs. In response to this situation, scientists proposed that genetic engineering can be used to express siRNA targeting viral key genes in crops, conferring them against viral abilities. At present, similar strategies have been implemented in bell peppers and other diseases, and have achieved good resistance to viruses such as CMV. In cucumber, this technology also has application prospects. In addition, long-chain non-coding RNA (lncRNA) can also be used as so-called "ceRNA", that is, competitive endogenous RNA, and compete with miRNA to bind, thereby releasing disease-resistant gene transcripts that should be inhibited by miRNA. Although cucumber disease-resistant ceRNA network has not been established yet, we can imagine that when pathogens invade, some lncRNA expression is upregulated, and by adsorbing miR482, etc., it protects the mRNA of NBS-LRR disease-resistant genes from being degraded, and ultimately improves disease-resistant protein accumulation. This hypothesis deserves further study.
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