Molecular Pathogens, 2025, Vol.16, No.4, 193-206 http://microbescipublisher.com/index.php/mp 197 However, the relationship between SA and JA pathways is not always opposite. In some cases, there is a synergy between the two. For example, for some semibiotype pathogens that have both nutritional and cell-killing stages, plants may require both SA and JA-mediated defenses. The study found that when cucumbers deal with complex infections such as root knot nematodes, SA and JA signals will be induced together, and the combination of the two can achieve a stronger resistance effect. In addition, in the initial PTI immunity of plants, the SA and JA pathways are often involved at the same time: on the one hand, SA triggers local defense genes, and on the other hand, JA promotes harm-related responses and provides protection from different levels. 4.2 Interaction between ET and JA in pathogen defense Ethylene (ET) and jasmonic acid (JA) often show synergies in plant immunity, jointly building a second line of defense against necrotic pathogens and pests. Unlike the SA pathway that mainly targets biological pathogens, the JA and ET pathways are often classified into the same camp, synergistically inducing resistance responses to tissue destructive invaders (Yang et al., 2020). In cucumbers, a large amount of evidence suggests a positive interaction between JA and ET signals: when the JA pathway is triggered by pests, it is often accompanied by an increase in ET synthesis, and the accumulation of ET can in turn amplify JA-mediated defense gene expression through transcriptional regulation. For example, when cucumbers suffer from grey mold infection or mechanical damage, both JA and ET levels rise significantly, and the plants initiate a series of coordinated defense reactions, including production of reactive oxygen species, thickening of cell walls and synthetic defense enzymes, etc., thereby effectively mitigating pathogen expansion. Molecular mechanism studies show that the JA/ET pathway shares some downstream transcription factors, such as ERF1/ORA59, etc. These transcription factors require the simultaneous activation of JA and ET signals to fully function (Huang et al., 2022). ERF1 is regarded as a molecular node for JA-ET synergistic response. In plants such as cucumber, ERF1 can directly bind to promoter to initiate defense gene expression. Overexpression of ERF1 increases resistance to necrotic pathogens. 4.3 Interaction between hormone signal and environmental factors The effect of the phytohormone signaling pathways depends not only on their own interactions, but also significantly affected by environmental conditions. In cucumber disease resistance studies, there is growing evidence that environmental factors can alter plant disease resistance performance by affecting hormone balance. Temperature is one of the important influencing factors. The significant decrease in resistance to powdery mildew in cucumber plants under high temperature conditions is believed to be related to the limited SA accumulation and reduced PR gene expression at high temperatures; at the same time, high temperature will increase the release of ethylene, making cucumber plants more susceptible to symptoms of premature aging and aggravate the development of the disease (Figure 1) (Liang et al., 2023). Light is also a key factor affecting hormone resistance. Full light helps the normal operation of the JA and SA pathways, and there is a cross between the light signal and the defense signal: light regulators (such as HY5) can directly regulate the expression of disease-resistant genes and enhance the immune response (Amin et al., 2023). Humidity mainly affects the opening of stomata and the levels of ethylene and ABA. High humidity environments are prone to cause downy mildew epidemics. At this time, the cucumber itself will increase the ABA level to close some stomata, but high ABA may inhibit the SA pathway, thereby weakening the disease resistance to a certain extent (Wang et al., 2024). The impact of rising atmospheric CO₂ concentration on plant disease resistance has also attracted much attention. Some studies have cultivated cucumbers under high CO₂ conditions and found that the basal content of SA and JA in the plants has changed, which has affected the disease occurrence pattern: at higher CO₂ concentrations, some diseases that rely on SA resistance are more likely to occur, while JA-related resistance is enhanced. This is a question that needs to be considered in future climate change scenarios. 5 Genes and Molecular Mechanisms Related to Cucumber's Disease Resistance 5.1 Identification and function of disease-related (R) genes Disease-resistant genes (R genes) are key genes for plants to recognize pathogens and trigger defense responses, and have always been the focus of research on the genetic improvement of cucumbers for disease resistance.
RkJQdWJsaXNoZXIy MjQ4ODYzNA==