Molecular Pathogens, 2025, Vol.16, No.6, 266-275 http://microbescipublisher.com/index.php/mp 268 advance for "battle", but the processes are somewhat different (Yu et al., 2022). For instance, SAR requires the accumulation of salicylic acid (SA) and also the activation of disease-related (PR) genes. ISR, on the other hand, relies more on the jasmonic acid (JA) and ethylene (ET) signaling pathways and can often be activated without the SA and PR genes. Of course, there are exceptions. Sometimes the two pathways may "visit each other", and ISR may also be related to SA. Rashid and Chung (2017) mentioned that both immune mechanisms actually enable plants to enter a standby state in advance, allowing them to respond more quickly when they are actually attacked. 3.2 Activation of defense signaling pathways (e.g., JA/ET, SA) triggered by beneficial microbes Not all microorganisms can activate plant immunity, and not all activations follow a single pathway. JA/ET is the most common pathway in the ISR reaction, through which many beneficial microorganisms express defense genes such as PDF1.2 and induce the synthesis of some enzymes or defense substances (Wang et al., 2025). On the SAR side, it is more accustomed to following the SA line. After PR-like genes like PR1, PR2, and PR5 are activated, their resistance is significantly enhanced. However, there are also some microorganisms that "act on two boats" and can simultaneously trigger SA and JA/ET. For instance, Bacillus cereus AR156 has been proven to bring about a stronger disease resistance effect (Yang et al., 2023). There is also NPR1, which acts like a coordinator. No matter which path you take, many signals still have to be aggregated here in the end to be effectively executed (Martin-Rivilla et al., 2020). 3.3 Epigenetic regulation in plant immunity mediated by interactions with Rhizosphere microbiota Whether a plant has a good memory or not sometimes depends on whether its epigenetic system is effective. Recent studies have found that factors such as small RNA and chromatin modification are not absent when plants interact with beneficial microorganisms. Small Rnas can precisely regulate which immune genes should be activated and which should be temporarily dormant. This "sense of proportion" is actually one of the key steps to activate ISR (Romera et al., 2019). More interestingly, some defense states activated by microorganisms can not only last for a relatively long time but may also be passed on to the next generation, which opens up new doors for resistance breeding using the microbiome (Darshita et al., 2025). It can be said that this is not a simple immune response, but rather more like a subtle "immune training". 4 Influence of Host Plant Genetic Background on Microbial Recruitment 4.1 Genotype-specific preferences for particular rhizosphere microorganisms Different plant varieties do not treat microorganisms equally. Even if they are planted in the same field, different genotypes can lead to significant differences in the structure of their rhizosphere microbial communities. For crops such as wheat, barley, carrots, cotton and chrysanthemums, studies have shown that their respective cultivated varieties or ecological types often form bacterial or fungal communities with "variety labels". Not all microorganisms are welcome. Drought-tolerant wheat prefers a wide variety of fungi with complex functions and is significantly more "selective" than drought-tolerant varieties (Yue et al., 2024). In the recruitment of Pseudomonas, varieties of barley and carrots also show considerable individuality, especially modern cultivated species, which seem to be more adept at attracting bacteria that match the secretions of their root systems. However, not all community differences can be explained by genetic distance. For example, the composition of fungal groups is not directly influenced by host genetics as bacteria do (Rotoni et al., 2022). 4.2 Mechanisms of microbe recruitment mediated by plant-derived metabolites The key to whether microorganisms will recruit and who they will recruit lies in what plants secrete. The roots of plants release a large amount of substances, such as sugar, amino acids, organic acids, and various secondary metabolites. Behind these combinations, it is actually genes at work (Anderson et al., 2024). The quantity and types of metabolites vary greatly among different varieties. Specific metabolites such as coumarin (Arabidopsis thaliana) or hexose (barley) are "invitations" to some microorganisms and may be "prohibitions" to others (Pacheco-Moreno et al., 2024). Root morphology is not insignificant either. It determines whether microorganisms can "take root" smoothly. Of course, things are not set in stone. If plants encounter stresses such as diseases or drought, the chemicals secreted by their roots will also change, which invisibly regulates their microbial circle of friends (Sharma et al., 2023).
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