Legume Genomics and Genetics 2025, Vol.16, No.5, 225-233 http://cropscipublisher.com/index.php/lgg 229 and growth of root tumor organ morphology. This kind of interaction is like the steering wheel when driving. If there is the slightest deviation, the system can pull it back. 5 Environmental Influences on Epigenetic Regulation 5.1 Nutrient availability and epigenetic plasticity During the growth process of plants, nutritional conditions are not constant, especially for a key element like nitrogen, which sometimes is abundant and sometimes scarce. Such fluctuations are inevitable. In fact, the response of plants to such changes is far more direct than simply "absorbing less or more". They can also indirectly affect the activity of chromatin modification enzymes by altering the supply of metabolites. Metabolites like S-adenosylmethionine (SAM) are not only part of cellular metabolism but also serve as the "raw materials" for DNA and histone methylation. When nutritional conditions change, the supply of these substrates and cofactors will also be adjusted accordingly, thereby affecting the status of DNA methylation and histone modification (Huo et al., 2021). This endows plants with a certain degree of "response flexibility", enabling them to regulate gene expression in response to changes in the external environment, and may even indirectly affect the formation efficiency of root nodules. 5.2 Abiotic stress and nodule development Not all epigenetic changes result from nutrition; sometimes, environmental stress is the main cause. Drought, salt damage, high or low temperatures-these abiotic stresses often force plants to make adjustments within a short period of time. And such adjustments are not always dominated by hormones or transcription factors; many times, they are accomplished through rewriting at the chromatin level (Figure 2). For instance, under stress conditions, DNA methylation levels may rise overall or change locally, and the modification patterns of histones may also undergo rearrangement. Such changes can alter the expression profiles of a series of genes, thereby helping plants "enter emergency mode". However, there is also a problem: symbiotic signals and stress response signals may "clash". This means that when plants attempt to cope with stress, the normal development and function of root nodules may be disrupted to a certain extent. 5.3 Transgenerational epigenetic inheritance Usually, we think that environmental stimuli are merely the "memories" of contemporary plants, but this is not always the case. Sometimes, these epigenetic imprints do not automatically erase at the end of their life cycle but are stably passed on to the next generation. In other words, the environment experienced by the grandparents, such as drought or nutrient deficiency, may leave "traces" in the offspring-these traces are not due to a change in the genetic code itself, but rather the continuation of epigenetic markers. This kind of intergenerational transmission phenomenon may enhance the efficiency of offspring in responding to certain repetitive environments, and may also affect their root tumor development and symbiotic performance. However, whether such mechanisms are applicable to all leguminous plants still requires more experimental data to support them. 6 Case Study: Epigenetic Modulation in Medicago truncatula 6.1 Dynamic methylation landscape during nodulation During the development of root nodules in Alfalfa terrestris, the state of DNA methylation is not fixed but is constantly "rewritten" at different stages. Research has found that the expression level of demethylase DEMETER (DME) in the differentiation region of root nodules is very high, and this phenomenon is not accidental. Its existence seems to be directly related to whether the root tumor can develop normally. In fact, whole-genome bisulfite sequencing has long revealed that during the development of root nodules, the methylation levels in many regions are changing. In these regions, many are related to root tumor-specific genes, such as those sites encoding cysteine-rich peptides (Pecrix et al., 2022). If the function of the DME is lost, methylation of some key genes will increase, followed by down-regulation of expression, and the differentiation of plants and symbiotic bacteria will also be hindered as a result. And it is not only demethylation that plays a role; mechanisms like de novo methylation are equally important-especially when the root tumor development enters the later stage. This indicates that the methylation pattern of alfalfa root nodules is not simply static but constantly changing over time.
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