Legume Genomics and Genetics 2025, Vol.16, No.5, 225-233 http://cropscipublisher.com/index.php/lgg 230 Figure 2 The exchange of chemical signals underlying the initiation of the symbiosis process. NodD-Nodulation protein D; NF-Nod Factor; NFR1 and NFR5-Nod factor receptors 1 and 5; EPR3-exopolysaccharide receptor 3; ABC transporter-ATP-binding cassette transporter (Adopted from Nuc and Olejnik, 2025) 6.2 Role of histone marks in nodule organogenesis Methylation is not the only means of epigenetic regulation; histone modification also plays a role in the development of root nodules. Genome-wide analysis of alfalfa terlus reveals a complete set of enzymes related to methylation and acetylation, such as methyltransferases, demethylases, acetyltransferases, and deacetylases, some of which are expressed particularly high only in root nodules. A rather special finding is that, compared with the roots, within the root nodules, the levels of H3K27me3 (a common inhibitory marker) on the gene clusters known as "symbiotic islands" have actually decreased. This might imply that the chromatin in these regions has become looser, which is conducive to the activation of genes. Previous functional studies on HDAC (histone deacetylase) and HAT (acetyltransferase) have also shown that if these enzymes are absent, meristem is difficult to develop normally and the structure of root nodules is also affected (Pecrix et al., 2018).
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