Epigenetics’ Role in the Common Bean (Phaseolus vulgaris L.) and Soybean (Glycine max (L.) Merr.) Nodulation: a Review

Nitrogen-fixing bacteria establish symbiosis with legumes for the biological nitrogen fixation process in specific root structures called nodules. Among these legumes, common bean and soybean are two of the most economically relevant crops relying on the process for efficient nitrogen acquisition. This review proposes to answer two problem questions: what are the already known epigenetic mechanisms involved in the regulation in common bean and soybean nodulation and how they act during the nodulation process. DNA methylation, histone modification, and small RNAs were the epigenetic mechanisms found. The adenine methylation pattern of the bacterial genome changes during the symbiosis, and rhizobial tRNA-derived small RNA fragments are transferred to plant cells to cleave mRNA-specific mRNA targets, increasing the nodule number. On the other side of the interaction, plant DNA methylation is essential during the nodule’s life cycle, and histone lysine methyltransferases can also influence both bacterial colonization and nodule number. The microRNAs are the best-studied epigenetic modifiers that regulate legume nodulation. These epigenetic mechanisms can also be used for bacterial or plant autoregulation of nodulation since biological nitrogen fixation is a mandatory but energy-consuming process. This review article summarizes the state of the art of the epigenetic mechanisms involved in the regulatory network during common bean and soybean nodulation. Understanding of its mechanisms is essential to modulate and enhance nodulation in legume crops.