Transcriptional Networks in Medicago truncatula: Genomic and Functional Overview During Root Nodule Symbiosis

Root nodule symbiosis (RNS) is a mutualistic interaction between rhizobia and host legumes. Genetic and genomic resources generated by the Medicago community provide an ideal platform for understanding the symbiotic interaction between Medicago truncatula and its symbiotic partner Sinorhizobium (Ensifer) sp.. “Nod Factor,” the chemical messenger secreted by rhizobia activates developmental cues to establish RNS. Root nodule development consists of a series of events starting from rhizobial recognition and infection at the root epidermal cells, transcriptomic changes in the symbiotically hallmarked root cells followed by the activation of several parallel signal transduction pathways. These coordinate the connectivity of the epidermal to inner root cell layers to initiate nodule primordia formation. Root nodule development is a finely tuned process where nodule organogenesis is synchronized with rhizobial infection, and the nodule number is controlled by a systemic signal. RNS is governed by the gene regulatory network driven by transcription factors. In this chapter, we present an overview of how transcription factor-guided transcriptional networks control cascade of events during nitrogen-fixing root nodule genesis.