Root nodule symbiosis (RNS) is the pillar behind sustainable agriculture and plays a pivotal role in the environmental nitrogen cycle. Most of the genetic, molecular, and cell-biological knowledge on RNS come from model legumes that exhibit a root-hair mode of bacterial infection in contrast to the Dalbergoid legumes exhibiting crack-entry of rhizobia. As a step towards understanding this important group of legumes, we have combined microscopic analysis and temporal transcriptome to obtain a dynamic view of plant gene expression during Arachis hypogaea (peanut) nodule development. We generated a comprehensive transcriptome data by mapping the reads to A. hypogaea, and two diploid progenitor genomes. Additionally, we performed BLAST searches to identify nodule-induced yet-to-be annotated peanut genes. Comparison between peanut, Medicago truncatula, Lotus japonicus, and Glycine max showed upregulation of 61 peanut orthologs among 111 tested known RNS-related genes, indicating conservation in mechanisms of nodule development among members of the Papilionoid family. Unlike model legumes, recruitment of class 1 phytoglobin derived symbiotic hemoglobin (SymH) in peanut indicates diversification of oxygen scavenging mechanisms in the Papilionoid family. Finally, absence of cysteine-rich motif-1 containing-NCRs, but the recruitment of defensin like NCRs suggest a diverse molecular mechanism of terminal bacteroid differentiation. In summary, our work describes genetic conservation and diversification in legume-rhizobial symbiosis in the Papilionoid family, as well as among members of the Dalbergoid legumes.

Legumes can host nitrogen-fixing rhizobia inside root nodules. In model legumes, rhizobia enter via infection threads (ITs) and develop nodules in which the infection zone contains a mixture of infected and uninfected cells. Peanut (Arachis hypogaea) diversified from model legumes c. 50-55 million years ago. Rhizobia enter through 'cracks' to form nodules in peanut roots where cells of the infection zone are uniformly infected. Phylogenomic studies have indicated symbiosis as a labile trait in peanut. These atypical features prompted us to investigate the molecular mechanism of peanut nodule development.Combining cell biology, genetics and genomic tools, we visualized the status of hormonal signaling in peanut nodule primordia. Moreover, we dissected the signaling modules of Nodule INception (NIN), a master regulator of both epidermal infection and cortical organogenesis.Cytokinin signaling operates in a broad zone, from the epidermis to the pericycle inside nodule primordia, while auxin signaling is narrower and focused. Nodule INception is involved in nodule organogenesis, but not in crack entry. Nodulation Pectate Lyase, which remodels cell walls during IT formation, is not required. By contrast, Nodule enhanced Glycosyl Hydrolases (AhNGHs) are recruited for cell wall modification during crack entry.While hormonal regulation is conserved, the function of the NIN signaling modules is diversified in peanut.

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