Molecular cartography of root nodule organogenesis: insights into the role of transcription factors.

Nodule organogenesis is a complex developmental process which is induced post-embryonically following rhizobia infection and relies on coordinated action of a suite of transcription factors at multiple stages starting from its initiation, de novo meristem establishment and differentiation. The rewiring of nitrate-responsive NIN-LIKE PROTEIN (NLP) homolog, NODULE INCEPTION (NIN) transcription factor through genetic adaptations in the common ancestor of the nitrogen-fixing clade is crucial to the origin of nodulation trait. Moreover, nodule formation emerged through the co-option of evolutionary ancient root/lateral root developmental pathways mediated by conserved transcriptional regulators such as NF-Y, LBD16, SHR/SCR, WOX5, PLT and KNOX. Importantly, NIN has acquired functions to integrate signals from rhizobia infection and divert the existing developmental pathways towards nodule organogenesis, thereby acts as a master regulator of root nodule symbiosis. Nodule specific innovations in the existing developmental pathway genes through mechanisms like genetic adaptations in their cis-regulatory region, paralog retention, changes in spatio-temporal gene expression pattern and functional changes to the protein are crucial for imparting developmental novelty during nodule organogenesis. Here, we provide a consolidated idea on transcription factor-mediated genetic modules regulating distinct stages of nodule organogenesis with an emphasis on their nodule specific innovations. This knowledge gain is particularly important to engineer nitrogen-fixing nodules into non-nodulation crop plants which will eventually minimize the reliance on synthetic nitrogen fertilizers and thereby favors a sustainable agricultural system.