From Inception to Maturation: Recent Insights in Nodule Organogenesis

Root branching and lateral root formation aided successful colonization of land plants in terrestrial ecosystems, eventually resulting in the origin of an adaptive trait called nodulation in specific land plant lineages. Nodule and lateral root formations involve de novo organogenesis, which is induced post-embryonically with coordinated steps of cell division and differentiation. There are substantial resemblances between the initiation of nodule and lateral root, including the types of root cell lineages that acquire mitotic competence to induce new organ formation and the pattern of formative division within the incipient organ primordia, both converging on a localized auxin response. The recruitment of specific genetic modules of lateral root developmental pathways during nodule organogenesis corroborates their evolutionary origin as modified lateral roots. However, such genetic modules are likely to be regulated differentially depending on the acquisition of nodulation-specific adaptations to ensure novelty in nodule organogenesis. Despite commonalities, these two lateral organs have differential morphology, vascular organizations, and functions due to the acquisition of different organ identities, suggesting involvement of distinct differentiation pathways during the maturation of both these organs. The current review provides a consolidated idea of the recent advances in nodule development with its resemblance to lateral root formation, emphasizing the shared and unique genetic regulators involved from their inception to maturation.