The roots of nodules: a shared genetic infrastructure of root lateral organs suggests a common origin

Nitrogen (N)-fixing root nodule symbiosis (RNS) is founded on a blend of genes borrowed from pre-existing processes. Infection and intracellular uptake of bacterial symbionts have long been associated with fungal accommodation in arbuscular mycorrhiza as putative genetic origin. This review focuses on the second key feature of RNS, the nodule organ. It investigates evidence for its genetic origin in lateral root and, more globally, lateral organ formation, and pinpoints a set of common organogenesis (COR) genes. The transcription factor gene NODULE INCEPTION (NIN) is discussed as a mediator of both infection and organogenesis, bridging both processes and harbouring the molecular key to an evolutionarily successful N-fixing association between Eurosid nodulators and bacterial microbionts. Comparing the hormonal and genetic mediators of lateral root and nodule priming, initiation and primordium formation, we scrutinize parallels and differences along their respective developmental trajectories. The transcription factors LOB-DOMAIN PROTEIN (LBD) 16 and SCARECROW (SCR)/SHORTROOT (SHR) are highlighted as mediators of both lateral root and nodule formation. Their roles as both recipients and activators of regulatory activity in either of these organs are analysed in the light of recent findings. Finally, we summarize and integrate novel insights on LIGHT DEPENDENT SHORT HYPOCOTYLS (LSH) 1/2 as master regulators of nodule versus root identity.