Functional genomics dissection of the nodulation autoregulation pathway (AON) in soybean (Glycine max)

The combination of mutation-based genetics and functional genomics allowed a detailed dissection of the nodulation-induction and Autoregulation of Nodulation (AON) pathways of soybean. Applicable to all legumes, nodulation was induced by Rhizobium/Bradyrhizobium produced lipopolysaccharides (Nod factors), perceived by Nod factor receptors (NFR1/NFR5 dimers), leading to cortical and pericycle cell divisions. These induce the production of CLE (Clavata3-like) peptides, that travel in the xylem to the shoot, where they are perceived by a receptor complex including a LRR receptor kinase, encoded by GmNARK, LjHAR1, MtSUNN and closely related receptors in other legumes like Phaseolus vulgaris (common bean), Pisum sativum (pea) and Glycine soja. This activated receptor complex negatively regulates by phosphorylation the constitutive synthesis of miR2111 in the shoot, that normally is translocated via the phloem to the entire plant body, initiating suppression of root-expressed receptor kinase ‘Too Much Love (TML)’, which in turn suppresses the nodule initiation cascade. Nodulation thus is permitted during a developmental window between the induction and progress of the nodulation/cell division/infection cascade during the first few days after inoculation and the functional ’readiness’ of the AON cascade, delayed by root-shoot-root loop. Loss-of-function mutations in GmNARK and TML result in excessive nodulation (supernodulation/hypernodulation/supernummary nodulation) as well as localised tolerance to externally applied nitrate. Recent analyses indicate interaction with gibberellin signalling, plant immunity as well as lateral root formation. Further details of the parallel functions of key points in this regulatory loop remain to be elucidated.