Autoregulation of nodulation (AON) in Pisum sativum (pea) involves signalling events associated with both nodule primordia development and nitrogen fixation

To define the signalling events required for the activation of AON, we utilised approach grafts between wild-type pea plants and their mutants defective at successive stages of nodule formation. AON signalling strength was monitored by prior inoculation of mutant root portions (as so-called ‘sensor’) and quantifying nodule formation on connected roots of delayed inoculated wild type (the ‘reporter’). Detectable AON sensing and associated signal exchange between root and shoot started after root hair curling but before the initiation of visible cortical and pericycle cell divisions. The strength of AON signalling was correlated with the stage of nodule development and size of nodule, with mature nitrogen-fixing nodules possessing the strongest AON-inducing signal. We demonstrated that the pea supernodulating mutant nod3 may function pre-NARK in the root. A model for the activation of AON signalling and its potential relationship with cell division, nitrogen fixation and/or cytokinin signal transduction are presented.