The Ethylene Responsive Factor Required for Nodulation 1 (ERN1) Transcription Factor Is Required for Infection-Thread Formation in Lotus japonicus

Several hundred genes are transcriptionally regulated during infection-thread formation and development of nitrogen-fixing root nodules. We have characterized a set of Lotus japonicus mutants impaired in root-nodule formation and found that the causative gene, Ernl, encodes a protein with a characteristic APETALA2/Ethylene Responsive Factor (AP2/ERF) transcription factor domain. Phenotypic characterization of four ernl alleles shows that infection pockets are formed but root-hair infection threads are absent. Formation of root-nodule primordia is delayed and no normal transcellular infection threads are found in the infected nodules. Corroborating the role of ERN1. (ERF Required for Nodulation1) in nodule organogenesis, spontaneous nodulation induced by an autoactive CCaMK and cytokinin induced nodule primordia were not observed in end mutants. Expression of Ernl is induced in the susceptible zone by Nod factor treatment or rhizobial inoculation. At the cellular level, the pErn1:GUS reporter is highly expressed in root epidermal cells of the susceptible zone and in the cortical cells that form nodule primordia. The genetic regulation of this cellular expression pattern was further investigated in symbiotic mutants. Nod factor induction of Ernl in epidermal cells was found to depend on Nfr1, Cyclops, and Nsp2 but was independent of Nin and Nf-ya1. These results suggest that ERN1. functions as a transcriptional regulator involved in the formation of infection threads and development of nodule primordia and may coordinate these two processes.