The ERN 1 transcription factor gene is a target of the CC a MK / CYCLOPS complex and controls rhizobial infection in  Lotus japonicus

Bacterial accommodation inside living plant cells is restricted to the nitrogen‐fixing root nodule symbiosis. In many legumes, bacterial uptake is mediated via tubular structures called infection threads ( IT s). To identify plant genes required for successful symbiotic infection, we screened an ethyl methanesulfonate mutagenized population of Lotus japonicus for mutants defective in IT formation and cloned the responsible gene, ERN 1 , encoding an AP2/ERF transcription factor. We performed phenotypic analysis of two independent L. japonicus mutant alleles and investigated the regulation of ERN 1 via transactivation and DNA –protein interaction assays. In ern1 mutant roots, nodule primordia formed, but most remained uninfected and bacterial entry via IT s into the root epidermis was abolished. Infected cortical nodule cells contained bacteroids, but transcellular IT s were rarely observed. A subset exhibited localized cell wall degradation and loss of cell integrity associated with bacteroid spread into neighbouring cells and the apoplast. Functional promoter studies revealed that CYCLOPS binds in a sequence‐specific manner to a motif within the ERN 1 promoter and in combination with CC a MK positively regulates ERN 1 transcription. We conclude that the activation of ERN 1 by CC a MK / CYCLOPS complex is an important step controlling IT ‐mediated bacterial progression into plant cells.