The Symbiosis-Related ERN Transcription Factors Act 14 in Concert to Coordinate 15 Rhizobial Host Root Infection

Legumes improve their mineral 69 nutrition through nitrogen-fixing root nodule symbioses with 70 soil rhizobia. Rhizobial infection of legumes is regulated by a number of transcription factors, 71 including ERF Required for Nodulation 1 (ERN1). Medicago truncatula plants defective in 72 ERN1 are unable to nodulate, but still exhibit early symbiotic responses including rhizobial 73 infection. ERN1 has a close homolog, ERN2, which shows partially overlapping expression 74 patterns. Here we show ern2 mutants exhibit a later nodulation phenotype than ern1, being 75 able to form nodules but with signs of premature senescence. Molecular characterization of 76 the ern2-1 mutation reveals a key role for a conserved threonine for both DNA binding and 77 transcriptional activity. In contrast to either single mutant the double ern1-1 ern2-1 line is 78 completely unable to initiate infection or nodule development. The strong ern1-1 ern2-1 79 phenotype demonstrates functional redundancy between these two transcriptional regulators 80 and reveals the essential role of ERN1/ERN2 to coordinately induce rhizobial infection and 81 nodule organogenesis. While ERN1/ERN2 act in concert in the root epidermis, only ERN1 can 82 efficiently allow the development of mature nodules in the cortex, probably through an 83 independent pathway. Together, these findings reveal the key roles that ERN1/ERN2 play at 84 the very earliest stages of root nodule development.