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DNA Res. 2017 Apr 1;24(2):193-203. doi: 10.1093/dnares/dsw052.

Function and evolution of a Lotus japonicus AP2/ERF family transcription factor that is required for development of infection threads.

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National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8602, Japan.
Division of Symbiotic Systems, National Institute for Basic Biology, National Institute for Natural Sciences, Okazaki 444-8585, Japan.
Department of General Systems Studies, Graduate School of Arts and Sciences, the University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan.
Department of Biological Sciences, Graduate School of Science, the University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan.
Department of Basic Biology, School of Life Science, Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan.
Department of Life Science, Aichi University of Education, Kariya, Aichi 448-8542, Japan.
Kazusa DNA Research Institute, Kisarazu, Chiba 292-0812, Japan.
Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai 980-8577, Japan.


Legume-rhizobium symbiosis is achieved by two major events evolutionarily acquired: root hair infection and organogenesis. Infection thread (IT) development is a distinct element for rhizobial infection. Through ITs, rhizobia are efficiently transported from infection foci on root hairs to dividing meristematic cortical cells. To unveil this process, we performed genetic screening using Lotus japonicus MG-20 and isolated symbiotic mutant lines affecting nodulation, root hair morphology, and IT development. Map-based cloning identified an AP2/ERF transcription factor gene orthologous to Medicago truncatula ERN1. LjERN1 was activated in response to rhizobial infection and depended on CYCLOPS and NSP2. Legumes conserve an ERN1 homolog, ERN2, that functions redundantly with ERN1 in M. truncatula. Phylogenetic analysis showed that the lineages of ERN1 and ERN2 genes originated from a gene duplication event in the common ancestor of legume plants. However, genomic analysis suggested the lack of ERN2 gene in the L. japonicus genome, consistent with Ljern1 mutants exhibited a root hair phenotype that is observed in ern1/ern2 double mutants in M. truncatula. Molecular evolutionary analysis suggested that the nonsynonymous/synonymous rate ratios of legume ERN1 genes was almost identical to that of non-legume plants, whereas the ERN2 genes experienced a relaxed selective constraint.


ERN1; Lotus japonicus; infection thread development; molecular evolutionary analysis

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