Division of Symbiotic Systems, National Institute for Basic Biology, Okazaki, Aichi, 444-8585, Japan.
Department of Basic Biology, School of Life Science, SOKENDAI (The Graduate University for Advanced Studies), Okazaki, Aichi, 444-8585, Japan.
J Plant Res. 2019 Sep;132(5):641-653. doi: 10.1007/s10265-019-01122-w. Epub 2019 Jul 16.
Legumes engage in symbiosis with nitrogen-fixing soil bacteria, collectively called rhizobia, under nitrogen-limited conditions. In many legumes, the root invasion of rhizobia is mediated by infection threads (ITs), tubular invaginations of the host cell wall and plasma membrane, developed from infection foci of deformed root hairs. IT formation is regulated by a series of signal transduction in host root. Nodulation signals activate the host transcription factor (TF), CYCLOPS, which directly induces expression of two TF genes, ERF REQUIRED FOR NODULATION1 (ERN1) and NODULE INCEPTION (NIN), essential for IT development. Here, we explored the relationship among these three symbiotic TF genes in the model legume Lotus japonicus and examined how their interplay contributes to IT formation. qRT-PCR analysis showed that NIN expression induced by rhizobial infection was attenuated in ern1-1, and further declined in cyclops-3 ern1-1. ERN1 overexpression led to induction of NIN expression in cyclops-3 ern1-1 in the presence of rhizobia. Thus, in addition to CYCLOPS, ERN1 is able to increase the NIN expression level depending on infection. Furthermore, consistent with this transcriptional hierarchy, ectopic expression of ERN1 as well as NIN suppressed the IT-deficient cyclops-3 phenotype, but ERN1 failed to confer ITs in the nin-2 root. However, the ern1-1 symbiotic epidermal phenotype was not suppressed by the NIN ectopic expression. The cyclops-3 ern1-1 double mutant was less sensitive to rhizobial infection than the single mutants and defective in the symbiotic root hair response at earlier stages. This more severe phenotype of the double mutant suggests a role for ERN1 that independent of the CYCLOPS-mediated transcriptional regulation. We conclude that ERN1 is involved in regulating NIN expression in addition to CYCLOPS, and these TFs coordinately promote the symbiotic root hair response and IT development. Our data help to reveal the extensive role of ERN1 in root nodule symbiosis signaling.
豆科植物在氮限制条件下与固氮土壤细菌（统称为根瘤菌）共生。在许多豆科植物中，根瘤菌的根侵染是由感染丝（ITs）介导的，感染丝是宿主细胞壁和质膜的管状内陷，由变形根毛的感染焦点发育而来。IT 的形成受宿主根部一系列信号转导的调节。结瘤信号激活宿主转录因子（TF）CYCLOPS，其直接诱导两个 TF 基因的表达，即结瘤必需的 ERF（ERN1）和根瘤起始（NIN），这对于 IT 的发育是必不可少的。在这里，我们探索了模式豆科植物百脉根中这三个共生 TF 基因之间的关系，并研究了它们的相互作用如何促进 IT 的形成。qRT-PCR 分析表明，rhizobial 感染诱导的 NIN 表达在 ern1-1 中减弱，在 cyclops-3 ern1-1 中进一步下降。ERN1 过表达导致在 rhizobia 存在的情况下 cyclops-3 ern1-1 中 NIN 表达的诱导。因此，除了 CYCLOPS 之外，ERN1 还能够根据感染增加 NIN 的表达水平。此外，与这种转录层次结构一致，ERN1 和 NIN 的异位表达抑制了 IT 缺陷的 cyclops-3 表型，但 ERN1 不能在 nin-2 根中赋予 IT。然而，NIN 的异位表达并没有抑制 ern1-1 的共生表皮表型。cyclops-3 ern1-1 双突变体对根瘤菌感染的敏感性低于单突变体，并且在早期阶段对共生根毛反应缺陷。双突变体更严重的表型表明 ERN1 具有独立于 CYCLOPS 介导的转录调控的作用。我们得出结论，ERN1 除了 CYCLOPS 之外，还参与调节 NIN 的表达，这些 TF 共同促进共生根毛反应和 IT 的发育。我们的数据有助于揭示 ERN1 在根瘤共生信号转导中广泛的作用。
