Rhizobial infection of legume roots during development of nitrogen fixing root nodules occurs either intracellularly though plant derived infection threads traversing the epidermal and cortical cell layers to deliver the bacteria or intercellularly via bacterial entry between epidermal plant cells. Although, around 25% of all legume genera are postulated to be intercellularly infected, the pathways and mechanisms supporting this process has remained virtually unexplored due to lack of genetically amenable legumes that have this infection mode. In this study, we report that the model legume Lotus japonicus is infected intercellularly by Rhizobium sp. IRBG74 and demonstrate that the resources available in Lotus enable insight into the genetic requirements and the fine-tuning of the pathway governing intercellular infection. Inoculation of Lotus mutants shows that Ern1 and RinRK1 are dispensable for intercellular infection in contrast to intracellular infection. Other symbiotic genes, including Nfr5, SymRK, CCaMK, Epr3, Cyclops, Nin, Nsp1, Nsp2, Cbs and Vpy1 are equally important for both entry modes. Comparative RNAseq analysis of roots inoculated with IRBG74 revealed a distinctive transcriptome response compared to intracellular colonization. In particular, a number of cytokinin-related genes were differentially regulated. Corroborating this observation cyp735A and ipt4 cytokinin biosynthesis mutants were significantly affected in their nodulation with IRBG74 while lhk1 cytokinin receptor mutants did not form any nodules. These results indicate that a differential requirement for cytokinin signalling conditions intercellular rhizobial entry and highlight the distinct modalities of the inter- and intra-cellular infection mechanisms.

中文翻译：

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独特的信号传导途径介导了日本莲的细胞间和细胞内根瘤菌感染

固氮根瘤发育过程中，豆科植物根的根瘤菌感染发生在细胞内，尽管植物感染线穿过表皮和皮层细胞层传递细菌，或通过细菌进入表皮植物细胞之间进入细胞间。尽管假设所有豆科属中约有25％被细胞间感染，但由于缺乏具有这种感染模式的遗传上适合的豆类，支持这一过程的途径和机制实际上尚未被探索。在这项研究中，我们报告模型豆科植物日本莲被根际菌感染。IRBG74并证明Lotus中可用的资源使人们能够深入了解遗传需求和调控细胞间感染的途径。Lotus突变体的接种表明，与细胞内感染相反，Ern1和RinRK1对于细胞间感染是必不可少的。其他共生基因，包括Nfr5，SymRK，CCaMK，Epr3，Cyclops，Nin，Nsp1，Nsp2，Cbs和Vpy1对两种进入模式同样重要。用IRBG74接种的根的比较RNAseq分析显示，与细胞内定植相比，转录组反应独特。特别地，许多细胞分裂素相关基因受到差异调节。证实此观察结果的是，cyp735A和ipt4细胞分裂素生物合成突变体在其与IRBG74的结瘤中受到显着影响，而lhk1细胞分裂素受体突变体未形成任何结节。