Intensive research on nitrogen-fixing symbiosis in two model legumes has uncovered the molecular mechanisms, whereby rhizobial Nod factors activate a plant symbiotic signaling pathway that controls infection and nodule organogenesis. In contrast, the so-called Nod-independent symbiosis found between Aeschynomene evenia and photosynthetic bradyrhizobia, which does not involve Nod factor recognition nor infection thread formation, is less well known. To gain knowledge on how Nod-independent symbiosis is established, we conducted a phenotypic and molecular characterization of A. evenia lines carrying mutations in different nodulation genes. Besides investigating the effect of the mutations on rhizobial symbiosis, we examined their consequences on mycorrhizal symbiosis and in nonsymbiotic conditions. Analyzing allelic mutant series for AePOLLUX, Ca2+/calmodulin dependent kinase, AeCYCLOPS, nodulation signaling pathway 2 (AeNSP2), and nodule inception demonstrated that these genes intervene at several stages of intercellular infection and during bacterial accommodation. We provide evidence that AeNSP2 has an additional nitrogen-dependent regulatory function in the formation of axillary root hairs at lateral root bases, which are rhizobia-colonized infection sites. Our investigation of the recently discovered symbiotic actor cysteine-rich receptor-like kinase specified that it is not involved in mycorrhization; however, it is essential for both symbiotic signaling and early infection during nodulation. These findings provide important insights on the modus operandi of Nod-independent symbiosis and contribute to the general understanding of how rhizobial-legume symbioses are established by complementing the information acquired in model legumes.

中文翻译：

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基于突变体的豆科植物 Aeschynomene evenia 中与 Nod 无关的共生关系的分析。

对两种模式豆科植物固氮共生的深入研究揭示了根瘤菌Nod因子激活控制感染和根瘤器官发生的植物共生信号通路的分子机制。相比之下，在 Aeschynomene evenia 和光合慢生根瘤菌之间发现的所谓的不依赖 Nod 的共生关系，不涉及 Nod 因子识别或感染线形成，鲜为人知。为了获得有关如何建立不依赖于 Nod 的共生关系的知识，我们对携带不同结瘤基因突变的 A. evenia 系进行了表型和分子表征。除了研究突变对根瘤菌共生的影响外，我们还研究了它们对菌根共生和非共生条件的影响。分析 AePOLLUX 的等位基因突变系列，Ca2+/钙调蛋白依赖性激酶、AeCYCLOPS、结瘤信号通路 2 (AeNSP2) 和结节开始表明这些基因在细胞间感染的几个阶段和细菌适应过程中进行干预。我们提供的证据表明，AeNSP2 在侧根基部腋根毛的形成中具有额外的氮依赖性调节功能，侧根基部是根瘤菌定植的感染部位。我们对最近发现的共生分子富含半胱氨酸受体样激酶的研究表明它不参与菌根化；然而，它对于结瘤期间的共生信号传导和早期感染都是必不可少的。