A bstractThe term ``actinorhiza'' refers both to the filamentous bacteria Frankia, an actinomycete, and to the root location of nitrogen-fixing nodules. Actinorhizal plants are classified into four subclasses, eight families, and 25 genera comprising more than 220 species. Although ontogenically related to lateral roots, actinorhizal nodules are characterized by differentially expressed genes, supporting the idea of the uniqueness of this new organ. Two pathways for root infection have been described for compatible Frankia interactions: root hair infection or intercellular penetration. Molecular phylogeny groupings of host plants correlate with morphologic and anatomic features of actinorhizal nodules. Four clades of actinorhizal plants have been defined, whereas Frankia bacteria are classified into three major phylogenetic groups. Although the phylogenies of the symbionts are not fully congruent, a close relationship exists between plant and bacterial groups. A model for actinorhizal specificity is proposed that includes different levels or degrees of specificity of host-symbiont interactions, from fully compatible to incompatible. Intermediate, compatible, but delayed or limited interactions are also discussed. Actinorhizal plants undergo feedback regulation of symbiosis involving at least two different and consecutive signals that lead to a mechanism controlling root nodulation. These signals mediate the opening or closing of the window of susceptibility for infection and inhibit infection and nodule development in the growing root, independently of infection mechanism. The requirement for at least two molecular recognition steps in the development of actinorhizal symbioses is discussed.

中文翻译：

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菌根共生

摘要术语“放线菌”既指丝状细菌 Frankia（一种放线菌），也指固氮根瘤的根部位置。放线植物分为4个亚纲、8个科、25个属，220余种。尽管在个体发生上与侧根相关，但放线根瘤的特征在于差异表达的基因，支持这一新器官的独特性。对于兼容的 Frankia 相互作用，已经描述了两种根感染途径：根毛感染或细胞间渗透。寄主植物的分子系统发育分组与放线根瘤的形态学和解剖学特征相关。已经定义了四个放线根植物进化枝，而 Frankia 细菌被分为三个主要的系统发育组。尽管共生体的系统发育并不完全一致，但植物群和细菌群之间存在密切的关系。提出了一种放线菌特异性模型，其中包括宿主-共生体相互作用的不同水平或程度的特异性，从完全相容到不相容。还讨论了中间、兼容但延迟或有限的交互。Actinorhizal 植物经历共生的反馈调节，涉及至少两个不同且连续的信号，导致控制根结瘤的机制。这些信号介导感染易感窗口的打开或关闭，并独立于感染机制抑制生长根中的感染和根瘤发育。