Some plants fix atmospheric nitrogen by hosting symbiotic diazotrophic rhizobia or Frankia bacteria in root organs known as nodules. Such nodule symbiosis occurs in 10 plant lineages in four taxonomic orders: Fabales, Fagales, Cucurbitales, and Rosales, which are collectively known as the nitrogen-fixing clade. Nodules are divided into two types based on differences in ontogeny and histology: legume-type and actinorhizal-type nodules. The evolutionary relationship between these nodule types has been a long-standing enigma for molecular and evolutionary biologists. Recent phylogenomic studies on nodulating and nonnodulating species in the nitrogen-fixing clade indicated that the nodulation trait has a shared evolutionary origin in all 10 lineages. However, this hypothesis faces a conundrum in that legume-type and actinorhizal-type nodules have been regarded as fundamentally different. Here, we analyzed the actinorhizal-type nodules formed by Parasponia andersonii (Rosales) and Alnus glutinosa (Fagales) and found that their ontogeny is more similar to that of legume-type nodules (Fabales) than generally assumed. We also show that in Medicago truncatula, a homeotic mutation in the co-transcriptional regulator gene NODULE ROOT1 (MtNOOT1) converts legume-type nodules into actinorhizal-type nodules. These experimental findings suggest that the two nodule types have a shared evolutionary origin.

一些植物通过在称为根瘤的根器官中寄生共生固氮营养根瘤菌或Frankia细菌来固定大气中的氮。这种根瘤共生发生在四个分类目的 10 个植物谱系中：蚕豆目、山毛榉目、葫芦目和蔷薇目，统称为固氮进化枝。根据个体发育和组织学的差异，结节分为两种类型：豆科结节和放线根结节。这些结节类型之间的进化关系长期以来一直是分子和进化生物学家的一个谜。最近对固氮进化枝中结瘤和非结瘤物种的系统发育研究表明，结瘤性状在所有 10 个谱系中具有共同的进化起源。然而，这一假说面临着一个难题，即豆科植物型和放线根型结核被认为是根本不同的。在这里，我们分析了由Parasponia andersonii (Rosales) 和Alnus glutinosa (Fagales) 形成的放线菌型根瘤，发现它们的个体发育比一般假设的更类似于豆科植物型根瘤(Fabales)。我们还发现，在蒺藜苜蓿中，共转录调节基因NODULE ROOT1 ( MtNOOT1 ) 的同源突变可将豆科植物型根瘤转化为放线菌型根瘤。这些实验结果表明，这两种结核类型具有共同的进化起源。