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.

一些植物通过共生重氮根瘤菌或Frankia来固定大气中的氮根器官中的细菌称为结节。这种结节共生以四个分类顺序出现在10个植物谱系中：Fabales，Fagales，葫芦科和Rosales，这些植物统称为固氮进化枝。根据个体发育和组织学的不同，结节分为两种类型：豆类和结节型结节。这些结节类型之间的进化关系一直是分子生物学和进化生物学家的长期难题。最近对固氮进化枝中根瘤和非根瘤物种的种系研究表明，根瘤性状在所有10个谱系中都有共同的进化起源。然而，该假设面临一个难题，因为豆科植物型和放线菌型结核被认为是根本不同的。这里，Parasponia andersonii（Rosales的）和桤木（山毛榉目），并发现它们的个体发育更类似于豆科植物型结节（豆目）比通常假设的。我们还显示，在Medi藜苜蓿中，共转录调节基因NODULE ROOT1（MtNOOT1）的同源突变将豆类结节转变为放线型结节。这些实验结果表明，两种根瘤类型具有共同的进化起源。