Table_2_A novel function of the key nitrogen-fixation activator NifA in beta-rhizobia: Repression of bacterial auxin synthesis during symbiosis.XLSX

Rhizobia fix nitrogen within root nodules of host plants where nitrogenase expression is strictly controlled by its key regulator NifA. We recently discovered that in nodules infected by the beta-rhizobial strain Paraburkholderia phymatum STM815, NifA controls expression of two bacterial auxin synthesis genes. Both the iaaM and iaaH transcripts, as well as the metabolites indole-acetamide (IAM) and indole-3-acetic acid (IAA) showed increased abundance in nodules occupied by a nifA mutant compared to wild-type nodules. Here, we document the structural changes that a P. phymatum nifA mutant induces in common bean (Phaseolus vulgaris) nodules, eventually leading to hypernodulation. To investigate the role of the P. phymatum iaaMH genes during symbiosis, we monitored their expression in presence and absence of NifA over different stages of the symbiosis. The iaaMH genes were found to be under negative control of NifA in all symbiotic stages. While a P. phymatum iaaMH mutant produced the same number of nodules and nitrogenase activity as the wild-type strain, the nifA mutant produced more nodules than the wild-type that clustered into regularly-patterned root zones. Mutation of the iaaMH genes in a nifA mutant background reduced the presence of these nodule clusters on the root. We further show that the P. phymatum iaaMH genes are located in a region of the symbiotic plasmid with a significantly lower GC content and exhibit high similarity to two genes of the IAM pathway often used by bacterial phytopathogens to deploy IAA as a virulence factor. Overall, our data suggest that the increased abundance of rhizobial auxin in the non-fixing nifA mutant strain enables greater root infection rates and a role for bacterial auxin production in the control of early stage symbiotic interactions.

根瘤菌在宿主植物的根瘤中进行固氮作用，其固氮酶的表达受到关键调控因子NifA的严格调控。近期研究揭示了在受β根瘤菌菌株Paraburkholderia phymatum STM815感染的根瘤中，NifA调控了两种细菌性生长素合成基因的表达。iaaM和iaaH的转录物，以及代谢物吲哚乙酰胺（IAM）和吲哚-3-乙酸（IAA）在nifA突变体根瘤中的含量均高于野生型根瘤。本研究记录了P. phymatum nifA突变体在普通菜豆（Phaseolus vulgaris）根瘤中诱导的结构变化，最终导致瘤结过度生长。为探究P. phymatum iaaMH基因在共生过程中的作用，我们监测了其在共生不同阶段NifA存在与不存在条件下的表达。研究发现，iaaMH基因在所有共生阶段均受到NifA的负调控。尽管P. phymatum iaaMH突变体产生的瘤结数量和固氮酶活性与野生型菌株相同，但nifA突变体产生的瘤结数量多于野生型，并聚集于规则的根区。在nifA突变体背景下突变iaaMH基因降低了这些瘤结在根上的分布。进一步研究表明，P. phymatum iaaMH基因位于共生质粒中GC含量显著较低的区域，并且与IAM途径中的两个基因高度相似，该途径常被细菌性植物病原体用作IAA作为致病因子。总体而言，我们的数据表明，非固氮nifA突变体菌株中根瘤菌生长素的增加丰度使得根感染率更高，并且细菌生长素的生产在早期共生相互作用控制中发挥了一定作用。