Bradyrhizobium japonicum E109 is a bacterium widely used for inoculants production in Argentina. It is known for its ability to produce several phytohormones and degrade indole-3-acetic acid (IAA). The genome sequence of B. japonicum E109 was recently analyzed and it showed the presence of genes related to the synthesis of IAA by indole-3-acetonitrile, indole-3-acetamide and tryptamine pathways. Nevertheless, B. japonicum E109 is not able to produce IAA and instead has the ability to degrade this hormone under saprophytic culture conditions. This work aimed to study the molecular and physiological features of IAA degradation and identify the genes responsible of this activity. In B. japonicum E109 we identified two sequences coding for a putative 3-phenylpropionate dioxygenase (subunits α and β) responsible for the IAA degradation that were homologous to the canonical cluster of iacC and iacD of Pseudomonas putida 1290. These genes form a separate cluster together with three additional genes with unknown functions. The degradation activity was found to be constitutively expressed in B. japonicum E109. As products of IAA degradation, we identified two compounds, 3-indoleacetic acid 2,3-oxide and 2-(2-hydroperoxy-3-hydroxyindolin-3-yl) acetic acid. Our report proposes, for the first time, a model for IAA degradation in Bradyrhizobium.

Bradyrhizobium japonicum E109 是一种在阿根廷广泛用于接种剂生产的细菌。它以产生多种植物激素和降解吲哚-3-乙酸 (IAA) 的能力而闻名。最近分析了B. japonicum E109的基因组序列，它显示存在与通过 indole-3-乙腈、indole-3-乙酰胺和色胺途径合成 IAA 相关的基因。然而， B. japonicum E109 不能产生 IAA，而是能够在腐生培养条件下降解这种激素。这项工作旨在研究 IAA 降解的分子和生理特征，并确定负责该活动的基因。在B. japonicumE109 我们鉴定了两个序列编码推定的 3-苯基丙酸双加氧酶（亚基 α 和 β），负责 IAA 降解，它们与恶臭假单胞菌1290的iac C 和iac D的典型簇同源。这些基因与三个额外的功能未知的基因。发现降解活性在B. japonicum E109 中组成型表达。作为 IAA 降解的产物，我们确定了两种化合物，3-吲哚乙酸 2,3-氧化物和 2-(2-氢过氧-3-羟基吲哚-3-基) 乙酸。我们的报告首次提出了慢生根瘤菌中IAA 降解的模型。