Rhizobia and legumes form a symbiotic relationship resulting in the formation of root structures known as nodules, where bacteria fix nitrogen. Legumes release flavonoids that are detected by the rhizobial nodulation (Nod) protein NodD, initiating the transcriptional activation of nod genes and subsequent synthesis of Nod Factors (NFs). NFs then induce various legume responses essential for this symbiosis. Although evidence suggests differential regulation of nodD expression and NF biosynthesis during symbiosis, the necessity of this regulation for the formation of nitrogen‐fixing nodules remains uncertain. Here, we demonstrate that deletion of the Rlv3841 NodD regulatory domain results in a constitutively active protein (NodDFI) capable of activating NF biosynthesis gene expression without the presence of flavonoids. Optimised constitutive expression of nodDFI or nodD3 in nodD null mutants led to wild‐type levels of nodulation and nitrogen fixation in pea and M. truncatula, respectively, indicating that flavonoid‐regulated nodD expression is not essential for supporting symbiosis. These findings illustrate that transcriptional control of flavonoid‐independent NodD regulators can be employed to drive NF biosynthesis, which holds potential for engineering symbiosis between rhizobia and cereals equipped with reconstituted NF receptors.

中文翻译：

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通过类黄酮非依赖性 NodD 调节根瘤菌结瘤基因支持与豆科植物的固氮共生


根瘤菌和豆类形成共生关系，导致形成称为根瘤的根结构，细菌在其中固定氮。豆类释放出由根瘤结瘤 （Nod） 蛋白 NodD 检测到的类黄酮，从而启动 Nod 基因的转录激活和随后的 Nod 因子 （NF） 合成。然后 NF 诱导对这种共生至关重要的各种豆科植物反应。尽管有证据表明共生过程中 nodD 表达和 NF 生物合成的调节不同，但这种调节对固氮结节形成的必要性仍不确定。在这里，我们证明 Rlv3841 NodD 调节结构域的缺失导致组成型活性蛋白 （NodDFI） 能够在不存在类黄酮的情况下激活 NF 生物合成基因表达。nodD 无效突变体中 nodDFI 或 nodD3 的优化组成型表达分别导致豌豆和元后蚜虫的野生型结瘤和固氮水平，表明类黄酮调节的 nodD 表达对于支持共生不是必需的。这些发现表明，非类黄酮依赖性 NodD 调节因子的转录控制可用于驱动 NF 生物合成，这具有在根瘤菌和配备重组 NF 受体的谷物之间实现工程共生的潜力。