Haskett Timothy L, Cooke Louise, Green Patrick, Poole Philip S
Department of Biology, University of Oxford, Oxford, UK.
Environ Microbiol. 2025 Jan;27(1):e70014. doi: 10.1111/1462-2920.70014.
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 (NodD) capable of activating NF biosynthesis gene expression without the presence of flavonoids. Optimised constitutive expression of nodD 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.
根瘤菌与豆科植物形成共生关系，导致根结构上形成根瘤，细菌在根瘤中固定氮。豆科植物释放类黄酮，根瘤菌的结瘤（Nod）蛋白NodD能检测到这些类黄酮，从而启动nod基因的转录激活以及随后的Nod因子（NFs）合成。NFs随后诱导这种共生关系所必需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生物合成存在差异调节，但这种调节对固氮根瘤形成的必要性仍不确定。在这里，我们证明Rlv3841 NodD调节域的缺失导致一种组成型活性蛋白（NodD），该蛋白能够在没有类黄酮的情况下激活NF生物合成基因的表达。在nodD缺失突变体中优化nodD或nodD3的组成型表达分别导致豌豆和蒺藜苜蓿中结瘤和固氮达到野生型水平，这表明类黄酮调节的nodD表达对于支持共生并非必不可少。这些发现表明，可利用对不依赖类黄酮的NodD调节因子的转录控制来驱动NF生物合成，这为构建配备重组NF受体的根瘤菌与谷物之间的共生关系提供了潜力。
