Shanghai Key Laboratory of Bio-energy Crops, School of Life Sciences, Plant Science Center, Shanghai University, Shanghai, 200444, China.
School of Communication & Information Engineering, Shanghai University, Shanghai, 200444, China.
Environ Microbiol. 2017 Dec;19(12):5130-5145. doi: 10.1111/1462-2920.13992. Epub 2017 Nov 29.
The development of legume nitrogen-fixing nodules is regulated by reactive oxygen species (ROS) produced by symbionts. Several regulators from Rhizobium are involved in ROS sensing. In a previous study, we found that Sinorhizobium meliloti LsrB regulates lipopolysaccharide production and is associated with H O accumulation in alfalfa (Medicago sativa) nodules. However, its underlying regulatory mechanism remains unclear. Here, we report that the cysteine residues in LsrB are required for adaptation to oxidative stress, gene expression, alfalfa nodulation and nitrogen fixation. Moreover, LsrB directly activated the transcription of lrp3 and gshA (encoding γ-glutamylcysteine synthetase, responsible for glutathione synthesis) and this regulation required the cysteine (Cys) residues in the LsrB substrate-binding domain. The Cys residues could sense oxidative stress via the formation of intermolecular disulfide bonds, generating LsrB dimers and LsrB-DNA complexes. Among the Cys residues, C238 is a positive regulatory site for the induction of downstream genes, whereas C146 and C275 play negative roles in the process. The lsrB mutants with Cys-to-Ser substitutions displayed altered phenotypes in respect to their adaptation to oxidative stress, nodulation and nitrogen fixation-related plant growth. Our findings demonstrate that S. meliloti LsrB modulates alfalfa nodule development by directly regulating downstream gene expression via a post-translational strategy.
豆科植物氮固定根瘤的发育受共生体产生的活性氧（ROS）调节。根瘤菌中有几种调节剂参与 ROS 感应。在之前的一项研究中，我们发现根瘤菌 Sinorhizobium meliloti LsrB 调节脂多糖的产生，并与苜蓿（Medicago sativa）根瘤中 H2O2 的积累有关。然而，其潜在的调控机制尚不清楚。在这里，我们报告 LsrB 中的半胱氨酸残基对于适应氧化应激、基因表达、苜蓿结瘤和固氮是必需的。此外，LsrB 直接激活了 lrp3 和 gshA（编码γ-谷氨酰半胱氨酸合成酶，负责合成谷胱甘肽）的转录，这种调控需要 LsrB 底物结合域中的半胱氨酸残基。半胱氨酸残基可以通过形成分子间二硫键来感知氧化应激，从而产生 LsrB 二聚体和 LsrB-DNA 复合物。在这些半胱氨酸残基中，C238 是诱导下游基因表达的正调控位点，而 C146 和 C275 在该过程中起负作用。半胱氨酸到丝氨酸取代的 lsrB 突变体在适应氧化应激、结瘤和与氮固定相关的植物生长方面表现出不同的表型。我们的研究结果表明，S. meliloti LsrB 通过直接调节下游基因的表达来调节苜蓿根瘤的发育，这是一种翻译后策略。
