Lipa Paulina, Janczarek Monika
Department of Genetics and Microbiology, Institute of Biological Sciences, Maria Curie-Sklodowska University Lublin, Lublin, Poland.
PeerJ. 2020 Feb 11;8:e8466. doi: 10.7717/peerj.8466. eCollection 2020.
Symbiotic bacteria, commonly called rhizobia, lead a saprophytic lifestyle in the soil and form nitrogen-fixing nodules on legume roots. During their lifecycle, rhizobia have to adapt to different conditions prevailing in the soils and within host plants. To survive under these conditions, rhizobia fine-tune the regulatory machinery to respond rapidly and adequately to environmental changes. Symbiotic bacteria play an essential role in the soil environment from both ecological and economical point of view, since these bacteria provide Fabaceae plants (legumes) with large amounts of accessible nitrogen as a result of symbiotic interactions (i.e., rhizobia present within the nodule reduce atmospheric dinitrogen (N) to ammonia, which can be utilized by plants). Because of its restricted availability in the soil, nitrogen is one of the most limiting factors for plant growth. In spite of its high content in the atmosphere, plants are not able to assimilate it directly in the N form. During symbiosis, rhizobia infect host root and trigger the development of specific plant organ, the nodule. The aim of root nodule formation is to ensure a microaerobic environment, which is essential for proper activity of nitrogenase, i.e., a key enzyme facilitating N fixation. To adapt to various lifestyles and environmental stresses, rhizobia have developed several regulatory mechanisms, e.g., reversible phosphorylation. This key mechanism regulates many processes in both prokaryotic and eukaryotic cells. In microorganisms, signal transduction includes two-component systems (TCSs), which involve membrane sensor histidine kinases (HKs) and cognate DNA-binding response regulators (RRs). Furthermore, regulatory mechanisms based on phosphoenolopyruvate-dependent phosphotranspherase systems (PTSs), as well as alternative regulatory pathways controlled by Hanks-type serine/threonine kinases (STKs) and serine/threonine phosphatases (STPs) play an important role in regulation of many cellular processes in both free-living bacteria and during symbiosis with the host plant (e.g., growth and cell division, envelope biogenesis, biofilm formation, response to stress conditions, and regulation of metabolism). In this review, we summarize the current knowledge of phosphorylation systems in symbiotic nitrogen-fixing bacteria, and their role in the physiology of rhizobial cells and adaptation to various environmental conditions.
共生细菌，通常被称为根瘤菌，在土壤中过着腐生生活，并在豆科植物根部形成固氮根瘤。在其生命周期中，根瘤菌必须适应土壤中和宿主植物体内普遍存在的不同条件。为了在这些条件下生存，根瘤菌对调节机制进行微调，以便对环境变化做出快速而充分的反应。从生态和经济角度来看，共生细菌在土壤环境中起着至关重要的作用，因为这些细菌通过共生相互作用为豆科植物（豆类）提供大量可利用的氮（即根瘤内的根瘤菌将大气中的二氮（N₂）还原为氨，植物可以利用氨）。由于氮在土壤中的可用性有限，它是植物生长最限制的因素之一。尽管大气中氮含量很高，但植物无法直接以N₂形式吸收它。在共生过程中，根瘤菌感染宿主根并触发特定植物器官根瘤的发育。根瘤形成的目的是确保微需氧环境，这对于固氮酶（即促进氮固定的关键酶）的正常活性至关重要。为了适应各种生活方式和环境压力，根瘤菌已经发展出几种调节机制，例如可逆磷酸化。这种关键机制调节原核细胞和真核细胞中的许多过程。在微生物中，信号转导包括双组分系统（TCSs），它涉及膜传感器组氨酸激酶（HKs）和同源DNA结合反应调节因子（RRs）。此外，基于磷酸烯醇丙酮酸依赖性磷酸转移酶系统（PTSs）的调节机制，以及由汉克斯型丝氨酸/苏氨酸激酶（STKs）和丝氨酸/苏氨酸磷酸酶（STPs）控制的替代调节途径，在自由生活细菌以及与宿主植物共生期间（例如生长和细胞分裂、包膜生物合成、生物膜形成、对压力条件的反应以及代谢调节）的许多细胞过程调节中发挥重要作用。在这篇综述中，我们总结了共生固氮细菌中磷酸化系统的当前知识，以及它们在根瘤菌细胞生理学和适应各种环境条件中的作用。
