Nascimento Francisco X, Tavares Maria J, Rossi Márcio J, Glick Bernard R
Departamento de Microbiologia, Universidade Federal de Santa Catarina, Florianópolis, SC, 88040-900, Brazil.
Department of Biology, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
Heliyon. 2018 Dec 20;4(12):e01068. doi: 10.1016/j.heliyon.2018.e01068. eCollection 2018 Dec.
Ethylene plays an important role in regulating the rhizobial nodulation process. Consequently, numerous strains of rhizobia possess the ability to decrease plant ethylene levels by the expression of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase or via the production of rhizobitoxine, thus, leading to an increased ability to nodulate leguminous plants. Nevertheless, not much is understood about the prevalence of these ethylene modulation genes in different rhizobial groups nor their role in the evolution of the symbiotic process. In this work, we analyze the prevalence and evolution of the enzymes ACC deaminase (AcdS) and dihydrorhizobitoxine desaturase (RtxC) in 395 NodC genomes from different rhizobial strains isolated from a wide range of locations and plant hosts, and discuss their importance in the evolution of the symbiotic process. The obtained results show that AcdS and RtxC are differentially prevalent in rhizobial groups, indicating the existence of several selection mechanisms governed by the rhizobial strain itself and its evolutionary origin, the environment, and, importantly, the leguminous plant host (co-evolution). Moreover, it was found that the prevalence of AcdS and RtxC is increased in and , and lower in other groups. Data obtained from phylogenetic, evolutionary as well as gene localization analysis support the previous hypotheses regarding the ancient origin of the nodulation abilities in and and brings a new perspective for the importance of ethylene modulation genes in the development of the symbiotic process. The acquisition of AcdS by horizontal gene transfer and a positive selection in other rhizobial groups indicates that this enzyme plays an important role in the nodulation process of many rhizobia. On the other hand, RtxC is negatively selected in most symbioses. Understanding the evolution of ethylene modulation genes in rhizobia may be the key to the development of new strategies aiming for an increased nodulation and nitrogen fixation process.
乙烯在调节根瘤菌结瘤过程中发挥着重要作用。因此，许多根瘤菌菌株能够通过表达1-氨基环丙烷-1-羧酸（ACC）脱氨酶或产生根瘤毒素来降低植物乙烯水平，从而增强对豆科植物的结瘤能力。然而，对于这些乙烯调节基因在不同根瘤菌群中的普遍性及其在共生过程进化中的作用，我们了解得并不多。在这项研究中，我们分析了从广泛的地点和植物宿主中分离出的395个根瘤菌菌株的NodC基因组中ACC脱氨酶（AcdS）和二氢根瘤毒素去饱和酶（RtxC）的普遍性和进化情况，并讨论了它们在共生过程进化中的重要性。所得结果表明，AcdS和RtxC在根瘤菌群中的普遍性存在差异，这表明存在几种由根瘤菌菌株本身及其进化起源、环境以及重要的豆科植物宿主（共同进化）所控制的选择机制。此外，研究发现AcdS和RtxC在[具体菌属1]和[具体菌属2]中的普遍性增加，而在其他菌群中较低。从系统发育、进化以及基因定位分析中获得的数据支持了先前关于[具体菌属1]和[具体菌属2]中结瘤能力古老起源的假设，并为乙烯调节基因在共生过程发展中的重要性带来了新的视角。通过水平基因转移获得AcdS以及在其他根瘤菌群中存在正选择，表明该酶在许多根瘤菌的结瘤过程中发挥着重要作用。另一方面，RtxC在大多数共生关系中受到负选择。了解根瘤菌中乙烯调节基因的进化可能是开发旨在增强结瘤和固氮过程的新策略的关键。
