Rhizobium leguminosarum bv. viciae 3841 Adapts to 2,4-Dichlorophenoxyacetic Acid with “Auxin-Like” Morphological Changes, Cell Envelope Remodeling and Upregulation of Central Metabolic Pathways

There is a growing need to characterize the effects of environmental stressors at the molecular level on model organisms with the ever increasing number and variety of anthropogenic chemical pollutants. The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), as one of the most widely applied pesticides in the world, is one such example. This herbicide is known to have non-targeted undesirable effects on humans, animals and soil microbes, but specific molecular targets at sublethal levels are unknown. In this study, we have used Rhizobium leguminosarum bv. viciae 3841 (Rlv) as a nitrogen fixing, beneficial model soil organism to characterize the effects of 2,4-D. Using metabolomics and advanced microscopy we determined specific target pathways in the Rlv metabolic network and consequent changes to its phenotype, surface ultrastructure, and physical properties during sublethal 2,4-D exposure. Auxin and 2,4-D, its structural analogue, showed common morphological changes in vitro which were similar to bacteroids isolated from plant nodules, implying that these changes are related to bacteroid differentiation required for nitrogen fixation. Rlv showed remarkable adaptation capabilities in response to the herbicide, with changes to integral pathways of cellular metabolism and the potential to assimilate 2,4-D with consequent changes to its physical and structural properties. This study identifies biomarkers of 2,4-D in Rlv and offers valuable insights into the mode-of-action of 2,4-D in soil bacteria.

随着人为源化学污染物的种类与数量与日俱增，在分子层面解析环境胁迫因子对模式生物的影响已成为愈发迫切的研究需求。除草剂2,4-二氯苯氧乙酸（2,4-dichlorophenoxyacetic acid, 2,4-D）作为全球应用最广泛的农药之一，正是这类污染物的典型案例。已知该除草剂会对人类、动物及土壤微生物产生非靶向的不良影响，但亚致死浓度下其具体的分子作用靶点仍未明确。本研究以豌豆根瘤菌（Rhizobium leguminosarum bv. viciae 3841，以下简称Rlv）这一具备固氮功能的有益土壤模式生物为研究对象，旨在解析2,4-D对其产生的生物学效应。借助代谢组学与先进显微成像技术，本研究明确了Rlv在亚致死浓度2,4-D暴露过程中，其代谢网络内的特异性靶向通路，以及随之发生的表型、表面超微结构与物理性质变化。生长素及其结构类似物2,4-D在体外实验中呈现出一致的形态变化，该变化与从植物根瘤中分离得到的类菌体的形态变化高度相似，这表明此类变化与固氮作用所需的类菌体分化过程密切相关。Rlv对该除草剂展现出显著的适应能力：其细胞代谢的核心通路发生重塑，且具备同化2,4-D的潜在能力，进而导致其物理与结构性质产生相应改变。本研究成功鉴定出Rlv中针对2,4-D的生物标志物，并为解析2,4-D在土壤细菌中的作用模式提供了极具价值的科学见解。