Division of Chemistry and Chemical Engineering, California Institute of Technologygrid.20861.3d, Pasadena, California, USA.
B CUBE Center for Molecular Bioengineering, Technische Universität Dresden, Dresden, Germany.
J Bacteriol. 2022 Jul 19;204(7):e0044221. doi: 10.1128/jb.00442-21. Epub 2022 Jun 3.
Rhizobia are a group of bacteria that increase soil nitrogen content through symbiosis with legume plants. The soil and symbiotic host are potentially stressful environments, and the soil will likely become even more stressful as the climate changes. Many rhizobia within the clade, like Bradyrhizobium diazoefficiens, possess the genetic capacity to synthesize hopanoids, steroid-like lipids similar in structure and function to cholesterol. Hopanoids are known to protect against stresses relevant to the niche of . Paradoxically, mutants unable to synthesize the extended class of hopanoids participate in symbioses with success similar to that of the wild type, despite being delayed in root nodule initiation. Here, we show that in B. diazoefficiens, the growth defects of extended-hopanoid-deficient mutants can be at least partially compensated for by the physicochemical environment, specifically, by optimal osmotic and divalent cation concentrations. Through biophysical measurements of lipid packing and membrane permeability, we show that extended hopanoids confer robustness to environmental variability. These results help explain the discrepancy between previous in-culture and results and indicate that hopanoids may provide a greater fitness advantage to rhizobia in the variable soil environment than the more controlled environments within root nodules. To improve the legume-rhizobium symbiosis through either bioengineering or strain selection, it will be important to consider the full life cycle of rhizobia, from soil to symbiosis. Rhizobia, such as , play an important role in the nitrogen cycle by making nitrogen gas bioavailable through symbiosis with legume plants. As climate change threatens soil health, this symbiosis has received increased attention as a more sustainable source of soil nitrogen than the energy-intensive Haber-Bosch process. Efforts to use rhizobia as biofertilizers have been effective; however, long-term integration of rhizobia into the soil community has been less successful. This work represents a small step toward improving the legume-rhizobium symbiosis by identifying a cellular component-hopanoid lipids-that confers robustness to environmental stresses rhizobia are likely to encounter in soil microenvironments as sporadic desiccation and flooding events become more common.
根瘤菌是一组通过与豆科植物共生来增加土壤氮含量的细菌。土壤和共生宿主是潜在的应激环境,随着气候变化,土壤可能会变得更加紧张。在 分支内的许多根瘤菌,如慢生根瘤菌,具有合成甾族化合物样脂类的遗传能力,这些脂类在结构和功能上与胆固醇相似。已知甾族化合物样脂类可以抵御与 生境相关的应激。矛盾的是,尽管根瘤起始延迟,但无法合成扩展类甾族化合物样脂类的突变体能够成功参与共生,类似于野生型。在这里,我们表明在慢生根瘤菌中,扩展型甾族化合物样脂类缺陷突变体的生长缺陷至少可以部分通过物理化学环境来补偿,特别是通过最佳的渗透压和二价阳离子浓度。通过脂质包装和膜通透性的生物物理测量,我们表明扩展的甾族化合物样脂类赋予了对环境变化的稳健性。这些结果有助于解释先前在培养中和 结果之间的差异,并表明甾族化合物样脂类可能为根瘤菌在多变的土壤环境中提供比根瘤内更受控制的环境更大的适应性优势。为了通过生物工程或菌株选择来改善豆科植物-根瘤菌共生关系,考虑根瘤菌的完整生命周期,从土壤到共生关系,将非常重要。 如慢生根瘤菌,通过与豆科植物共生使氮气生物可用,在氮循环中发挥重要作用。随着气候变化威胁到土壤健康,这种共生关系作为比能源密集型哈伯-博世过程更可持续的土壤氮源而受到更多关注。利用根瘤菌作为生物肥料的努力已经取得了成效;然而,将根瘤菌长期整合到土壤群落中则不太成功。这项工作代表着通过鉴定细胞成分-甾族化合物样脂类-赋予根瘤菌对土壤微环境中可能遇到的环境压力的稳健性,朝着改善豆科植物-根瘤菌共生关系迈出了一小步,因为零星的干旱和洪水事件变得越来越普遍。