Abstract：
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 Bradyrhizobium 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 B. diazoefficiens. 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 in planta 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. IMPORTANCE Rhizobia, such as B. diazoefficiens, 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.
摘要：
根瘤菌是一组通过与豆科植物共生增加土壤氮含量的细菌。土壤和共生宿主是潜在的压力环境，随着气候变化，土壤可能会变得更加紧张。缓生根瘤菌进化枝内的许多根瘤菌，像缓生根瘤菌重氮，具有合成类大麻的遗传能力，类固醇类脂质在结构和功能上与胆固醇相似。已知类Hopanoids可以防止与重氮芽孢杆菌的生态位相关的压力。矛盾的是,无法合成扩展类的类野果的突变体成功地参与了共生，与野生型相似，尽管根瘤起始延迟。这里,我们表明，在B.重氮效率中，扩展的hopanoid缺陷突变体的生长缺陷可以至少部分地通过物理化学环境来补偿，具体来说,通过最佳的渗透和二价阳离子浓度。通过生物物理测量脂质包装和膜通透性，我们表明，扩展的类胡豆素赋予环境变异性的鲁棒性。这些结果有助于解释先前的培养和植物结果之间的差异，并表明，在可变的土壤环境中，与根瘤中更受控的环境相比，类野果可以为根瘤中的根瘤菌提供更大的适应性优势。通过生物工程或菌株选择改善豆科植物-根瘤菌共生关系，重要的是要考虑根瘤菌的整个生命周期，从土壤到共生。重要性根瘤菌,如B.重氮杂效苷，通过与豆科植物共生使氮气生物可利用，在氮循环中发挥重要作用。气候变化威胁着土壤健康，与能源密集型Haber-Bosch过程相比，这种共生作为更可持续的土壤氮源受到了越来越多的关注。使用根瘤菌作为生物肥料的努力是有效的；然而，将根瘤菌长期整合到土壤群落中的效果较差。这项工作通过确定一种细胞成分-类hopanoid脂质，向改善豆科植物-根瘤菌共生迈出了一小步，该成分赋予了环境胁迫的鲁棒性，随着零星的干燥和洪水事件变得越来越普遍，根瘤菌可能会在土壤微环境中遇到。
