The use of plant growth-promoting bacteria represents an alternative to the massive use of mineral fertilizers in agriculture. However, some abiotic stresses commonly found in the environment, like salinity, can affect the efficiency of this approach. Here, we investigated the key mechanisms involved in the response of the plant growth-promoting bacterium Gluconacetobacter diazotrophicus to salt stress by using morphological and cell viability analyses, comparative proteomics, and reverse genetics. Our results revealed that the bacteria produce filamentous cells in response to salt at 100 mM and 150 mM NaCl. However, such a response was not observed at higher concentrations, where cell viability was severely affected. Proteomic analysis showed that salt stress modulates proteins involved in several pathways, including iron uptake, outer membrane efflux, osmotic adjustment, cell division and elongation, and protein transport and quality control. Proteomic data also revealed the repression of several extracytoplasmic proteins, especially those located at periplasm and outer membrane. The role of such pathways in the tolerance to salt stress was analyzed by the use of mutant defectives for Δtbdr (iron uptake), ΔmtlK and ΔotsA (compatible solutes synthesis), and ΔdegP (quality control of nascent extracytoplasmic proteins). ΔdegP presented the highest sensitivity to salt stress, Δtbdr, andΔmtlK also showed increased sensitivity, but ΔotsA was not affected. This is the first demonstration that DegP protein, a protease with minor chaperone activity, is essential for tolerance to salt stress in G. diazotrophicus. Our data contribute to a better understanding of the molecular bases that control the bacterial response/tolerance to salt stress, shedding light on quality control of nascent extracytoplasmic proteins.

使用促进植物生长的细菌替代了在农业中大量使用矿物肥料。但是，环境中常见的一些非生物胁迫（例如盐度）会影响这种方法的效率。在这里，我们通过形态和细胞生存力分析，比较蛋白质组学和反向遗传学，研究了促进植物生长的细菌重氮糖杆菌对盐胁迫的响应的关键机制。我们的结果表明，细菌会在100 mM和150 mM NaCl的盐浓度下产生丝状细胞。然而，在较高浓度下未观察到这种反应，在较高浓度下细胞活力受到严重影响。蛋白质组学分析表明，盐胁迫可调节涉及多种途径的蛋白质，包括铁的吸收，外膜外排，渗透调节，细胞分裂和伸长，蛋白质转运和质量控制。蛋白质组学数据还揭示了几种胞浆外蛋白的抑制，特别是位于周质和外膜的蛋白。通过使用Δ突变缺陷来分析此类途径在耐盐胁迫中的作用TBDR（铁摄取），Δ MTLK和Δ奖励计划（相容性溶质合成），和Δ的degP（新生细胞质外的蛋白质的质量控制）。Δ的degP呈现对盐胁迫，Δ最高灵敏度TBDR，和δ MTLK也显示增加的敏感性，但Δ奖励计划并没有受到影响。这是第一个证明DegP蛋白（一种具有较小分子伴侣活性的蛋白酶）对于重氮营养菌中盐胁迫的耐受性至关重要。。我们的数据有助于更好地了解控制细菌对盐胁迫的响应/耐受性的分子碱基，从而有助于对新生胞质蛋白的质量控制。