在全球范围内，干旱所引发的作物减产总量已超越其他气候因素造成的减产量总和。植物根际促生菌（Plant growth-promoting rhizobacteria，PGPR）作为兼具环境友好性与功能特异性的微生物资源，其通过分泌胞外多糖、调控激素水平及激活抗氧化系统来提升植物耐旱性而备受关注。相较于单一菌株，复合菌群因其功能多样、菌间协同效应以及强生态适应性，在提高大豆生物量、缓解干旱损伤方面的潜力，成为国内外研究热点。然而，针对复合菌群提升鲜食大豆耐旱性的作用机制研究仍较为匮乏。为此，本文开展以下研究。

为了获得提高鲜食大豆耐旱性的复合菌群，以PEG-6000模拟干旱胁迫，以胞外多糖（Exopolysaccharide, EPS）产量为核心筛选指标，构建耐旱菌群。筛选到两株耐旱芽孢杆菌：Bacillus mojavensis WB3和Priestia megaterium NG127，分别与Ensifer adhearens NER9构建两组复合菌群：NER9-WB3组合和NER9-NG127组合。在0-20% PEG-6000的渗透胁迫下，两个复合菌群的EPS产量分别较单菌处理显著提高39.11-308%和45.98-216%，展现出显著的协同效应。

（2）为了探究高产EPS菌群对鲜食大豆耐旱性的提升效果，本文通过盆栽试验，分析了干旱胁迫下两个菌群对不同品种鲜食大豆的生长发育、含水量、抗氧化酶活性和土壤特性的影响。盆栽试验结果表明，两个复合菌群较不接菌对照显著提升鲜食大豆地上部干重（45.84-51.43%）、籽粒产量（218-246%）、根瘤数量（216-242%）和叶片相对含水量（41.80-44.44%）。与不接菌对照相比，两个复合菌群显著提高鲜食大豆叶片脯氨酸（87.63-88.27%）、根瘤豆血红蛋白含量（49.87-55.73%），显著降低叶片丙二醛含量（33.98-36.24%），同时显著增强叶片和土壤抗氧化酶活性。此外，干旱条件下复合菌群较不接菌处理，显著提高鲜食大豆根际土壤多糖含量（135-147%），改变土壤团聚体比例。此外，NER9-WB3从植物渗透调节、抗氧化系统和根际土壤改良这三个方面对鲜食大豆耐旱性的提升效果优于单菌及NER9-NG127处理。

（3）为了阐明NER9-WB3复合菌群增强鲜食大豆耐旱性的分子机制，采用实时荧光定量PCR技术系统检测菌群EPS合成基因、植物响应干旱胁迫相关基因的表达，并通过高通量测序分析根际土壤细菌群落结构与功能。结果表明，与单菌处理相比，复合菌群处理显著增加了NER9的基因1817（多糖丙酮酰转移酶家族蛋白）和PA1229（胞外多糖产生蛋白ExoZ）、WB3的基因3387（多糖脱乙酰酶）和2670（胞外多糖产生蛋白ExoZ）4个EPS合成关键基因的表达量。其中在0-30% PEG胁迫下复合菌群处理中ExoZ基因表达较NER9处理显著增加，而比WB3显著下降，表明NER9菌株在复合菌群EPS合成中占主导地位。干旱条件下，相较于单菌处理，复合菌群显著上调苗期和成熟期大豆根和叶组织中水通道蛋白基因GmSIP（2.75-10.39倍）、植物珠蛋白基因GmPgb1（4.09-7.79倍）、锌指蛋白基因GmZnF1（2.45-12.63倍）和阴离子通道蛋白基因GmVDAC（1.81-7.45倍）的相对表达量，并且显著上调苗期根瘤结瘤基因GmLB1（4.01倍）、GmEnod40（4.15倍）、GmPTF1（5.04倍）的相对表达量。鲜食大豆根际土壤细菌高通量测序结果表明，在属水平上，与不接菌处理相比，NER9-WB3处理组在正常和干旱条件下分别显著富集Bacillus、Arthrobacter、Ensifer、Azoarcus、Azohydromonas等具有结瘤固氮功能的植物促生菌和Bacillus、Streptomyces、Microvirga、Lysobacter等具有耐旱和固氮能力的促生菌。冗余分析结果表明，Lysobacter、Bacillus、Arthrobacter与叶片和土壤抗氧化酶呈显著正相关，推测这些优势菌属是造成鲜食大豆各种耐旱相关指标表现差异的关键类群。

综上所述，高产胞外多糖的复合菌群NER9-WB3协同调控鲜食大豆渗透调节系统和抗氧化能力，提高根际土壤中具有耐旱、结瘤固氮功能的根际促生菌Bacillus、Lysobacter、Streptomyces等细菌的丰度，协助鲜食大豆抵御干旱胁迫。本研究为耐旱型微生物菌剂的研发提供理论依据，为旱作农业区大豆产量稳定与根际土壤健康维持提供了技术支撑，助力农业应对气候变化的可持续发展。

Globally, the total reduction in crop yields caused by drought has exceeded the combined reduction in yields caused by other climatic factors. Plant growth-promoting rhizobacteria (PGPR), as a microbial resource that is both environmentally friendly and functionally specific, has attracted much attention for its ability to enhance plant drought tolerance by secreting exopolysaccharides, regulating hormone levels, and activating the antioxidant system. Compared with single strains, microbiota, due to their diverse functions, synergistic effects among bacteria, and strong ecological adaptability, have become a research hotspot both at home and abroad for their potential in increasing soybean biomass and alleviating drought damage. However, there is very little research on the mechanism by which composite microbial communities improve the drought tolerance of vegetable soybeans.

(1)In order to obtain a microbiota that can enhance the drought tolerance of vegetable soybeans, PEG-6000 was used to simulate drought stress. And the yield of exopolysaccharide (EPS) was taken as the core screening index to construct a drought-tolerant microbiota. Two strains of drought-tolerant bacilli, Bacillus mojavensis WB3 and Priestia megaterium NG127, were screened out. They were respectively combined with Ensifer adhearens NER9 to establish two groups of microbiota: the NER9-WB3 combination and the NER9-NG127 combination. Under the osmotic stress of 0-20% PEG-6000, the EPS yields of the two microbiota were significantly increased by 39.11-308% and 45.98-216% respectively compared with the treatments of single strains, demonstrating significant synergistic effects.

(2)In order to explore the effect of the high-EPS-producing microbiota on improving the drought tolerance of vegetable soybeans, this paper analyzed the effects of the two microbiota on the growth and development, water content, antioxidant enzyme activities, and soil properties of different varieties of vegetable soybeans under drought stress through a pot experiment. The results showed that, compared with the control without inoculation of bacteria, the treatment with the two microbiota significantly increased the aboveground dry weight (45.84-51.43%), grain yield (218-246%), the number of root nodules (216-242%), and the relative water content of leaves (41.80-44.44%) of vegetable soybeans. Compared with the non-inoculation treatment, the two microbiota significantly increased the proline content in the leaves of vegetable soybeans (87.63-88.27%) and the leghemoglobin content in  nodules (49.87-55.73%), and significantly reduced the malondialdehyde content in the leaves (33.98-36.24%). At the same time, they significantly enhanced the antioxidant enzyme activities in both the leaves and the soil. In addition, under drought conditions, compared with the treatment without inoculation of bacteria, the microbiota significantly increased the polysaccharide content in the rhizosphere soil of vegetable soybeans (135-147%) and changed the proportion of soil aggregates. Moreover, the results of the experiment indicated that, from the three aspects of plant osmotic adjustment, the antioxidant system, and rhizosphere soil improvement, the NER9-WB3 treatment was superior to the single-strain treatment and the NER9-NG127 treatment in improving the drought tolerance of vegetable soybeans.

(3)In order to elucidate the molecular mechanism by which the NER9-WB3 microbiota enhances the drought tolerance of vegetable soybeans, real-time fluorescence quantitative PCR technology was used to systematically detect the expression of genes related to the synthesis of EPS by the microbial community and the plant's response to drought stress. Additionally, high-throughput sequencing was used to analyze the structure and function of the bacterial community in the rhizosphere soil.The results showed that, compared with the treatment of single strains, the treatment with the microbiota significantly increased the expression levels of four key genes involved in EPS synthesis: gene1817 (polysaccharide pyruvyl transferase family protein) and genePA1229 (exopolysaccharide-producing protein ExoZ) of NER9. gene3387 (polysaccharide deacetylase) and gene2670 (exopolysaccharide-producing protein ExoZ) of WB3. Among them, under the stress of 0-30% PEG, the expression of the ExoZ gene in the treatment with the microbiota was significantly higher than that in the NER9 treatment, but significantly lower than that in the WB3 treatment, indicating that the NER9 strain played a dominant role in the EPS synthesis of the microbiota.Under drought conditions, compared with the treatment of single strains, the microbiota significantly upregulated the relative expression levels of the aquaporin gene GmSIP (2.75-10.39 times), the plant globin gene GmPgb1 (4.09-7.79 times), the zinc finger protein gene GmZnF1 (2.45-12.63 times), and the anion channel protein gene GmVDAC (1.81-7.45 times) in the root and leaf tissues of vegetable soybeans at the seedling and mature stages. Moreover, it significantly upregulated the relative expression levels of the nodulation genes GmLB1 (4.01 times), GmEnod40 (4.15 times), and GmPTF1 (5.04 times) in nodules at the seedling stage.The results of high-throughput sequencing of the bacteria in the rhizosphere soil of vegetable soybeans showed that, at the genus level, compared with the control without inoculation of bacteria group, the treatment group with NER9-WB3 was significantly enriched with PGPR with nodulation and nitrogen fixation functions, such as Bacillus, Arthrobacter, Ensifer, Azoarcus, and Azohydromonas, under normal and drought conditions, as well as plant growth-promoting bacteria with drought tolerance and nitrogen fixation abilities, such as Bacillus, Streptomyces, Microvirga, and Lysobacter. Redundancy analysis showed that Lysobacter, Bacillus, and Arthrobacter had a significant positive correlation with the antioxidant enzymes in the leaves and soil. It is speculated that these dominant genera are the key groups contributing to the differences in various drought tolerance-related indicators of vegetable soybeans.

In conclusion, the microbiota NER9-WB3 with high exopolysaccharide production synergistically regulates the osmotic adjustment system and antioxidant capacity of vegetable soybeans, increases the abundance of PGPR such as Bacillus, Lysobacter, and Streptomyces with functions of drought tolerance, nodulation, and nitrogen fixation in the rhizosphere soil, and helps vegetable soybeans resist drought stress. This study provides a theoretical basis for the research and development of drought-tolerant microbial agents, offers technical support for the stable yield of soybeans and the maintenance of rhizosphere soil health in dry farming areas, and contributes to the sustainable development of agriculture in response to climate change.

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