Plant growth-promoting rhizobacteria(PGPR)and its mechanisms against plant diseases for sustainable agriculture and better productivity

Plant growth-promoting rhizobacteria(PGPR)are specialized bacterial communities inhabiting the root rhizosphere and the secretion of root exudates helps to,regulate the microbial dynamics and their interactions with the plants.These bacteria viz.,Agrobacterium,Arthobacter,Azospirillum,Bacillus,Burkholderia,Flavobacterium,Pseudomonas,Rhizobium,etc.,play important role in plant growth promotion.In addition,such symbiotic associations of PGPRs in the rhizospheric region also confer protection against several diseases caused by bacterial,fungal and viral pathogens.The biocontrol mechanism utilized by PGPR includes direct and indirect mechanisms direct PGPR mechanisms include the production of antibiotic,siderophore,and hydrolytic enzymes,competition for space and nutrients,and quorum sensing whereas,indirect mechanisms include rhizomicrobiome regulation via.secretion of root exudates,phytostimulation through the release of phytohormones viz.,auxin,cytokinin,gibberellic acid,1-aminocyclopropane-1-carboxylate and induction of systemic resistance through expression of antioxidant defense enzymes viz.,phenylalanine ammonia lyase(PAL),peroxidase(PO),polyphenyloxidases(PPO),superoxide dismutase(SOD),chitinase andβ-glucanases.For the suppression of plant diseases potent bio inoculants can be developed by modulating the rhizomicrobiome through rhizospheric engineering.In addition,understandings of different strategies to improve PGPR strains,their competence,colonization efficiency,persistence and its future implications should also be taken into consideration.

Application of PCR primer sets for detection of <i>Pseudomonas</i>sp. functional genes in the plant rhizosphere

Plant growth promoting pseudomonads play an important role in disease suppression and there is considerable interest in development of bio-marker genes that can be used to monitor these bacteria in agricultural soils. Here, we report the application ofa PCR primer sets targeting genes encoding the main antibiotic groups. Distribution of the genes was variably distributed across type strains of 28 species with no phylogenetic groupingfor the detected antibioticsgenes, phlD for 2,4-diacetylphloroglucinol (2,4-DAPG) and phzCD for phenazine-1-carboxylic acid or hcnBC for hydrogen cyanide production. Analysis of field soils showed that primer sets for phlD and phzCD detected these genes in a fallowed neutral pH soil following wheat production, but that the copy numbers were below the detection limits in bulk soils having an acidic pH. In contrast, PCR products for the phzCD, pltc and hcnBc genes were detectable in mature root zones following plantingwith wheat. The ability to rapidly characterize populations of antibiotics producers using specific primer sets will improve our ability to assess the impacts of management practices on the functional traits of Pseudomonas spp. populations in agricultural soils.

根际促生细菌(PGPR)对冬枣根际土壤微生物数量及细菌多样性影响

从6年生冬枣根际土壤中筛选出1株根际促生细菌(PGPR),以发酵鸡粪(DC)为吸附载体制成PGPR生物肥(PF),利用传统的氯仿熏蒸法和现代的T-RFLP技术,从冬枣根际土壤中可培养微生物的数量、细菌群落多样性和微生物量碳的动态变化3个方面,分析PF、普通生物肥料(NF)和DC对冬枣根际土壤微生物特征的影响。结果表明:同NF处理相比,PF处理中的细菌数量和微生物总量显著增加,真菌数量显著减少,但放线菌数量差异不显著。PF处理根际土壤具有较高的丰富度指数、多样性指数和均匀度指数。基于T-RFLP的主成分分析结果表明:PF处理的细菌群落结构成为1个独立的群,NF,DC和CK处理构成1个相对独立的群。此外,同其他试验处理相比,PF可相对稳定地提供冬枣生长周期内的微生物生物量碳。PGPR生物肥料的施用可提高可培养微生物数量,提高根际土壤中微生物多样性,有效改善冬枣根际土壤的微生态环境。

太阳能消毒与植物根际促生菌(PGPR)联合对黄瓜根腐病的防治效果

为明确太阳能土壤消毒与植物根际促生菌(PGPR)联合对黄瓜根腐病防治效果,在对温室进行太阳能土壤消毒之后,采用蘸根和灌根的方式测定了解磷、抑菌效果较好的3个菌株JPG-5、LWG-5和YB-4对黄瓜根腐病的田间防治效果及其对黄瓜根围养分的影响。结果表明:1)YB-4300倍蘸根对黄瓜根腐病防治效果最高,病指防效为89.49%,显著高于LWG-5300倍和JPG-5500倍的防治效果。2)LWG-5500倍灌根对黄瓜根腐病防治效果最高,为89.47%,显著高于LWG-5800倍和JPG-5500倍防治效果。3)不管是灌根还是蘸根,JPG-5500倍和YB-4300倍均能显著提高黄瓜根围有效磷含量,分别为99.68、86.81 mg/kg和84.30、86.56 mg/kg。因此,PGPR菌株在田间促进养分转化效果和对根腐病的防治效果综合评判,用YB-4300倍蘸根、JPG-5500倍灌根和LWG-5500倍灌根效果较好。

Mutual Information Flow between Beneficial Microorganisms and the Roots of Host Plants Determined the Bio-Functions of Biofertilizers

Modern agriculture is facing new challenges in which ecological and molecular approaches are being integrated to achieve higher crop yields while minimizing negative impacts on the environment. The application of biofertilzers could meet this requirement. Biofertilizer is a natural organic fertilizer that helps to provide all the nutrients required by the plants and helps to increase the quality of the soil with a natural microorganism environment. This paper reviewed the types of biofertilzers, the biological basic of biofertilizers in plant growth promotion. This paper also assayed the bidirectional information exchange between plant-microbes in rhizoshpere and the signal pathway of plant growth- promoting rhizobacteria (PGPR) and plant growth-promoting fungi (PGPF) in the course of plant infection. At last, the challenges of the application and the promising future of biofertilizers were also discussed.

根际微生物群落与促生菌多样性及其筛选策略

根际是地球上最大的生态系统,在生物圈功能中发挥着非常显著的作用,微生物和植物在根际环境中形成的复杂网络式关系会直接或间接地影响植物生长,深入了解并利用这种互作关系对于提高农业产出投入比以及筛选获得更高效、广适的促生菌尤为必要。综述了根际微生物群落与促生菌多样性以及筛选策略,分析了研究中尚存在的一些问题,并对今后的研究进行了展望。

连作大豆根际促生菌的筛选鉴定及促生效果研究

为获得抗(耐)连作障碍的植物根际促生菌(plant growth-promoting rhizobacteria,PGPR),解决连作障碍对大豆的不良影响,从黑龙江大庆市林甸县试验基地采集连作多年的大豆根际土,从中分离筛选具有解磷和固氮的功能的菌株,进行形态学、生理生化指标、16S rDNA分子生物学鉴定,并对菌株的解磷、分泌IAA(3-吲哚乙酸)能力及ACC脱氨酶活性进行测定,对菌株对种子萌发和苗期的促生作用进行研究和分析。结果得到22株PGPR菌株经鉴定分别属于假单胞菌属(Pseudomonas),肠杆菌属(Enterobacter),土壤杆菌属(Agrobacterium),柠檬酸杆菌属(Citrobacter),沙雷氏菌属(Serratia),克雷伯氏菌属(Klebsiella)。菌株的有效溶磷量范围为0.19~17.49 mg·L^(-1);菌株均有固氮能力;菌株的IAA产量范围为17.72~88.21μg·mL^(-1);菌株的ACC脱氨酶活性范围为0.33~0.67 U·mg^(-1)。一株菌株对大豆种子萌发有显著促进作用;两株菌株对大豆苗期促生效果明显。研究结果可为后续PGPR微生物菌肥的研制与开发提供试验基础。

Characterization of 1-Aminocyclopropane-1-Carboxylate(ACC) Deaminase-Containing Pseudomonas spp.in the Rhizosphere of Salt-Stressed Canola

When exposed to biotic or abiotic stress conditions, plants produce ethylene from its immediate precursor 1-aminocyclopropane-1- carboxylate （ACC）, leading to retarded root growth and senescence. Many plant growth-promoting rhizobacteria contain the enzyme ACC deaminase and this enzyme can cleave ACC to form a-ketobutyrate and ammonium, thereby lowering levels of ethylene. The aim of this study was to isolate and characterize ACC deaminase-producing bacteria from the rhizosphere of salt-stressed canola （Brassica napus L.）. Out of 105 random bacterial isolates, 15 were able to utilize ACC as the sole source of nitrogen. These 15 isolates were also positive for indole acetic acid （IAA） production. Phylogenetic analysis based on partial 16S rDNA sequences showed that all isolates belonged to fluorescent Pseudomonas spp. In the canola rhizosphere investigated in this study, Pseudomonas fluorescens was the dominant ACC deaminase-producing species. Cluster analysis based on BOX-AIR-based repetitive extragenic palindromic-polymerase chain reaction （BOX-PCR） patterns suggested a high degree of genetic variability in ACC deaminase-producing P. fluorescens strains. The presence of indigenous ACC-degrading bacteria in the rhizosphere of canola grown in saline soils indicates that these bacteria may contribute to salinity tolerance.

Siderophore-Producing Rhizobacteria as a Promising Tool for Empowering Plants to Cope with Iron Limitation in Saline Soils:A Review

Iron(Fe) bioavailability to plants is reduced in saline soils;however, the exact mechanisms underlying this effect are not yet completely understood. Siderophore-expressing rhizobacteria may represent a promising alternative to chemical fertilizers by simultaneously tackling salt-stress effects and Fe limitation in saline soils. In addition to draught, plants growing in arid soils face two other major challenges: high salinity and Fe deficiency. Salinity attenuates growth, affects plant physiology, and causes nutrient imbalance,which is, in fact, one of the major consequences of saline stress. Iron is a micronutrient essential for plant development, and it is required by several metalloenzymes involved in photosynthesis and respiration. Iron deficiency is associated with chlorosis and low crop productivity. The role of microbial siderophores in Fe supply to plants and the effect of plant growth-promoting rhizobacteria(PGPR) on the mitigation of saline stress in crop culture are well documented. However, the dual effect of siderophore-producing PGPR, both on salt stress and Fe limitation, is still poorly explored. This review provides a critical overview of the combined effects of Fe limitation and soil salinization as challenges to modern agriculture and intends to summarize some indirect evidence that argues in favour of siderophore-producing PGPR as biofertilization agents in salinized soils. Recent developments and future perspectives on the use of PGPR are discussed as clues to sustainable agricultural practices in the context of present and future climate change scenarios.

Mining rhizobacteria from indigenous halophytes to enhance alfalfa(Medicago sativa L.)growth and soil reclamation in saline soils of Northwest China

Enhancing the growth of alfalfa(Medicago sativa L.)through inoculation with rhizobacteria represents a sustainable strategy for reclaiming saline soils.However,the lack of suitable strains and practical application guidelines poses significant challenges to the utilization of Plant Growth-Promoting Rhizobacteria(PGPR)in saltaffected soils of Northwest China.In this study,we selected four PGPR strains derived from indigenous halophytes based on their growth-promoting characteristics.These strains underwent further selection via a petri dish assay.Subsequently,the effects of the selected PGPR strains on alfalfa growth and soil fertility were rigorously examined through pot trials.The results demonstrated that Bacillus filamentosus HL3,B.filamentosus HL6,Bacillus subtilis subsp.stercoris HG12,and Paenibacillus peoriae HG24 significantly produced indole-3-acetic acid(IAA),solubilized phosphorus,and fixed nitrogen(except for B.filamentosus HL6,which did not significantly fix nitrogen).Compared to non-inoculated plants,B.filamentosus HL6 and B.subtilis subsp.stercoris HG12 significantly enhanced seed germination,root elongation,and seedling biomass in a 150 mmol/L NaCl saline solution.In saline-alkaline soils,PGPR inoculation under brackish water irrigation did not restore alfalfa growth to the levels observed under freshwater irrigation.Principal Component Analysis(PCA)condensed ten indicators into two indices,explaining 86.85%of the variance.Using these two indices as weights,an evaluation model for the PGPR-alfalfa symbiosis indicated that B.subtilis subsp.stercoris HG12 had the most substantial effect under freshwater irrigation,while co-inoculation with B.subtilis subsp.stercoris HG12 and B.filamentosus HL6 had the most significant impact on alfalfa growth and soil improvement under brackish water irrigation.Available phosphorus was identified as the primary factor influencing alfalfa growth,contributing 82.3%to the growth variation.These findings provide suitable microbial strains for the utilization of saline-alkali land and underscore the potential of applying indigenous PGPR-alfalfa symbiotic techniques to improve soil fertility and crop yield in the arid regions of Northwest China.

Bacillus amyloliquefaciens Strain W19 can Promote Growth and Yield and Suppress Fusarium Wilt in Banana Under Greenhouse and Field Conditions

Plant growth-promoting rhizobacteria （PGPR） are considered to be the most promising agents for cash crop production via increasing crop yields and decreasing disease occurrence. The Bacillus amyloliquefaciens strain W19 can produce secondary metabolites （iturin and bacillomycin D） effectively against Fusarium oxysporum f. sp. cubense （FOC）. In this study, the ability of a bio-organic fertilizer （BIO） containing strain W19 to promote plant growth and suppress the Fusarium wilt of banana was evaluated in both pot and field experiments. The results showed that application of BIO significantly promoted the growth and fruit yield of banana while suppressing the banana Fusariurn wilt disease. To further determine the beneficial mechanisms of the strain, the colonization of green fluorescent protein-tagged strain W19 on banana roots was observed using confocal laser scanning microscopy and scanning electron microscopy. The effect of banana root exudates on the formation of biofilm of strain W19 indicated that the banana root exudates may enhance colonization. In addition, the strain W19 was able to produce indole-3-acetic acid （IAA）, a plant growth-promoting hormone. The results of these experiments revealed that the application of strain W19-enriched BIO improved the banana root colonization of strain W19 and growth of banana and suppressed the Fusarium wilt. The PGPR strain W19 can be a useful biocontrol agent for the production of banana under field conditions.

植物根际促生菌作用机制研究进展

植物根际促生菌(plant growth promoting rhizobacteria,PGPR)对促进植物生长和维持土壤健康具有重要意义,然而对其作用机制的认识仍存在不足,使其应用受限;近年来PGPR研究取得了不少进展,但尚缺乏综合解析.为此,针对PGPR,从定殖和促生机制两大方面进行国内外研究进展综述:在定殖机制方面,植物通过分泌有机物来招募特异的PGPR,随后PGPR通过趋化性和固体表面运动等机制感知根际分泌物并向根系迁移,最后通过植物、环境条件、竞争等进一步筛选定殖;在促生机制方面,PGPR通过分泌有机酸、产生螯合物、质子挤出或者酶促反应等机制促进氮、磷、铁等营养元素转化,并分泌植物激素和各种代谢产物来应对各种非生物胁迫和生物胁迫.总体而言PGPR-植物相互作用的过程多样而结果共赢.未来需要聚焦根际分泌物及PGPR代谢产物的作用方式与机理,更加关注PGPR微生物组在根际定殖、促生及缓解植物压力方面的作用,并积极推进PGPR的潜在应用,为PGPR的科学应用和农业可持续发展提供理论支撑.

Biosurfactant-assisted phytoremediation of potentially toxic elements in soil:Green technology for meeting the United Nations Sustainable Development Goals

Biosurfactants are biomolecules produced by microorganisms, low in toxicity, biodegradable, and relatively easy to synthesize using renewable waste substrates. Biosurfactants are of great importance with a wide and versatile range of applications, including the bioremediation of contaminated sites. Plants may accumulate soil potentially toxic elements(PTEs), and the accumulation efficacy may be further enhanced by the biosurfactants produced by rhizospheric microorganisms. Occasionally, the growth of bacteria slows down in adverse conditions, such as highly contaminated soils with PTEs. In this context,the plant's phytoextraction capacity could be improved by the addition of metal-tolerant bacteria that produce biosurfactants. Several sources, categories,and bioavailability of PTEs in soil are reported in this article, with the focus on the cost-effective and sustainable soil remediation technologies, where biosurfactants are used as a remediation method. How rhizobacterial biosurfactants can improve PTE recovery capabilities of plants is discussed, and the molecular mechanisms in bacterial genomes that support the production of important biosurfactants are listed. The status and cost of commercial biosurfactant production in the international market are also presented.

Phosphorus-Mobilizing Rhizobacterial Strain Bacillus cereus GS6 Improves Symbiotic Efficiency of Soybean on an Aridisol Amended with Phosphorus-Enriched Compost

Legume plants are an essential component of sustainable farming systems.Phosphorus(P) deficiency is a significant constraint for legume production, especially in nutrient-poor soils of arid and semi-arid regions.In the present study, we conducted a pot experiment to evaluate the effects of a phosphorus-mobilizing plant-growth promoting rhizobacterial strain Bacillus cereus GS6, either alone or combined with phosphate-enriched compost(PEC) on the symbiotic(nodulation-N_2 fixation) performance of soybean(Glycine max(L.) Merr.) on an Aridisol.The PEC was produced by composting food waste with addition of single super phosphate.The bacterial strain B.cereus GS6 showed considerable potential for P solubilization and mobilization by releasing carboxylates in insoluble P(rock phosphate)-enriched medium.Inoculation of B.cereus GS6 in combination with PEC application significantly improved nodulation and nodule N_2 fixation efficiency.Compared to the control(without B.cereus GS6 and PEC), the combined application of B.cereus GS6 with PEC resulted in significantly higher accumulation of nitrogen(N), P, and potassium(K) in grain, shoot, and nodule.The N:P and P:K ratios in nodules were significantly altered by the application of PEC and B.cereus GS6, which reflected the important roles of P and K in symbiotic performance of soybean.The combined application of PEC and B.cereus GS6 also significantly increased the soil dehydrogenase and phosphomonoesterase activities, as well as the soil available N, P, and K contents.Significant positive relationships were found between soil organic carbon(C) content, dehydrogenase and phosphomonoesterase activities, and available N, P, and K contents.This study suggests that inoculation of P-mobilizing rhizobacteria, such as B.cereus GS6, in combination with PEC application might enhance legume productivity by improving nodulation and nodule N_2 fixation efficiency.

Extracts from Marine Macroalgae and Opuntia ficus-indica Cladodes Enhance Halotolerance and Enzymatic Potential of Diazotrophic Rhizobacteria and Their Impact on Wheat Germination Under Salt Stress

Soil salinity, which affects more than 6% of the earth’s land surface and more than 20% of its irrigated areas, is a major threat to agriculture. Diazotrophic bacteria are among the functional groups of soil microbiota that are threatened by this abiotic stress, as their activity is mostly inhibited by salt stress. Seventy bacterial strains with distinct characteristics were isolated from soils by using N-free Jensen’s selective medium. Based on their ability to produce metabolites of agricultural interest, four strains were selected and identified as Flavobacterium johnsoniae, Pseudomonas putida, Achromobacter xylosoxidans, and Azotobacter chroococcum. The selected strains were grown at different NaCl concentrations (0–600 mmol L^(-1) in N-free broth and 0–2 000 mmol L^(-1) in Luria-Bertani medium) in the presence and absence of glycine betaine (GB), aqueous and hydro-alcoholic extracts from marine macroalgae, Ulva lactuca and Enteromorpha intestinalis, and Opuntia ficus-indica cladodes. The selected bacterial strains, GB, and the aforementioned extracts were tested for their ability to promote the germination of wheat (Triticum durum) seeds at 0–300 mmol L^(-1) NaCl. Compared with the results obtained with the synthetic osmoprotectant GB, the extracts from O. ficus-indica, U. lactuca, and E. intestinalis significantly promoted bacterial growth and seed germination under salt stress.