Plant growth-promoting bacteria that decrease heavy metal toxicity in plants

Kluyvera ascorbata SUD165 and a siderophore-overproducing mutant of this bacterium, K. ascorbata SUD165/26, were used to inoculate tomato, canola, and Indian mustard seeds which were then grown in soil for 25-42 days in the presence of either nickel, lead, or zinc. The parameters that were monitored included plant wet and dry weight, protein and chlorophyll content in the plant leaves, and concentration of heavy metal in the plant roots and shoots. As indicated by a decrease in the measured values of these parameters, in all instances, plant growth was inhibited by the presence of the added metal. Both bacterial strains were effective, although not always to a statistically significant extent, at relieving a portion of the growth inhibition caused by the metals. In most cases, the siderophore overproducing mutant K. ascorbata 165/26 exerted a more pronounced effect on plant growth than did the wild-type bacterium K. ascorbata SUD165. The data suggest that the ability of these bacteria to protect plants against the inhibitory effects of high concentrations of nickel, lead, and zinc is related to the bacteria providing the plants with sufficient iron.     

A Plant Growth-Promoting Bacterium That Decreases Nickel Toxicity in Seedlings

A plant growth-promoting bacterium, Kluyvera ascorbata SUD165, that contained high levels of heavy metals was isolated from soil collected near Sudbury, Ontario, Canada. The bacterium was resistant to the toxic effects of Ni²⁺, Pb²⁺, Zn²⁺, and CrO₄⁻, produced a siderophore(s), and displayed 1-aminocyclopropane-1-carboxylic acid deaminase activity. Canola seeds inoculated with this bacterium and then grown under gnotobiotic conditions in the presence of high concentrations of nickel chloride were partially protected against nickel toxicity. In addition, protection by the bacterium against nickel toxicity was evident in pot experiments with canola and tomato seeds. The presence of K. ascorbata SUD165 had no measurable influence on the amount of nickel accumulated per milligram (dry weight) of either roots or shoots of canola plants. Therefore, the bacterial plant growth-promoting effect in the presence of nickel was probably not attributable to the reduction of nickel uptake by seedlings. Rather, it may reflect the ability of the bacterium to lower the level of stress ethylene induced by the nickel.     

Colonization and persistence of a plant growth-promoting bacterium Pseudomonas fluorescens strain CS85, on roots of cotton seedlings

Pseudomonas fluorescens CS85, which was previously isolated from the rhizosphere of cotton seedlings, acts as both a plant growth-promoting bacterium and a biocontrol agent against cotton pathogens, including Rhizoctonia solani, Colletotrichum gossypii, Fusarium oxysporum f sp. vasinfectum, and Verticillium dahliae. Strain CS85 was labeled separately with luxAB and gusA. The labeled strains were stably maintained and had high levels of expression of the marker genes, luxAB and gusA, after successive transfers on nonselective medium, long-term preservation, and after recovery from soil. The labeled strains displayed similar biocontrol characteristics (e.g., antibiosis, effects of growth-promotion and disease-control) to the original strain. The labeled strains colonized all surfaces of the young plant root zones, such as roots hairs and lateral roots, although the distribution of the labeled strains on the root surfaces was not uniform. Moreover, the population densities of the labeled strains on the root surface were stably maintained at high levels during the first 2 weeks of plant growth in the native soil, so that about 10(7)-10(8) CFU/g root were detected, then decreased gradually. Nevertheless, approximately 10(6) CFU/g root of the labeled strains were observed on the root surfaces 35 d after planting.     

Burkholderia sp. SCMS54 reduces cadmium toxicity and promotes growth in tomato

Cadmium (Cd) can enter soil through the use of fertilisers, calcareous, pesticides and industrial and/or domestic effluents. Cd can leach into groundwater and be taken up by plants, potentially leading to reductions in plant growth and yield. In soil, plant roots interact with heavy metal (HM)‐tolerant microorganisms that may promote plant growth. Soil microorganisms may also be able to solubilise or mobilise soil metals, thereby acting as bioremediators. A better understanding of the interaction among plants, metals, microorganisms and soil will lead to improved plant tolerance. Two multi‐tolerant bacteria from the Burkholderia genus were isolated from Cd‐contaminated and Cd‐uncontaminated soil of a coffee plantation. In addition to its high tolerance to Cd, the strain SCMS54 produces indole‐acetic acid (IAA), solubilises inorganic phosphate and produces siderophores, demonstrating its potential to contribute to beneficial plant–microorganism interactions. When interacting with tomato plants exposed to Cd, the bacterium led to decreases in plant peroxide and chlorosis levels, promoted relative plant growth and decreased the root absorption of Cd, resulting in increased plant tolerance to this highly toxic HM. The results indicated that the inoculation of tomato plants with Burkholderia sp. SCMS54 promotes better growth in plants cultivated in the presence of Cd. This phenomenon appears to be attributed to a mechanism that decreases Cd concentrations in the roots via a beneficial interaction between the bacteria and the plant roots.     

The viable but nonculturable state of Ralstonia solanacearum may be involved in long-term survival and plant infection

The role of the dormant-like viable but nonculturable (VBNC) condition in the etiology of bacterial infection was examined using a plant system. The plant-pathogenic bacterium Ralstonia solanacearum was first shown to enter into the VBNC state both in response to cupric sulfate when in a saline solution and when placed in autoclaved soil. To determine if the VBNC condition is related to pathogenesis, the physiological status of bacteria recovered from different regions of inoculated tomato plants was determined at different stages of infection. The fraction of in planta bacteria that were VBNC increased during infection and became greater than 99% by the late stage of disease. The possibility that soil-dwelling VBNC bacteria may resuscitate and infect plants was also examined. When tomato seeds were germinated in sterile soil that contained VBNC but no detectable culturable forms of R. solanacearum cells, resuscitation was observed to occur in soil adjacent to plant roots; these resuscitated bacteria were able to infect plants. This is the first report of R. solanacearum entering the VBNC state and of resuscitation of any VBNC plant-pathogenic bacteria and provides evidence that the VBNC state may be involved in explaining the persistent nature of some infections.     

Effect of Paenibacillus alvei, strain K165, on the germination of Verticillium dahliae microsclerotia in planta

Verticillium wilt is a devastating disease of a wide range of herbaceous and woody plant hosts, incited by the soilborne fungus Verticillium dahliae. In the present study, the effect of the potential biocontrol isolate Paenibacillus alvei, strain K165, on the germination of V. dahliae microsclerotia (msc) was investigated. Strain K165 was isolated from tomato root tips and its activity against V. dahliae has been shown in glasshouse and field experiments. In the present study, the application of K165 resulted in the reduction of msc germination of V. dahliae, in the root tips and the zone of elongation, of eggplants by 50% compared to the control treatment; whereas 10 and 12 cm away from root tips and in soil without plants the percentage of msc germination was reduced by 26% and 40%, respectively. However, K165 did not significantly affect the number and length of hyphae per germinated msc. In a split-root system, K165 triggered induced systemic resistance in eggplants against V. dahliae by reducing disease severity and msc germination by 27% and 20%, respectively. In addition, K165 colonised the rhizosphere of eggplants and soil in a population density of 5 and 3 log₁₀ cfu g⁻¹, 7 dpi, respectively. This is the first report of evaluating the direct/indirect effect of a rhizospheric bacterium on msc germination in the rhizosphere of eggplants, indicating that strain K165 reduces msc germination.     

Evidence for mutualism between a plant growing in a phosphate-limited desert environment and a mineral phosphate solubilizing (MPS) rhizobacterium

Alkaline desert soils are high in insoluble calcium phosphates but deficient in soluble orthophosphate (Pi) essential for plant growth. In this extreme environment, one adaptive strategy could involve specific associations between plant roots and mineral phosphate solubilizing (MPS) bacteria. The most efficient MPS phenotype in Gram-negative bacteria results from extracellular oxidation of glucose to gluconic acid via the quinoprotein glucose dehydrogenase. A unique bacterial population isolated from the roots of Helianthus annus jaegeri growing at the edge of an alkaline dry lake in the Mojave Desert showed no MPS activity and no gluconic acid production. Addition of a concentrated solution containing material washed from the roots to these bacteria in culture resulted in production of high levels of gluconic acid. This effect was mimicked by addition of the essential glucose dehydrogenase redox cofactor 2,7,9-tricarboxyl-1H-pyrrolo[2,3]-quinoline-4,5-dione (PQQ) but the bioactive component was not PQQ. DNA hybridization data confirmed that this soil bacterium carried a gene with homology to the Escherichia coli quinoprotein glucose dehydrogenase. These data suggest that expression of the direct oxidation pathway in this bacterium may be regulated by signaling between the bacteria and the plant root. The resultant acidification of the rhizosphere may play a role in nutrient availability and/or other ecophysiological parameters essential for the survival of this desert plant.     

The Viable But Nonculturable State of Ralstonia solanacearum May Be Involved in Long-Term Survival and Plant Infection

The role of the dormant-like viable but nonculturable (VBNC) condition in the etiology of bacterial infection was examined using a plant system. The plant-pathogenic bacterium Ralstonia solanacearum was first shown to enter into the VBNC state both in response to cupric sulfate when in a saline solution and when placed in autoclaved soil. To determine if the VBNC condition is related to pathogenesis, the physiological status of bacteria recovered from different regions of inoculated tomato plants was determined at different stages of infection. The fraction of in planta bacteria that were VBNC increased during infection and became greater than 99% by the late stage of disease. The possibility that soil-dwelling VBNC bacteria may resuscitate and infect plants was also examined. When tomato seeds were germinated in sterile soil that contained VBNC but no detectable culturable forms of R. solanacearum cells, resuscitation was observed to occur in soil adjacent to plant roots; these resuscitated bacteria were able to infect plants. This is the first report of R. solanacearum entering the VBNC state and of resuscitation of any VBNC plant-pathogenic bacteria and provides evidence that the VBNC state may be involved in explaining the persistent nature of some infections.     

Utilizing pyrene-degrading endophytic bacteria to reduce the risk of plant pyrene contamination

Aims

Endophytic bacteria are ubiquitous in plants, but little information is available on the influence of endophytic bacteria on the uptake and metabolism of PAH by plants. Thus, we seek to investigate whether the colonization of a target plant by a PAH-degrading endophytic bacterium would improve the PAH metabolism of the plant and reduce the risk of plant PAH contamination.

Methods

A pyrene-degrading endophyte was isolated from PAH-contaminated plants using enrichment culture. After root inoculation with the isolated bacterium, greenhouse container experiments were conducted. Pyrene residues in soil and plant samples were analyzed by HPLC.

Results

A pyrene-degrading endophytic bacterium, Staphylococcus sp. BJ06, was isolated from Alopecurus aequalis and could degrade 56.0 % of pyrene (50 mg?·?L^?1) within 15 days. BJ06 grew and degraded pyrene efficiently under environmental conditions. The bacterium significantly promoted ryegrass growth and pyrene removal from contaminated soil in container experiments. The pyrene concentrations in ryegrass roots and shoots in endophyte-inoculated planted soil were reduced by 31.01 % and 44.22 %, respectively, compared with endophyte-free planted soil.

Conclusions

We have provided new perspectives on the regulation and control of plant uptake of organic contaminants with endophytic bacteria. The results of this study will be valuable to risk assessments of plant PAH contamination.     

华西雨屏区不同退耕模式细根(包括草根)分解过程中土壤酶动态

采用原状土芯(intact core)法, 探讨了四川洪雅柳江退耕模式——光皮桦(Betula luminifera)与扁穗牛鞭草(Hemarthria compressa)复合模式(HN)、扁穗牛鞭草草地模式(NC)、柳杉(Cryptameria fortunei)人工林模式(LS)、光皮桦人工林模式(H)细根(包括草根)分解过程中土壤酶动态。结果表明: 1) HN下的土壤脲酶、蔗糖酶、酸性磷酸酶活性较大, LS下的土壤脲酶、酸性磷酸酶活性最小, 显著低于其他模式(p < 0.05)。2) HN、NC和LS下的土壤脲酶与细根(包括草根)分解速率显著相关, HN的蔗糖酶、NC的酸性磷酸酶、LS的多酚氧化酶活性与细根(包括草根)分解速率也呈显著正相关关系(p < 0.05)。3) 除H外, 土壤脲酶活性与细根C/N、纤维素绝对含量呈显著负相关关系(p < 0.05); 除NC外, 多酚氧化酶活性与细根纤维素绝对含量呈显著负相关关系。4)土壤脲酶活性与需氧固氮细菌或与真菌数量显著相关, HN下的土壤蔗糖酶活性与细菌和纤维素分解菌数量呈正相关关系, H与NC下的土壤酸性磷酸酶还分别与细菌和纤维素分解菌数量呈正相关关系(p < 0.05)。以上结果显示: 由光皮桦与扁穗牛鞭草不同生活型植物构成的复合模式有利于土壤酶活性的提高; 土壤脲酶活性高低能够反映这几种退耕模式细根(包括草根)分解速率的快慢, 细根(包括草根)的C/N是影响土壤脲酶活性的一个重要因素; 土壤酶活性与土壤真菌、需氧固氮细菌、纤维分解菌及细菌数量有关。     

The Effect of Mode of Application of Bacillus subtilis RB14-C on its Efficacy as a Biocontrol Agent Against Rhizoctonia solani

Bacillus subtilis RB14‐C, which produces the antibiotic iturin A, was investigated for its effectiveness as a biocontrol agent against Rhizoctonia solani infecting tomato using seed coating and/or direct introduction of the bacteria to the soil. The ability of RB14‐C to colonize plant roots and produce iturin A in soil, depending on the method of bacterial application, was also determined. Seed coating and the combined treatment (soil and seed bacterization) did not protect seedlings against damping‐off caused by R. solani. By contrast, RB14 introduced only to the soil controlled the disease. The total number of RB14‐C bacteria on the roots of plants grown from coated seeds was significantly lower than on the roots of plants grown in soil mixed with the bacteria. In the combined treatment, application of B. subtilis with seeds to soil preinoculated with this bacterium, at first suppressed the population of RB14‐C in the soil. Then the colonization was generally uniform. The concentration of iturin A in non‐planted soil was highest at the beginning of the experiment (i.e. after application of the bacterial suspension) but then decreased, and was undetectable 3 days after incubation. However, after seed planting the antibiotic was produced again around young roots. Bacteria introduced to the soil as a seed coating also released the antibiotic around the seeds.     

Survival of GacS/GacA mutants of the biological control bacterium Pseudomonas aureofaciens 30-84 in the wheat rhizosphere

GacS/GacA comprises a two-component regulatory system that controls the expression of secondary metabolites required for the control of plant diseases in many pseudomonads. High mutation frequencies of gacS and gacA have been observed in liquid culture. We examined whether gacS/gacA mutants could competitively displace the wild-type populations on roots and thus pose a threat to the efficacy of biological control. The survival of a gac mutant alone and in competition with the wild type on roots was examined in the biological control strain Pseudomonas aureofaciens 30-84. In this bacterium, GacS/GacA controls the expression of phenazine antibiotics that are inhibitory to plant pathogenic fungi and enhance the competitive survival of the bacterium. Wheat seedlings were inoculated with strain 30-84, and bacteria were recovered from roots after 21 days in sterile or nonsterile soil to check for the presence of gacS or gacA mutants. Although no mutants were detected in the inoculum, gacS/gacA mutants were recovered from 29 out of 31 roots and comprised up to 36% of the total bacterial populations. Southern hybridization analysis of the recovered gacA mutants did not indicate a conserved mutational mechanism. Replacement series analysis on roots utilizing strain 30-84 and a gacA mutant (30-84.gacA) or a gacS mutant (30-84.A2) demonstrated that although the mutant population partially displaced the wild type in sterile soil, it did not do so in natural soil. In fact, in natural soil final rhizosphere populations of wild-type strain 30-84 starting from mixtures were at least 1.5 times larger than would be predicted from their inoculation ratio and generally were greater than or equal to the population of wild type alone despite lower inoculation rates. These results indicate that although gacS/gacA mutants survive in natural rhizosphere populations, they do not displace wild-type populations. Better survival of wild-type populations in mixtures with mutants suggests that mutants arising de novo or introduced within the inoculum may be beneficial for the survival of wild-type populations in the rhizosphere.     

Method for Establishing a Bacterial Inoculum on Corn Roots

Few bacteria from the corn rhizosphere grew in media with 50 μg of mancozeb per ml. A mancozeb-resistant Pseudomonas strain from the rhizosphere was serially subcultured in media containing mancozeb and spectinomycin until it was resistant to 175 μg of mancozeb and 850 μg of spectinomycin per ml. The population of the pseudomonad added to soil fell to low numbers in 6 days in unamended or glucose-amended soil, but its numbers exceeded 10⁵/g for at least 12 days if the soil was supplemented with mancozeb. The numbers of this organism remained small on corn roots derived from untreated, inoculated seeds, but the population was two or more orders of magnitude greater on roots derived from mancozeb-coated seeds. The abundance of the inoculum strain on the 3-cm portion of roots nearest the stem declined markedly after about 1 week, but applying urea to the foliage reduced or prevented the decline. The numbers of the pseudomonad on segments of roots 3- to 6- and 6- to 9-cm from the stem were higher on plants derived from the mancozeb-coated seeds. Applying spectinomycin to the foliage did not promote growth of the bacterium. This method is proposed as a means to establish an introduced bacterium on plant roots.     

Endophytic colonization of Typha australis by a plant growth-promoting bacterium Klebsiella oxytoca strain GR-3

AIMS: To isolate and characterize endophytic diazotrophic bacteria from a semi-aquatic grass (Typha australis) which grows luxuriantly with no addition of any nitrogen source. METHODS AND RESULTS: Ten endophytic diazotrophic bacteria from surface-sterilized roots and culm of T. australis were isolated and screened for plant growth-promoting activities employing standard methods. Based on the rate of nitrogenase activity, indole acetic acid (IAA) production and phosphate (P) solubilization, one root isolate namely GR-3 was found to be the most efficient one. This isolate was identified as Klebsiella oxytoca on the basis of 16S rDNA sequence analysis. Amplification of nifH by polymerase chain reaction (PCR) and detection of dinitrogenase reductase by western blot confirmed the diazotrophic nature of GR-3. It was tagged with gusA fused to a constitutive promoter and the resulting transconjugant was inoculated onto endophyte-free rice variety Malviya dhan-36 seedlings to express cross-infection ability which resulted in a significant increase in root/shoot length and chlorophyll a content. CONCLUSIONS: Roots and culm of T. australis harbour several endophytic diazotrophic bacteria. One root isolate, identified as K. oxytoca GR-3, seems to be an efficient plant growth-promoting bacterium. SIGNIFICANCE AND IMPACT OF THE STUDY: Plant growth-promoting properties of GR-3 suggest that this promising isolate merits further investigations for potential application in agriculture.     

Nutrients Can Enhance the Abundance and Expression of Alkane Hydroxylase CYP153 Gene in the Rhizosphere of Ryegrass Planted in Hydrocarbon-Polluted Soil

Plant-bacteria partnership is a promising strategy for the remediation of soil and water polluted with hydrocarbons. However, the limitation of major nutrients (N, P and K) in soil affects the survival and metabolic activity of plant associated bacteria. The objective of this study was to explore the effects of nutrients on survival and metabolic activity of an alkane degrading rhizo-bacterium. Annual ryegrass (Lolium multiflorum) was grown in diesel-contaminated soil and inoculated with an alkane degrading bacterium, Pantoea sp. strain BTRH79, in greenhouse experiments. Two levels of nutrients were applied and plant growth, hydrocarbon removal, and gene abundance and expression were determined after 100 days of sowing of ryegrass. Results obtained from these experiments showed that the bacterial inoculation improved plant growth and hydrocarbon degradation and these were further enhanced by nutrients application. Maximum plant biomass production and hydrocarbon mineralization was observed by the combined use of inoculum and higher level of nutrients. The presence of nutrients in soil enhanced the colonization and metabolic activity of the inoculated bacterium in the rhizosphere. The abundance and expression of CYP153 gene in the rhizosphere of ryegrass was found to be directly associated with the level of applied nutrients. Enhanced hydrocarbon degradation was associated with the population of the inoculum bacterium, the abundance and expression of CYP153 gene in the rhizosphere of ryegrass. It is thus concluded that the combination between vegetation, inoculation with pollutant-degrading bacteria and nutrients amendment was an efficient approach to reduce hydrocarbon contamination.     

Paenibacillus polymyxa invades plant roots and forms biofilms

Paenibacillus polymyxa is a plant growth-promoting rhizobacterium with a broad host range, but so far the use of this organism as a biocontrol agent has not been very efficient. In previous work we showed that this bacterium protects Arabidopsis thaliana against pathogens and abiotic stress (S. Timmusk and E. G. H. Wagner, Mol. Plant-Microbe Interact. 12:951-959, 1999; S. Timmusk, P. van West, N. A. R. Gow, and E. G. H. Wagner, p. 1-28, in Mechanism of action of the plant growth promoting bacterium Paenibacillus polymyxa, 2003). Here, we studied colonization of plant roots by a natural isolate of P. polymyxa which had been tagged with a plasmid-borne gfp gene. Fluorescence microscopy and electron scanning microscopy indicated that the bacteria colonized predominantly the root tip, where they formed biofilms. Accumulation of bacteria was observed in the intercellular spaces outside the vascular cylinder. Systemic spreading did not occur, as indicated by the absence of bacteria in aerial tissues. Studies were performed in both a gnotobiotic system and a soil system. The fact that similar observations were made in both systems suggests that colonization by this bacterium can be studied in a more defined system. Problems associated with green fluorescent protein tagging of natural isolates and deleterious effects of the plant growth-promoting bacteria are discussed.     

焦化厂地肤根内解芘细菌的筛选及促生潜力

从多环芳烃耐受植物根内分离具多环芳烃降解功能的内生细菌并研究其促生特性,为内生菌协同宿主植物修复多环芳烃污染土壤提供基础.以长期受多环芳烃污染的焦化厂区生长的地肤为材料,从其根内分离出以芘和1-氨基环丙烷-1-羧酸(ACC)为唯一碳源和氮源的内生细菌8株.通过芘降解试验,筛选得到3株高效芘降解内生细菌KSE4、KSE7和KSE8,经鉴定分别为芽孢杆菌属、假单胞菌属和鞘氨醇菌属.通过液体培养试验,研究了3株菌在芘胁迫下产ACC脱氨酶的能力和对地肤种子萌发的影响.结果表明: 随着芘浓度(0~15 mg·L^-1)的升高,ACC脱氨酶活性降低,其中KSE7的效果最好,在芘浓度为15 mg·L^-1时,地肤发芽率和芽长分别比对照提高了44.8%和61.1%,在地肤-微生物修复焦化厂污染土壤的修复中具有一定的应用潜力.     

Effectiveness of autochthonous bacterium and mycorrhizal fungus on Trifolium growth, symbiotic development and soil enzymatic activities in Zn contaminated soil

AIMS: This study investigates how autochthonous micro-organisms [bacterium and/or arbuscular mycorrhizal (AM) fungi] affected plant tolerance to Zn contamination. METHODS AND RESULTS: Zinc-adapted and -nonadapted Glomus mosseae strains protected the host plant against the detrimental effect of Zn (600 microg g(-1)). Zn-adapted bacteria increased root growth and N, P nutrition in plants colonized by adapted G. mosseae and decreased the specific absorption rate (SAR) of Cd, Cu, Mo or Fe in plants colonized by Zn-nonadapted G. mosseae. Symbiotic structures (nodule number and extraradical mycelium) were best developed in plants colonized by those Zn-adapted isolates that were the most effective in increasing plant Zn tolerance. The bacterium also increased the quantity and quality (metabolic characteristics) of mycorrhizal colonization, with the highest improvement for arbuscular vitality and activity. Inocula also enhanced soil enzymatic activities (dehydrogenase, beta-glucosidase and phosphatase) and indol acetic acid (IAA) accumulation, particularly in the rhizosphere of plants inoculated with Zn-adapted isolates. CONCLUSIONS: Glomus mosseae strains have a different inherent potential for improving plant growth and nutrition in Zn-contaminated soil. The bacterium increased the potential of mycorrhizal mycelium as inoculum. SIGNIFICANCE AND IMPACT OF THE STUDY: Mycorrhizal performance, particularly that of the autochthonous strain, was increased by the bacterium and both contributed to better plant growth and establishment in Zn-contaminated soils.     

Involvement of phytohormones in the development of interaction between wheat seedlings and endophytic <Emphasis Type="Italic">Bacillus subtilis</Emphasis> strain 11BM

Wheat (Triticum aestivum L., cv. Kazakhstanskaya 10) seedling growth activation by the endophytic 11BM strain of the bacterium Bacillus subtilis Cohn and the involvement of phytohormones in this process were studied. At the concentration used, bacteria enhanced wheat root and shoot growth as compared to control. Biotests demonstrated auxin-, cytokinin-, and gibberellin-like activities of the endophyte, but the result depended on the way of experiment performing. When wheat seeds were treated with the spores of B. subtilis 11BM strain, the concentrations of IAA and ABA increased transiently in the roots and shoots of the seedlings. The involvement of IAA oxidase in the plant response to inoculation with bacteria was shown; it was accompanied by a decrease in enzyme activity, which occurred later than auxin accumulation. It was concluded that observed changes in the hormonal status of wheat plants under the influence of the endophytic bacterial strain may be one of the mechanisms for seedling growth stimulation.     

土壤微生物膜生理生态功能研究进展

土壤微生物膜是由土壤细菌及其分泌物积聚形成的生物群落,是生物土壤结皮的初始形态和重要组成部分。作为土壤细菌生命过程中最典型的生存形式,土壤微生物膜不仅能保护基质内细胞生存,还可黏附于土壤颗粒和植物根系表面,发挥重要的生态功能。本文在解析土壤微生物膜结构与组成的基础上,从土壤质量与植物健康两个方面总结分析了土壤微生物膜生理生态功能:土壤微生物膜代谢活性高于游离细胞,可高效分泌胞外聚合物并且具有更强的有机物质转化速率,在提升土壤肥力,吸附、固持和降解土壤污染物和促进土壤团聚体形成方面具有重要意义;土壤微生物膜可通过多种微生物间协同作用、促进分泌多种促生物质与胞外聚合物以发挥固持作用等改善植物养分利用状况,增强植物抗逆性。揭示土壤微生物膜生态功能的微观机制、筛选和应用功能性土壤微生物膜是未来重要的发展方向。