Pseudomonas brassicacearum strain Am3 containing 1-aminocyclopropane-1-carboxylate deaminase can show both pathogenic and growth-promoting properties in its interaction with tomato

The role of bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity in the interaction between tomato (Lycopersicon esculentum=Solanum lycopersicum) and Pseudomonas brassicacearum was studied in different strains. The phytopathogenic strain 520-1 possesses ACC deaminase activity, an important trait of plant growth-promoting rhizobacteria (PGPR) that stimulates root growth. The ACC-utilizing PGPR strain Am3 increased in vitro root elongation and root biomass of soil-grown tomato cv. Ailsa Craig at low bacterial concentrations (10(6) cells ml-1 in vitro and 10(6) cells g-1 soil) but had negative effects on in vitro root elongation at higher bacterial concentrations. A mutant strain of Am3 (designated T8-1) that was engineered to be ACC deaminase deficient failed to promote tomato root growth in vitro and in soil. Although strains T8-1 and 520-1 inhibited root growth in vitro at higher bacterial concentrations (>10(6) cells ml-1), they did not cause disease symptoms in vitro after seed inoculation, or in soil supplemented with bacteria. All the P. brassicacearum strains studied caused pith necrosis when stems or fruits were inoculated with a bacterial suspension, as did the causal organism of this disease (P. corrugata 176), but the non-pathogenic strain Pseudomonas sp. Dp2 did not. Strains Am3 and T8-1 were marked with antibiotic resistance and fluorescence to show that bacteria introduced to the nutrient solution or on seeds in vitro, or in soil were capable of colonizing the root surface, but were not detected inside root tissues. Both strains showed similar colonization ability either on root surfaces or in wounded stems. The results suggest that bacterial ACC deaminase of P. brassicacearum Am3 can promote growth in tomato by masking the phytopathogenic properties of this bacterium.     

Suppression of Fusarium Colonization of Cotton Roots and Fusarium Wilt by Seed Treatments with Gliocladium virens and Bacillus subtilis

In a growth chamber at 25 C, the fungal antagonist Gliocladium virens colonized tap roots and secondary roots of cotton in non-sterile soil after seed treatment with preparations of G. virens. Colonization of tap roots by G. virens increased over time, and decreased with root depth. Seed treatments with G. virens strains G-4 and G-6 and with Bacillus subtilis strains GB03 and GB07 reduced the colonization of tap roots and secondary roots of cotton seedlings by Fusarium spp. Under greenhouse conditions, the same seed treatments suppressed the incidence and severity of fusarium wilt of cotton in soil infested with Fusarium oxysporum f. sp. vasinfectum and Meloidogyne incognita. Gliotoxin, produced by 'Q-group' strains of G. virens, inhibited F. oxysporum f. sp. vasinfectum in vitro. The seed treatments with G. virens strain G-6 and B. subtilis strains GB03 and GB07 did not affect the reproduction of root-knot nematodes. The results of this study may help to explain why the treatment of cotton seed with biocontrol agents often results in more vigorous and higher yielding plants, and indicate that there is potential for using G. virens and B. subtilis as seed treatments to control fusarium wilt of cotton.     

Root colonization of maize and lettuce by bioluminescent Rhizobium leguminosarum biovar phaseoli.

Two strains of Rhizobium leguminosarum bv. phaseoli and three other plant growth-promoting rhizobacteria (PGPR) were examined for the potential of maize and lettuce root colonization. All of these strains were selected in vitro for their phosphate-solubilizing abilities. Maize and lettuce seeds were treated with derivatives of all strains marked with lux genes for bioluminescence and resistance to kanamycin and rifampin prior to planting in nonsterile Promix and natural soil. The introduced bacterial strains were quantified on roots by dilution plating on antibiotic media together with observation of bioluminescence. Rhizobia were superior colonizers compared with other tested bacteria; rhizobial root populations averaged log 4.1 CFU/g (fresh weight) on maize roots 4 weeks after seeding and log 3.7 CFU/g (fresh weight) on lettuce roots 5 weeks after seeding. The average populations of the recovered PGPR strains were log 3.5 and log 3.0 CFU/g (fresh weight) on maize and lettuce roots, respectively. One of the three PGPR was not recovered later than the first week after seeding in Promix. Bioluminescence also permitted visualization of in situ root colonization in rhizoboxes and demonstrated the efficiency of rhizobial strains to colonize and survive on maize and lettuce roots.     

A new bio-formulation containing plant growth promoting rhizobacterial mixture for the management of sheath blight and enhanced grain yield in rice

Three plant growth promoting rhizobacterial (PGPR) strains, PF1,FP7 and PB2, were tested alone and in combinations for suppression ofrice sheath blight disease and promotion of plant growth underglasshouse and field conditions. The mixture of PGPR strainssignificantly reduced the sheath blight incidence when applied as eitherbacterial suspension through seed, root, foliar and soil application inglasshouse conditions, or as talc-based formulation under fieldconditions, compared to the respective individual strains. The averagemean of disease reduction was 29.2% for single strains and45.1% for mixtures. In addition to disease suppression, treatmentwith mixture of PGPR strains promoted plant growth in terms of increasedplant height and number of tillers, and ultimately grain yield. Theaverage increases in yield for single strains were 17.7%, and25.9% in case of mixture. Mixture of three PGPR strains reduceddisease and promoted growth to a level equivalent to two strainmixtures. Though seed treatment of either single strain or strainmixtures alone could reduce the disease, subsequent application to root,leaves or soil further reduced the disease and enhanced the plantgrowth. The mixture consisting of PF1 plus FP7 was the most effective inreducing the disease and in promoting plant growth and grainyield.     

Development of Multi-Component Transplant Mixes for Suppression of Meloidogyne incognita on Tomato (Lycopersicon esculentum)

The effects of combinations of organic amendments, phytochemicals, and plant-growth promoting rhizobacteria on tomato (Lycopersicon esculentum) germination, transplant growth, and infectivity of Meloidogyne incognita were evaluated. Two phytochemicals (citral and benzaldehyde), three organic amendments (pine bark, chitin, and hemicellulose), and three bacteria (Serratia marcescens, Brevibacterium iodinum, and Pseudomonas fluorescens) were assessed. Increasing rates of benzaldehyde and citral reduced nematode egg viability in vitro. Benzaldehyde was 100% efficacious as a nematicide against juveniles, whereas citral reduced juvenile viability to less than 20% at all rates tested. Benzaldehyde increased tomato seed germination and root weight, whereas citral decreased both. High rates of pine bark or chitin reduced plant growth but not seed germination, whereas low rates of chitin increased shoot length, shoot weight, and root weight; improved root condition; and reduced galling. The combination of chitin and benzaldehyde significantly improved tomato transplant growth and reduced galling. While each of the bacterial isolates contributed to increased plant growth in combination treatments, only Brevibacterium iodinum applied alone significantly improved plant growth.     

LuxAB标记的N2106-L在小麦根圈的定植动态

目的研究小麦PGPR（植物根际促生菌）菌株的个体生态学及其在小麦根圈的定植动态。方法采用三亲本杂交法将发光酶基因luxAB转人具有固氮能力的小麦根际促生菌Azotobacter N2106菌株中,获得标记菌株N2106-L,将标记菌株接种到灭菌和非灭菌的黄褐土、红壤和黄潮土中研究其存活状况,采用根盒试验追踪标记菌株在小麦根圈的定植动态。结果标记菌株N2106-L具有发光活性和对km、str、tet三种抗生素的抗性,且具有较好的遗传稳定性。N2106-L在灭菌土壤中的数量稍高于非灭菌土壤;在3种土壤中的数量依次为：黄褐土〉黄潮土〉红壤。N2106-L在小麦根表定植密度大于根际定植密度;在小麦根际,小麦播种10d时标记菌株在0-2cm深度根际土壤定植达到最大值（2．17±0．25）×10^6CFU／g土,20d时在2-4cm深度的根际土壤中达到最高定植水平（3．92±0．47）×10^5CFU／g土;在小麦根表,标记菌株在小麦播种10d时在所有深度的根段均达到最高定植水平,0-2cm根段定植密度为（3．60±0．60）×10^6CFU／g鲜根,12cm以下根段达到（2．78±0．56）×10^4CFU／g鲜根。结论标记菌株随着根的伸长不断向根尖方向扩散,且较为稳定地在小麦根圈定植,研究结果为小麦PGPR菌株的应用提供了可靠实验数据。     

Disease suppression and crop improvement through fluorescent pseudomonads isolated from cultivated soils

Three fluorescent pseudomonads isolated from rhizosphere/rhizoplane of crop plants showed in vitro antibiosis against seven fungal and two bacterial plant pathogens on iron-deficient KB medium. Seed bacterization of chick- pea (Cicer arientinum L.), egg plant (Solanum melongena L.), soybean (Glycine max Merr.) and tomato (Lycopersicon esculentum Mill.) with these organisms showed an increased seed germination, shoot height, root length, fresh weight, dry weight and yield. Seed bacterization with one of these strains, RB 8, reduced the number of chick-pea wilted plants in wilt-sick (Fusarium oxysporum f.sp. ciceris) soil. Addition of iron into the soil eliminated the disease suppression. The disease suppression and/or growth enhancement along with the positive root colonization by these organisms indicate their possible use as plant growth-promoting rhizobacteria (PGPR)/biocontrol agents against chick-pea wilt.     

影响引人微生物根部定殖的因素

从外界引入的各类有益微生物如生防菌(BCA)和根际促生菌或增产菌(PGPR,YIB)到种子表面随其生根发芽而蔓延或直接到根表沿根分布定殖．外来微生物在根际定殖的过程为与根尖接触,沿根分布,最后在根际建立自己的种群．定殖的位点以PGPR为例,是表皮细胞间隙,或侧根、根毛基部．外来微生物在根际定殖动态变化的原因,由于根际生物的和非生物的因素引起的．生物因子除去外来微生物本身的生理特性,还有根际土著微生物与外来微生物的相互作用,更重要的是植物基因型对微生物定殖的影响．非生物因子包括土壤环境、土壤结构和含水量,土壤温度和土壤pH值均能影响外来微生物在根部的定殖．     

Colonization of barley (Hordeum vulgare) with Salmonella enterica and Listeria spp

Colonization of barley plants by the food-borne pathogens Salmonella enterica serovar typhimurium and three Listeria spp. (L. monocytogenes, L. ivanovii, L. innocua) was investigated in a monoxenic system. Herbaspirillum sp. N3 was used as a positive control and Escherichia coli HB101 as a negative control for endophytic root colonization. Colonization of the plants was tested 1-4 weeks after inoculation by determination of CFU, specific PCR assays and fluorescence in situ hybridization (FISH) with fluorescently labelled oligonucleotide probes in combination with confocal laser scanning microscopy (CLSM). Both S. enterica strains were found as endophytic colonizers of barley roots and reached up to 2.3 x 10(6) CFU per g root fresh weight after surface sterilization. The three Listeria strains had 10-fold fewer cell numbers after surface sterilization on the roots and therefore were similar to the results of nonendophytic colonizers, such as E. coli HB101. The FISH/CSLM approach demonstrated not only high-density colonization of the root hairs and the root surface by S. enterica but also a spreading to subjacent rhizodermis layers and the inner root cortex. By contrast, the inoculated Listeria spp. colonized the root hair zone but did not colonize other parts of the root surface. Endophytic colonization of Listeria spp. was not observed. Finally, a systemic spreading of S. enterica to the plant shoot (stems and leaves) was demonstrated using a specific PCR analysis and plate count technique.     

Effectiveness of various Pseudomonas spp. and Burkholderia caryophylli containing ACC-deaminase for improving growth and yield of wheat (Triticum aestivum L.)

This study assessed the possible role of different traits in selected plant growth-promoting rhizobacteria (PGPR) for improving wheat growth and yield under natural conditions. Rhizobacteria exhibiting 1-aminocyclopropane-1-carboxylate (ACC)-deaminase activity were isolated and screened for their growth-promoting activity in wheat under axenic conditions. Five isolates belonging to Pseudomonas and one Burkholderia caryophylli isolate that showed promising performances under axenic conditions were selected and characterized for in vitro ACC-deaminase activity, chitinase activity, auxin production, P solubilization, and root colonization. These isolates were then used as inocula for wheat cultivated under natural conditions in pot and/or field trials. Significant increases in root elongation, root weight, tillers per pot, 1,000-grain weight, and grain and straw yields were observed in response to inoculation with PGPR in the pot trials. Inoculation with these PGPR was also effective under field conditions and increased the wheat growth and yield significantly. However, the efficacy of the strains was inconsistent under the axenic, pot, and field conditions. Pseudomonasfluorescens (ACC50), which exhibited a relatively high in vitro ACC-deaminase activity, chitinase activity, auxin production, and P solubilization and more intensive root colonization, was the most efficient isolate under the field conditions. Therefore, these results demonstrated that ACC-deaminase activity is an efficient parameter for the selection of promising PGPR under axenic conditions. However, additional traits of PGPR, including auxin production, chitinase activity, P solubilization, and root colonization, are also important for selecting PGPR as biofertilizers.     

Seed Treatment of Barley with Idriella bolleyi causes Systemically Enhanced Defence against Root and Leaf Infection by Bipolaris sorokiniana

Inoculation of barley seed with the saprophytic fungus Idriella bolleyi caused systemically improved resistance on both leaves and roots of young plants to subsequent infection with the necrotrophic fungus Bipolaris sorokiniana . Lesion sizes were reduced by up to 50% as a result of the seed treatment. Seed and root inoculation with I. bolleyi induced biochemical defence responses in the plants as shown by slight accumulation of pathogenesis-related (PR) proteins, but the accumulation was not as great as when the roots were inoculated with B. sorokinina . Conidia of I. bolleyi applied to seed survived well during storage, with no significant reduction in colony forming units (CFUs) occurring at 15°C after 2 months. However, storage at 28°C decreased the viability of the applied conidia. The fungus colonized the seed and roots during field conditions as indicated by high numbers of CFUs. Two months after sowing, frequencies of I. bolleyi were higher on plants treated with the fungus than on control plants where colonization occurred naturally from field soil. The implications of the results are discussed in relation to earlier reports of biological control of cereal root diseases by I. bolleyi and induced resistance.     

Screening plant growth-promoting rhizobacteria for improving growth and yield of wheat

AIMS: Plant growth promoting rhizobacteria (PGPR) are commonly used as inoculants for improving the growth and yield of agricultural crops, however screening for the selection of effective PGPR strains is very critical. This study focuses on the screening of effective PGPR strains on the basis of their potential for in vitro auxin production and plant growth promoting activity under gnotobiotic conditions. METHODS AND RESULTS: A large number of bacteria were isolated from the rhizosphere soil of wheat plants grown at different sites. Thirty isolates showing prolific growth on agar medium were selected and evaluated for their potential to produce auxins in vitro. Colorimetric analysis showed variable amount of auxins (ranging from 1.1 to 12.1 mg l-1) produced by the rhizobacteria in vitro and amendment of the culture media with l-tryptophan (l-TRP), further stimulated auxin biosynthesis (ranging from 1.8 to 24.8 mg l-1). HPLC analysis confirmed the presence of indole acetic acid (IAA) and indole acetamide (IAM) as the major auxins in the culture filtrates of these rhizobacteria. A series of laboratory experiments conducted on two cv. of wheat under gnotobiotic (axenic) conditions demonstrated increases in root elongation (up to 17.3%), root dry weight (up to 13.5%), shoot elongation (up to 37.7%) and shoot dry weight (up to 36.3%) of inoculated wheat seedlings. Linear positive correlation (r = 0.99) between in vitro auxin production and increase in growth parameters of inoculated seeds was found. Based upon auxin biosynthesis and growth-promoting activity, four isolates were selected and designated as plant growth-promoting rhizobacteria (PGPR). Auxin biosynthesis in sterilized vs nonsterilized soil inoculated with selected PGPR was also monitored that revealed superiority of the selected PGPR over indigenous microflora. Peat-based seed inoculation with selected PGPR isolates exhibited stimulatory effects on grain yields of tested wheat cv. in pot (up to 14.7% increase over control) and field experiments (up to 27.5% increase over control); however, the response varied with cv. and PGPR strains. CONCLUSIONS: It was concluded that the strain, which produced the highest amount of auxins in nonsterilized soil, also caused maximum increase in growth and yield of both the wheat cv. SIGNIFICANCE AND IMPACT OF STUDY: This study suggested that potential for auxin biosynthesis by rhizobacteria could be used as a tool for the screening of effective PGPR strains.     

Application of plant growth-promoting rhizobacteria to control Papaya ringspot virus and Tomato chlorotic spot virus

Papaya ringspot virus (PRSV-W) and Tomato chlorotic spot virus (TCSV) are responsible for severe losses in cucurbits and tomato production in south Florida and other regions in the USA. Traditional chemicals are not effective to control these viruses. Using plant growth-promoting rhizobacteria (PGPR) may present an alternative to control these viruses. Results from this study demonstrated that applying mixtures of PGPR strains is more efficient to control PRSV-W and TCSV compared to individual PGPR strain only. The application method significantly affected the efficiency of PGPR to control PRSV-W and TCSV. The highest reduction in disease severity of both PRSV-W and TCSV occurred in case of soil drenching with PGPR, followed by root dipping and seed coating treatments. Application of PGPR mixtures of (IN937a & SE34) or (IN937a &, SE34 & T4) were the most efficient methods to control these viral diseases.     

Survival and colonization of rhizobacteria in a tomato transplant system

Plant-growth-promoting rhizobacteria (PGPR) are used on crops most often as seed treatments; however, an alternative application method for transplanted vegetables is mixing PGPR into the soilless medium in which the transplants are grown. Studies were undertaken to compare root colonization and persistence of rifampicin-resistant mutants of PGPR strains Bacillus pumilus SE34 and Pseudomonas fluorescens 89B61, SE34r and 89B61r, on tomato as a function of application method. When the bacteria were incorporated into Promix soilless medium at log 6, 7, and 8 colony- forming units/g, populations of strain SE34r per gram of medium maintained the initial inoculum densities, while populations of 89B61r decreased approximately one to two orders of magnitude by 4 weeks after planting. The populations of each PGPR strain colonizing roots after application into the soilless medium showed a similar pattern at 6 weeks as that at 4 weeks after planting, with higher populations on the whole roots and lateral roots than on the taproots. Strain SE34r but not 89B61r moved upwards and colonized the phyllosphere when incorporated into the soilless medium. Following application as seed treatment, populations of SE34r were significantly higher on upper roots and on the taproot than were populations following application through the soilless medium. Conversely, populations were higher on lower roots and lateral roots following application through the soilless medium than were populations following application as seed treatment. While strain SE34 enhanced plant growth with application both to the medium and as seed treatment, the level of growth promotion was significantly greater with application in the soilless medium. The results indicate that PGPR can be successfully incorporated into soilless media in vegetable transplant production systems.     

The effects of traits of Bacillus megaterium onseed and root colonization and their correlation withthe suppression of Rhizoctonia root rot of soybean

Bacillus megaterium strainB153-2-2 is a potential bacterial biocontrol agentagainst Rhizoctonia solani isolate 2B12(ISG-2B). To study the role of antagonism (Ant),chemotaxis (Che), motility (Mot), and sporulation(Spo) of the biocontrol agent during seed and rootcolonization and the correlation between rootcolonization and the suppression of soybean (Glycine max) root rot caused by R. solani,strain B153-2-2(Che⁺Mot⁺Ant⁺⁺Spo⁺⁺) and the sevenderived mutants with altered antagonism, chemotaxis,motility, and/or sporulation were used. The bacterialcells were introduced into soil separately either asa soybean seed coating or soil application. Two soilmixtures defined as coarse and fine soil were used. The bacterial cell chemotactic response to soybeanroot and seed exudates and antagonism to R.solani were significantly (p = 0.05) correlatedwith root and seed colonization in some but not alltreatments. The sporulation-defective mutants had lowcell populations immediately after application and,therefore, reduced root colonization. The differencesin root colonization diminished among the mutants andstrain B153-2-2 when R. solani was present inthe soil or, as seedlings grew older. Soybean seedlingroots grown in coarse soil had significantly greatercolonization by B153-2-2 or its mutants and a lowerdisease index than that in fine soil. There was asignificant positive correlation (r ² = 0.78)between root colonization by strain B153-2-2 or itsmutants and suppression of Rhizoctonia root rot.     

The biofertilising effect of seed dressing with PGPR Bacillus amyloliquefaciens FZB 42 combined with two levels of mineral fertilising in African cotton production

With the help of a field experiment, the influence of PGPR FZB42 (Bacillus amyloliquefaciens) seed dressing combined with a low (39 kg N/ha) and usual (79 kg N/ha) mineral nitrogen plant fertilisation on the cotton growth and yield was tested. The plant growth promoting rhizobacteria (PGPR) and the N-fertilisation alone promoted the plant growth, by the PGPR similar to the low N-fertilising. The cotton yield showed no significant difference between both treatments. The combined use of PGPR with different nitrogen plant fertilisation techniques resulted in the promotion of plant growth significantly compared to the mineral fertilising alone but differed significantly depending on the applied N-amount. The cotton yield in the combined application of PGPR and low N-amount increased by 75%. In combination with high N-fertilisation the yield increased by 30% compared to each of the N-fertilisation techniques. The effect was found to be based mainly on the PGPR growth promotion activity, resulting in larger plant root and stem–leaf system, increasing the possibilities to uptake available mineral nutrients. The great ecological value of the observed effectiveness improving influence by PGPR on the mineral plant fertilisation is discussed.     

Application of plant growth-promoting rhizobacteria to soybean (Glycine max [L.] Merr.) increases protein and dry matter yield under short-season conditions

We previously reported that application of plant growth-promoting rhizobacteria (PGPR) increased soybean growth and development and, specifically, increased nodulation and nitrogen fixation over a range of root zone temperatures (RZTs) in controlled environment studies. In order to expand on the previous studies, field experiments were conducted on two adjacent sites, one fumigated with methyl bromide and one nonfumigated, in 1994. Two experiments were conducted at each site, one involving combinations of two soybean cultivars and two PGPR strains, the other involving the same factors, but also in combination with two strains Bradyrhizobium japonicum. Soybean grain yield and protein yield were measured. The results of these experiments indicated that co-inoculation of soybean with B. japonicum and Serratia liquefaciens 2-68 or Serratia proteamaculans 1-102 increased soybean grain yield, protein yield, and total plant protein production, compared to the nontreated controls, in an area with low spring soil temperatures. Interactions existed between PGPR application and soybean cultivar, suggesting that PGPRs applied to cultivars with higher yield potentials were more effective. PGPRs applied to the rhizosphere without addition of B. japonicum also increased only leaf area and seed number at the fumigated site. Overall, inoculation of soybean plants with PGPRs in the presence of B. japonicum increased soybean grain yield, grain protein yield, and total plant protein production under short season conditions.     

Influence of liming,inoculum level and inoculum placement on root colonization of subterranean clover

Arbuscular mycorrhizal (AM) fungi differ in their response to soil pH. Thus, change in soil pH may influence the relative abundance of mycorrhizal fungi inside roots. Root colonization by two AM fungi was studied in relation to addition of lime (CaCO3), quantity of inoculum and inoculum placement. Addition of CaCO3 to an acid soil decreased the colonization of roots by Acaulospora laevis but increased colonization by Glomus invermaium when both fungi were present. In acid soil (pH 4.7), almost all roots were colonized by A. laevis, while G. invermaium was dominant when soil pH was increased to pH 7.3. This occurred regardless of whether the inoculum was banded or mixed throughout the soil. There was no effect of CaCO3 on the relative abundance of fungi inside roots at intermediate rates of CaCO3 application (pH 5.3-6.3) when both fungi were inoculated together. In this experiment, both fungi colonized roots at all levels of CaCO3 when inoculated alone, except for A. laevis at the highest level of CaCO3. We conclude that soil pH affects the competitive ability of these two AM fungi during mycorrhiza formation primarily by affecting hyphae growth in soil and thus the relative abundance of hyphae at the root surface and subsequently inside the root.