Ecology, physiology, and phylogeny of deep subsurface Sphingomonas sp

Several new species of the genus Sphingomonas including S. aromaticivorans, S. stygia, and S. subterranea that have the capacity for degrading a broad range of aromatic compounds including toluene, naphthalene, xylenes, p-cresol, fluorene, biphenyl, and dibenzothiophene, were isolated from deeply-buried (>200 m) sediments of the US Atlantic coastal plain (ACP). In S. aromaticivorans F199, many of the genes involved in the catabolism of these aromatic compounds are encoded on a 184-kb conjugative plasmid; some of the genes involved in aromatic catabolism are plasmid-encoded in the other strains as well. Members of the genus Sphingomonas were common among aerobic heterotrophic bacteria cultured from ACP sediments and have been detected in deep subsurface environments elsewhere. The major source of organic carbon for heterotrophic metabolism in ACP deep aquifers is lignite that originated from plant material buried with the sediments. We speculate that the ability of the subsurface Sphingomonas strains to degrade a wide array of aromatic compounds represents an adaptation for utilization of sedimentary lignite. These and related subsurface Sphingomonas spp may play an important role in the transformation of sedimentary organic carbon in the aerobic and microaerobic regions of the deep aquifers of the ACP. Received 12 May 1999/ Accepted in revised form 25 July 1999     

Culture Independent Detection of Sphingomonas sp. EPA 505 Related Strains in Soils Contaminated with Polycyclic Aromatic Hydrocarbons (PAHs)

The Sphingomonas genus hosts many interesting pollutant-degrading strains. Sphingomonas sp. EPA505 is the best studied polycyclic aromatic hydrocarbon (PAH)-degrading Sphingomonas strain. Based on 16S rRNA gene sequence analysis, Sphingomonas sp. strain EPA505 forms a separate branch in the Sphingomonas phylogenetic tree grouping exclusively PAH-degrading isolates. For specific PCR detection and monitoring of Sphingomonas sp. EPA505 and related strains in PAH-contaminated soils, a new 16S rRNA gene-based primer set was designed. The new primer set was shown to be highly selective for Sphingomonas sp. strain EPA505 as it only amplified DNA from strain EPA505 and not from other tested Sphingomonas strains or soil bacteria not belonging to the Sphingomonas genus. Using DNA extracts of a variety of inoculated PAH-contaminated soils, the primer pair was able to detect EPA505 in concentrations as low as 10² cells per gram of soil. Applying the new primer set, 16S rRNA gene fragments which were 99–100% similar to the corresponding gene of strain EPA505 were amplified from four of five PAH-contaminated soils. On the other hand, no PCR products were obtained from any of five tested uncontaminated soils. The preferential presence of EPA505 related Sphingomonas strains in PAH-contaminated soils with very different contamination profiles and different origin suggests an important role of this type of Sphingomonas in the natural Sphingomonas community colonizing PAH-contaminated sites.     

Phylogeny of Sphingomonas species that degrade pentachlorophenol

Four pentachlorophenol (PCP)-degrading bacteria isolated from geographically diverse areas have been examined in detail as regards their physiology and phylogeny. According to traditional biochemical methods, these strains had been classified as members of the genera Arthrobacter, Flavobacterium, Pseudomonas, and Sphingomonas. The PCP degradation pathway has been studied extensively in Sphingomonas (Flavobacterium) sp strain ATCC 39723 and the first three degradation steps catalyzed by a PCP-4-monooxygenase (PcpB) and a reductive dehalogenase (PcpC) that functions twice are well established. A fourth step appears to involve ring-fission of the aromatic nucleus (PcpA). Molecular analyses revealed that the PCP degradation pathway in these four strains was rather conserved, leading to a phylogenetic analysis using 16S rDNA. The results revealed a much closer phylogenetic relationship between these organisms than traditional classification indicated, placing them into the more recently established genus Sphingomonas where they may even represent a single species. With 16S rDNA analysis, many bacterial isolates involved in degradation of xenobiotic compounds that were previously classified into diverse genera have been reclassified into the genus Sphingomonas. Received 14 April 1999/ Accepted in revised form 20 July 1999     

Degradation of polyvinyl alcohol by <Emphasis Type="Italic">Sphingomonas</Emphasis> sp. SA3 and its symbiote

A total of 800 samples was taken from Taegu province, Korea, where many textile factories provide a source of polyvinyl alcohol (PVA) waste. These samples were screened for PVA-degrading bacteria. A new strain, SA3, was discovered which formed yellow colonies and used PVA as the sole carbon and energy source. Strain SA3 was identified as a Sphingomonas sp., based on the partial nucleotide sequence analysis of 16S ribosomal RNA, the presence of 2-hydroxymyristic acid (14:O 2-OH) and sphingolipids with d-17:0, d-18:0, d-19:1, and d-20:1 as the main dihydrosphingosines. This genus has not previously been reported as a PVA-degrading bacterium. Sphingomonas sp. SA3 needs a symbiote strain, SA2, for PVA degradation as a growth factor producer. In mixed cultures of these strains, the optimum temperature for PVA biodegradation ranged from 30 °C to 35 °C. The optimum pH was 8.0 and the most effective nitrogen source was NH₄ ⁺. Electronic Publication     

Identity and Pathogenicity of Fungi Associated with Root and Crown Rot of Soft Red Winter Wheat Grown on the Upper Coastal Plain Land Resource Area of Mississippi

Seedling stand, disease severity and fungal incidence were determined from untreated ‘Wakefield’ soft red winter wheat planted on a Leeper silty clay loam in field tests conducted at the Mississippi Agricultural and Forestry Experiment Station, Plant Science Research Center, Mississippi State University, Starkville, Mississippi during the 1996–97 and 1997–98 growing seasons. Seedling stand was reduced by 40% each year in plots established with untreated seed. Cochliobolus sativus was the most frequently isolated fungus. Fusarium acuminatum, Fusarium equiseti and Fusarium solani were the most prevalent Fusarium spp. Seven other Fusarium spp. and 23 species of other fungal genera were isolated. Pathogenicity tests with three isolates each of C. sativus, Cochliobolus spicifer, F. acuminatum, F. solani, F. equiseti, Fusarium compactum, Embellisia chlamydospora and Microdochium bolleyi were performed in test tube culture and two isolates each of C. sativus, C. spicifer, F. acuminatum, E. chlamydospora and M. bolleyi under greenhouse conditions. In test tubes and in the greenhouse, seedlings infected with isolates of C. sativus developed seedling blight, discoloration and necrosis, primarily in seminal roots and crowns. In the greenhouse, C. sativus induced lesions on the lower leaf sheath and reduced seedling height, seedling emergence, dry and fresh weight of roots and shoots. Isolates of F. acuminatum, F. solani, F. equiseti, F. compactum, E. chlamydospora and M. bolleyi induced slight to moderate orange to light‐brown discoloration of crown and seminal roots in test tubes. Cochliobolus spicifer isolates had the most pre‐emergence activity, inducing black root discoloration and root pruning of wheat seedlings and reducing seedling emergence, root fresh weight and shoot dry weight. In the greenhouse, F. acuminatum reduced seedling height, seedling emergence and root and shoot dry weights. Microdochium bolleyi and E. chlamydospora reduced fresh and dry weight of roots, plant emergence and shoot dry weight. Fusarium acuminatum and C. spicifer reduced the growth rate of wheat seedlings. All fungi evaluated showed increased disease severity compared to the untreated control. The high frequency of isolation of C. sativus from crown and root tissues can be partially explained by the dry, warm conditions during the early stages of wheat seedling development in the Upper Coastal Plain Land Resource Area of Mississippi.     

Fusarium Head Blight Reactions and Accumulation of Deoxynivalenol (DON) and Some of its Derivatives in Kernels of Wheat, Triticale and Rye

Fifty-three commercially grown cultivars and germplasm lines of winter triticale (n = 18), wheat (n = 13), and rye (n = 5) and spring triticale (n = 8), wheat (n = 7) and rye (n = 2) were inoculated at mid anthesis with a spore suspension consisting of a mixture of Fusarium culmorum, Fusarium avenaceum and Fusarium graminearum isolates of known toxinogenic activity. Reactions to Fusarium head blight were measured as disease severity, reductions of kernel number/head, kernel weight/head and 1000 kernel weight, number of Fusarium-damaged kernels and kernel content of deoxynivalenol (DON) and its acetyl-derivatives 3-AcDON, 15-AcDON, and moniliformin. None of the cereal genotypes was completely resistant to Fusarium head blight. Wheat suffered from the largest kernel weight reductions, and accumulated the largest amounts of deoxynivalenol (up to 39.5 mg/kg) and 3AcDON (up to 6.0 mg/kg) in kernels. Deoxynivalenol was not detected in grain samples of winter rye cv. Dańkowskie Z?ote, and spring rye cv. Ludowe. 15-AcDON was only detected in genotypes of triticale, and 3AcDON only in a few genotypes of winter wheat and rye. Moniliformin was detected at low concentrations (up to 0.092 mg/kg) in kernels of some genotypes selected for the mycotoxin analysis. A moderately strong Pearson correlation was found between head blight severity parameters and the accumulation of deoxynivalenol and its derivatives in grain of the cereal genotypes studied. Fusarium head blight severity parameters were correlated with the percentage of Fusarium-damaged kernels and reductions of yield components. However, some head blight-susceptible genotypes realized their potential yields, but accumulated high levels of mycotoxins in kernels. Both Fusarium head blight resistant and susceptible genotypes of the three cereal species accumulated deoxynivalenol in kernels. This finding suggests that the system regulating deoxynivalenol accumulation may be independent of Fusarium head blight reaction.     

Fusarium Head Blight of Common Polish Winter Wheat Cultivars – Comparison of Effects of Fusarium avenaceum and Fusarium culmorum on Yield Components

The effects of Fusarium avenaceum and Fusarium culmorum on the reduction in yield components, after independent inoculation of 14 winter wheat cultivars, were investigated. Single isolates of F. avenaceum and F. culmorum were independently used in inoculations of winter wheat heads. Reductions in the following yield traits: 1000‐kernel weight (TKW), the weight (WKH) and number (NKH) of kernels per head after inoculation were analysed statistically. The results indicate differences between both pathogens in their effects on yield traits. The statistical calculations were performed using analysis of variance (a three‐factor experiment) for particular yield trait reductions and multivariate analysis of variance for the yield trait reductions jointly. Almost all of the univariate and multivariate hypotheses concerning no differences between pathogens (F. culmorum, F. avenaceum), climatic conditions (years) and cultivars as well as hypotheses concerning no interactions between factors (pathogens, years, cultivars) were rejected at least at P= 0.05 significance level. The reduction of yield traits indicated individual reactions of the tested winter wheat cultivars to different pathogens. Among the tested traits the highest influence on the rejection of the hypothesis concerning the equivalence of F. avenaceum and F. culmorum was observed for TKW and WKH. The effect of the pathogen on yield reduction was greater for F. avenaceum than for F. culmorum during 1996 and 1997. A comparison of the cultivars indicated that the Begra cultivar showed the highest tolerance to inoculation with both Fusarium pathogens. Moreover, this genotype as well as several others showed lower tolerance to F. avenaceum rather than to F. culmorum, whereas Elena was the only cultivar with the opposite tendency.     

Naturally occurring phenanthrene degrading bacteria associated with seeds of various plant species

Seeds of 11 of 19 plant species tested yielded naturally occurring phenanthrene degrading bacteria when placed on phenanthrene impression plates. Seed associated phenanthrene degrading bacteria were mostly detected on caragana, Canada thistle, creeping red fescue, western wheatgrass, and tall wheat grass. Based on 16S rRNA analysis the most common bacteria isolated from these seeds were strains belonging to the genera Enterobacteria, Erwinia, Burkholderia, Pantoea, Pseudomonas, and Sphingomonas. These plants may provide an excellent source of pre-adapted bacterial-plant associations highly suitable for use in remediation of contaminated soil environments.     

Management of soil microbial communities to enhance populations of Fusarium graminearum-antagonists in soil

Fusarium head blight (FHB), incited by Fusarium graminearum Schwabe is one of the most devastating diseases of wheat. Primary inoculum generated on crop residue is the driving force of FHB epidemics. Fusarium survival on crop residues is affected by soil microbial antagonists. The incorporation of green manures has been shown to increase the density and diversity of microbes in soils, particularly the density and the pathogen-inhibitory activity of specific bacteria and fungi. Evidence of increased streptomycete populations in soil as a response to green manure incorporation, and their negative effect on the survival of Fusarium oxysporum Schlechtendahl in soil, suggests their potential use to reduce the survival of related pathogens. There is, however, no precedent for the use of green manures to promote indigenous streptomycete populations to control FHB. This study investigated the use of green manures (sorghum–sudangrass hybrid [Sorghum bicolor (L.) Moench–S. bicolor (L.) Moench var. sudanense (Piper)] and common buckwheat [Fagopyrum esculentum (Moench)]) for reducing F. graminearum survival in association with wheat residues. Soil bacterial density, streptomycete density and the density and inhibitory activity of F. graminearum-antagonists were monitored from planting until 3 and 6 months following the incorporation of green manures in greenhouse and field experiments, respectively. The decomposition of wheat residues and survival of Fusarium in residues was also assessed. The use of green manures did not statistically impact the survival of F. graminearum in wheat residue. However, green manures promoted the development of higher densities and antagonistic abilities of F. graminearum-antagonists in soils. Additionally, streptomycete densities and F. graminearum-antagonist densities were significantly and positively correlated with reduced survival of Fusarium. The results of our study suggest that the use of green manures can enhance populations of indigenous soil microorganisms antagonistic to the survival of F. graminearum in wheat residue.     

A new concept for reduction of diffuse contamination by simultaneous application of pesticide and pesticide-degrading microorganisms

Pesticide residues and their transformation products are frequently found in groundwater and surface waters. This study examined whether adding pesticide-degrading microorganisms simultaneously with the pesticide at application could significantly reduce diffuse contamination from pesticide use. Degradation of the phenoxyacetic acid herbicides MCPA (4-chloro-2-methylphenoxyacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) was studied in soil microcosm experiments after simultaneous spraying of herbicide and herbicide-degrading bacteria on an agricultural soil and on a sand with low degradation potential. The latter represented pesticide use on non-agricultural soils poor in microbial activity. Degradation and possible loss of herbicidal effect were also tested in a system with plants and the amounts of bacteria needed to give satisfactory MCPA-degradation rate and the survival of degrading bacteria in formulated MCPA were determined. The results showed >80–99% degradation of 2,4-D and MCPA in soil within 1 day and >99% within 3 days after inoculation with 10⁵–10⁷ herbicide-degrading bacteria g⁻¹ dry weight of soil. Enhanced degradation of MCPA was also obtained in the presence of winter wheat and white mustard without loss of the intended herbicidal effect on white mustard. The survival of an isolated MCPA-degrading Sphingomonas sp. in three realistic concentrations of formulated MCPA was very poor, showing that in practical applications direct contact between the microorganisms and the pesticide formulation must be precluded. The applicability and economic feasibility of the method and the information needed to obtain a useable product for field use are discussed.     

Chemical structure and function of glycosphingolipids of Sphingomonas spp and their distribution among members of the α-4 subclass of Proteobacteria

Sphingomonas spp are phylogenetically placed in the α-4 subclass of Proteobacteria. They have glycosphingolipids (GSL) in their membranes instead of lipopolysaccharide (LPS) as in other Gram-negative bacteria. S. paucimobilis, the type species of the genus, has GSL-1, which contains only glucuronic acid (GlcA) as a sugar moiety, and GSL-4A, which contains a tetrasaccharide including GlcA. GSL-1 and GSL-4A form the outer membrane of S. paucimobilis with outer membrane proteins and phospholipids. In the outer membrane, GSLs are assumed to locate and function as does the LPS of other Gram-negative bacteria. Sphingomonas spp closely related to the type species contain both GSL-1 and the oligosaccharide-type GSL such as GSL-4A, but other Sphingomonas spp and other genera in the α-4 subclass of Proteobacteria contain only GSL-1. Structural variations of fatty acids and dihydrosphingosines in the GSL-1 are presented. Received 19 April 1999/ Accepted in revised form 18 June 1999     

Detection of sphingomonads and in situ identification in activated sludge using 16S rRNA-targeted oligonucleotide probes

The increasing significance of members of the genus Sphingomonas in biotechnological applications has led to an increased interest in the diversity, abundance and ecophysiological potential of this group of Gram-negative bacteria. This general focus provides a challenge to improve means for identification of sphingomonads; eg molecular genetic methods for rapid and specific detection could facilitate screening of new isolates. Here, fluorescently labeled oligonucleotide probes targeted against 16S rRNA were used to typify strains previously assigned to the genus. All 46 sphingomonads tested including type strains of 21 Sphingomonasspecies could be detected with a probe originally designed for the genus and all but one with a probe designed for the alpha-4 subgroup of the Proteobacteria. The two probes are suitable for direct detection of sphingomonads in pure and mixed cultures as well as in environmental samples of unknown composition. The probes were used to identify sphingomonads in situ in activated sludge samples. Sphingomonads were rather abundant accounting for about 5–10% of the total cells in municipal sludges. Distinct patterns in aggregation of the cells suggest that these organisms could be involved in the formation process of sludge flocs. Received 27 May 1999/ Accepted in revised form 22 August 1999     

草假单胞菌HT1在蚕豆根和茎的定殖特性及对内生细菌多样性的影响

[背景] 关于蚕豆内生生防细菌的定殖规律及其对内生细菌微生物多样性的研究鲜有报道。[目的] 明确草假单胞菌HT1在蚕豆的定殖特性以及对根茎部内生细菌群落多样性的影响。[方法] 采用抗生素标记法测定菌株HT1的定殖特性,利用高通量测序技术分析该菌灌根处理与对照处理蚕豆根茎部内生细菌的群落多样性。[结果] 菌株HT1的定殖数量为根>茎>叶,呈先升后降的趋势,根部、茎部、叶部在第7天达到最大值,在第83天仍能检测到标记菌株。灌根处理降低了内生细菌的丰富度,提高了根内生细菌物种多样性;灌根处理组根部厚壁菌门、放线菌门、拟杆菌门丰度显著提高,茎部变形菌门的相对丰度有所升高但无显著影响;在属水平上,灌根处理后,根部如Romboutsia、Mitsuaria、乳杆菌属、德沃斯氏菌属等属的相对丰度明显升高,茎部假单胞菌、Muribaculaceae、Elstera、鞘氨醇单胞菌属等属的相对丰度明显升高。[结论] HT1菌株能在蚕豆植株体内定殖达83 d以上,并且可以影响蚕豆植株内部的微生物群落结构组成,提高相关微生物的相对丰度。     

On-farm experiments over 5 years in a grain maize/winter wheat rotation: effect of maize residue treatments on <Emphasis Type="Italic">Fusarium graminearum</Emphasis> infection and deoxynivalenol contamination in wheat

Over the course of 5 years, different maize residue treatments were conducted on 14 zero tillage on-farm sites in Switzerland to evaluate their effect on the development of Fusarium head blight (FHB) and the contamination with the mycotoxin deoxynivalenol (DON) in winter wheat grains and wheat straw following grain maize. Two experimental series with three and five different treatments were carried out, respectively. Fusarium graminearum (Schwabe) was the predominant FHB-causing species with an overall incidence of 15% infected wheat grains. A significant correlation between symptoms in the field, F. graminearum incidence and DON content in wheat grains and wheat straw was observed. The average DON content in both wheat grains and wheat straw was approximately 5,000 μg/kg and thus several times higher than the European maximum limit of 1,250 μg/kg for unprocessed small-grain cereals for human consumption. Of all grain samples, 74% were above the maximum limit. Pooled over both experimental series, the average reduction of DON in grains through treatments of the maize residue compared with a control treatment ranged between 21 and 38%. The effect of various other factors, including the year, the wheat variety, the site, the maize hybrid and the production system was evaluated as well. The year and the wheat variety were the most important FHB influencing factors. Over all treatments, the variety Levis showed a fivefold higher average DON content compared with the variety Titlis. From different categories of maize residue particles, intact pieces of 5–15 cm length were strongly correlated with F. graminearum incidence and DON content in grains. During the time course of this study, the recommendation from a preliminary version of the internet-based DON forecasting system FusaProg to apply or to omit a fungicide treatment was correct in 32 out of 42 cases. The results are currently being used to optimise the FusaProg models. This study has shown that in a grain maize/winter wheat rotation, the DON content in wheat grains frequently exceeded the European maximum limit, even with a thorough treatment of maize residues and less susceptible wheat varieties. Hence, in order to reduce the contamination risk in a zero tillage system, the crop rotation needs to be modified.     

Isolates of Fusarium graminearum collected 40–320?meters above ground level cause Fusarium head blight in wheat and produce trichothecene mycotoxins

The aerobiology of fungi in the genus Fusarium is poorly understood. Many species of Fusarium are important pathogens of plants and animals and some produce dangerous secondary metabolites known as mycotoxins. In 2006 and 2007, autonomous unmanned aerial vehicles (UAVs) were used to collect Fusarium 40–320 m above the ground at the Kentland Farm in Blacksburg, Virginia. Eleven single-spored isolates of Fusarium graminearum (sexual stage Gibberella zeae) collected with autonomous UAVs during fall, winter, spring, and summer months caused Fusarium head blight on a susceptible cultivar of spring wheat. Trichothecene genotypes were determined for all 11 of the isolates; nine isolates were DON/15ADON, one isolate was DON/3ADON, and one isolate was NIV. All of the isolates produced trichothecene mycotoxins in planta consistent with their trichothecene genotypes. To our knowledge, this is the first report of a NIV isolate of F. graminearum in Virginia, and DON/3ADON genotypes are rare in populations of the fungus recovered from infected wheat plants in the eastern United States. Our data are considered in the context of a new aerobiological framework based on atmospheric transport barriers, which are Lagrangian coherent structures present in the mesoscale atmospheric flow. This framework aims to improve our understanding of population shifts of F. graminearum and develop new paradigms that may link field and atmospheric populations of toxigenic Fusarium spp. in the future.     

A major QTL for resistance against Fusarium head blight in European winter wheat

We report on the verification of a resistance quantitative trait locus (QTL) on chromosome 1BL (now designated Qfhs.lfl-1BL) which had been previously identified in the winter wheat cultivar Cansas. For a more precise estimation of the QTL effect and its influence on plant height and heading date lines with a more homogeneous genetic background were created and evaluated in four environments after spray inoculation with Fusarium culmorum. Qfhs.lfl-1BL reduced FHB severity by 42% relative to lines without the resistance allele. This QTL did not influence plant height, but significantly delayed heading date by one day. All of the most resistant genotypes of the verification population carried this major QTL displaying its importance for disease resistance. This resistance QTL has not only been found in the cultivar Cansas, but also in the three European winter wheat cultivars Biscay, History and Pirat. A subsequent meta-analysis confirmed the presence of a single QTL on the long arm of chromosome 1B originating from the four mentioned cultivars. Altogether, the results of the present study indicate that Qfhs.lfl-1BL is an important component of FHB resistance in European winter wheat and support the view that this QTL would be effective and valuable in backcross breeding programmes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Rhizosphere bacteria of maize with chitinolytic activity and its potential in the control of phytopathogenic fungi

The chitinase enzyme was identified in isolated bacteria of maize rhizosphere as well as its potential for the biological control of fungi associated at seeds of the same plant. The production of chitinase enzyme was found in the genera identified as Acinetobacter, Bacterium, Burkholderia, Paenibacillus, Pseudomonas, Rhizobium, Shewanella, Sphingomonas and Stenotrophomonas. Bacterial isolates with ability to degrade fungal mycelium from maize fungi as Fusarium and Alternaria among others, were detected. Bacterial chitinase activity and the presence of the chiA gene were determined. The inoculation of chitinolytic bacteria showed a positive effect in the control of fungi in maize seeds. The results support the potential use of chitinase enzyme producing bacteria on the control of phytopathogenic fungi.     

Incidence of inhibitory pseudomonads in the Pacific Northwest

Incidence of pseudomonads inhibitory to the root growth of till and no-till seeded crops winter wheat (Triticum aestivum L.), pea (Pisum sativum), lentil (Lens culinaris), and no-till winter barley (Hordeum vulgare), top and bottom of a seeded slope, and on the weed downy brome (Bromus tectorum) was investigated. Pseudomonads on the rhizoplane of these plants ranged from 10⁶ to 10⁸ colony-forming units (cfu) per gram dry weight of root. Neither tillage management nor site on a seeded slope affected colonizing numbers. Total numbers of pseudomonads were reduced in a second sampling, particularly on winter barley roots. However, more inhibitory pseudomonads were found in the second sampling. Several of the isolates, both inhibitory and stimulatory from different host plants, were bioassayed against winter wheat seedlings. Generally, the effect was different on the winter wheat than on the host plant indicating the organisms had some specificity. Several pseudomonads were isolated that severely reduced downy brome root growth and not that of winter wheat. Contribution from Agric. Res. Serv., U S Dep. of Agric., in cooperation with the College of Agric, and Home Economics, Agric. Res. Center, Washington State Univ., Pullman, WA 99164. Scientific Paper No. 7491.     

Benzoxazinoid concentrations show correlation with Fusarium Head Blight resistance in Danish wheat varieties

Fusarium Head Blight (FHB) is a destructive disease that affects the grain yield and quality of cereals. The relationship between the natural defense chemicals benzoxazinoids and the FHB resistance of field grown winter wheat varieties was investigated. FHB resistance was assessed by the inoculation of wheat ears with mixtures of Fusarium avenaceum, Fusarium culmorum, Fusarium graminearum, and Microdochium nivale.     

Response of fluoranthene-degrading bacteria to surfactants

A prerequisite for surfactant-enhanced biodegradation is that the microorganisms survive, take up substrate and degrade it in the presence of the surfactant. Two Mycobacterium and two Sphingomonas strains, degrading fluoranthene, were investigated for their sensitivity towards non-ionic chemical surfactants. The effect of Triton X-100 and Tween 80 above their critical micelle concentration on mineralization of [¹⁴C]-glucose and [¹⁴C]-fluoranthene was measured in shaker cultures. Tween 80 had no toxic effect on any of the tested strains. The surfactant inhibited fluoranthene mineralization by the hydrophobic Mycobacterium spp. slightly, but more than doubled that by the two less hydrophobic Sphingomonas strains. Triton X-100 inhibited fluoranthene mineralization by all strains, yet this was more pronounced for the Sphingomonas spp. Both surfactants caused cell wall permeabilization, as shown by transient colouring of surfactant-containing media. Inhibition of glucose mineralization, indicating non-specific toxic effects of Triton X-100, was observed only for the Sphingomonas strains and the toxicity was caused by micelle-to-cell interactions. These strains, however, appeared to recover from initial Triton X-100 toxicity within 50–500 h of exposure. The ratio of surfactant concentration to initial cell density was found to determine critically the bacterial response to surfactants. For both Sphingomonas and Mycobacterium strains, this work indicates that fluoranthene solubilized in surfactant micelles is only partially available for mineralization by the bacteria tested. However, our results suggest that optimal conditions for polycyclic aromatic hydrocarbon mineralization can be developed by selection of the proper surfactant, bacterial strains, cell density and incubation conditions. Received: 6 February 1998 / Received revision: 19 June 1998 / Accepted: 19 June 1998