Populations Of Fluorescent Pseudomonads Research Articles

Fluorescent pseudomonadscontribute to the enhanced growth and yield of rice cultivated under system of rice intensification (SRI)

The System of Rice Intensification (SRI) is an innovative method of rice (Oryza sativa) cultivation that combines many farm practices. Though the benefits of SRI are obvious, the underlying principles in enhanced yields are not yet scientifically analyzed. Two important components of SRI are keeping the rice field moist without flooding and frequent weeding out practices that enhance the aerobic conditions which in turn improve soil biological activity including enhanced root growth and activity of aerobic soil organisms. We have taken up the present investigations, with the premise that soil microorganisms especially fluorescent pseudomonads (FLPs), whose role in enhancement of plant growth is unequivocal, may contribute to the enhanced growth and yield of rice cultivated under SRI.The results of present investigations revealed that rice cultivated under SRI harboured more the population of FLPs in rhizosphere than non-rhizosphere soil. Screening of rhizospheric FLPs isolates has revealed that many of the isolates possessed the ability of producing growth promoting substances like IAA, GA, siderophores, ‘p’solubilization. Some selected strains have also shown resistance towards heavy metals, salts and pH. They have also exhibited significant antifungal activity and enhanced the seed germination and efficient root colonization. Further, artificial inoculations have also clearly shown to enhance the growth in terms of height, dry weight of shoot and root. Thus, the results substantiate the role of FLPs for the enhanced growth and yield of rice cultivated under SRI.

Symptoms of Fern Distortion Syndrome resulting from inoculation with opportunistic endophytic fluorescent Pseudomonas spp.

BackgroundFern Distortion Syndrome (FDS) is a serious disease of Leatherleaf fern (Rumohra adiantiformis). The main symptom of FDS is distortion of fronds, making them unmarketable. Additional symptoms include stunting, irregular sporulation, decreased rhizome diameter, and internal discoloration of rhizomes. We previously reported an association of symptoms with increased endophytic rhizome populations of fluorescent pseudomonads (FPs). The aim of the current study was to determine if FPs from ferns in Costa Rica with typical FDS symptoms would recreate symptoms of FDS.Methodology and FindingsGreenhouse tests were conducted over a 29-month period. Micro-propagated ferns derived from tissue culture were first grown one year to produce rhizomes. Then, using an 8×9 randomized complete block experimental design, 8 replicate rhizomes were inoculated by dipping into 9 different treatments before planting. Treatments included water without bacteria (control), and four different groups of FPs, each at a two concentrations. The four groups of FPs included one group from healthy ferns without symptoms (another control treatment), two groups isolated from inside rhizomes of symptomatic ferns, and one group isolated from inside roots of symptomatic ferns. Symptoms were assessed 12 and 17 months later, and populations of FPs inside newly formed rhizomes were determined after 17 months. Results showed that inoculation with mixtures of FPs from ferns with FDS symptoms, but not from healthy ferns, recreated the primary symptom of frond deformities and also the secondary symptoms of irregular sporulation, decreased rhizome diameter, and internal discoloration of rhizomes.ConclusionsThese results suggest a model of causation of FDS in which symptoms result from latent infections by multiple species of opportunistic endophytic bacteria containing virulence genes that are expressed when populations inside the plant reach a minimum level.

Microbiological analysis of Panchagavya, vermicompost, and FYM and their effect on plant growth promotion of pigeon pea (Cajanus cajan L.) in India

Organic farming systems are based on principles of nutrient cycling and the minimal use of external inputs. In India, Panchagavya, vermicompost, and farmyard manure are integral components of nutrient management in organic farming systems. The aim of this study was to characterize these three different organic preparations with respect to microbiological quality and impacts on early crop growth. Among the three preparations, Panchagavya had the highest population of total bacteria (22 × 109 cfu ml−1), actinomycetes (60 × 104 cfu ml−1), phosphate solubilizers (103 × 106 cfu ml−1), fluorescent pseudomonads (151 × 105 cfu ml−1), and nitrifiers (5.4 × 106 cfu ml−1). Dehydrogenase activity (6.61 μg g−1 h−1) and microbial biomass carbon (89.6 μg g−1) were also found to be higher in Panchagavya. The short-term plant growth test with Cajanus cajan seeds treated with Panchagavya showed enhanced length of root (19.4 cm) and shoot (16.9 cm), dry mass (147 mg), leaf area (14.57 cm2), chlorophyll content (23 spad units), and photosynthetic activity (18.8 μmol m−2 s−1) after 15 days of sowing. Hence, these tests suggest that Panchagavya can be used as a low-cost preparation to support plant growth in organic agriculture.

Culture-Based Assessment of Microbial Communities in Soil Suppressive to Potato Common Scab.

A field in East Lansing, MI, showed a decline of potato common scab compared with an adjacent potato field. To confirm that the decline was due to biological factors, the soil was assayed. In the greenhouse, putative common-scab-suppressive soil (SS) was either treated with various temperatures or mixed with autoclaved SS at various ratios. Pathogenic Streptomyces scabies was incorporated into the treated soil at 106 CFU/cm3 of soil, followed by planting of either potato or radish. Disease severity was negatively correlated with the percentage of SS in the mixture and positively correlated with temperature above 60°C. The soil was screened for four groups of potential antagonists (general bacteria, streptomycetes, fluorescent pseudomonads, and bacilli) pairing in culture with S. scabies. The frequency of antagonistic bacteria in SS was higher than common-scab-conducive soil (CS) in all four groups but only pseudomonads and streptomycetes were significantly higher. The population of pathogenic Streptomyces spp. in the rhizosphere of CS was significantly higher than SS. Dilution plating of CS and SS samples showed no clear trends or differences in populations of total fungi, total bacteria, streptomycetes, fluorescent pseudomonads, and bacilli but terminal restriction fragment polymorphism analysis revealed two distinct microbial communities were present in SS and CS.

Coconut Leaf Vermiwash as a Component of Organic Farming for Sustaining Soil Health and Fertility

Vermiwash, produced from actively vermicomposting substrates of coconut leaf + cowdung by the action of Eudrilus sp., has an alkaline pH, contains major and minor nutrients, growth hormones, humic acid and plant beneficial bacteria. Application of appropriately diluted coconut leaf vermiwash has shown to increase germination and seedling vigour index of cow pea and paddy seeds in laboratory bioassays. Field trials with cowpea, maize and bhendi (okra) in CPCRI farm showed its capacity to increase biomass and yield of the crops accompanied by enhanced soil microbial activities. Application of the vermiwash in red sandy loam soil produced an increase of 36% of fresh biomass weight, nodule numbers and 43% nodule fresh weight in cowpea at 1:10 dilution compared to control. In maize, increase in cob yield by 5-10%, fresh cob weight increase of 29-64% and in bhendi 22 to 33% increase in yield were recorded when vermiwash was applied at 1:5 dilution. To validate these results, field trials were taken up in farmers’ plots at Majal and Edneer area in Kasaragod. Bitter gourd and cowpea crops were taken up at Majal while amaranthus, cowpea and green chillies at Edneer. The soil type was sandy loam at Majal and lateritic at Edneer. Vermiwash produced at CPCRI was used as treatment along with farmers’ practice as control. The mode of application of vermiwash, dilution rates and intervals of application were carried out by the farmers as per recommendations. Observations on yield of the crops in vermiwash applied plots were recorded by the farmers. A yield of 75 kg cowpea and 200 kg bitter gourd at Majal; 55 kg amaranthus, 155 kg cowpea and 10kg dried chilli at Edneer plot were recorded from vermiwash applied plots.
 In both the places, observations indicated that application of vermiwash resulted in yield of crops on par or slightly lower than the plots that received regular fertilizer inputs as per farmers’ practice. However, in vermiwash treated plots, the farmers listed many other important points viz. healthy plant and root hair growth, lesser pest and disease damage, larger leaf size and deep leaf colour, longer ability of plants to stay without wilting in field as well as longer time of remaining fresh in case of amaranthus after harvesting, etc. The soil analysis also revealed that application of vermiwash in both types of soils increased the bacterial, fungal, actinomycetes, free-living nitrogen fixing, phosphate solubilzing and fluorescent pseudomonad population, and dehydrogenase, urease and phosphatase enzyme activities when compared to fertilizer treated plots. This clearly established the fact that vermiwash enhanced the soil fertility status for production of quality yield of agronomic and horticulture crops.

Increased populations of deleterious fluorescent pseudomonads colonizing rhizomes of leatherleaf fern (Rumohra adiantiformis) and expression of symptoms of fern distortion syndrome after application of Benlate systemic fungicide

The overall objective of the current study was to test two hypotheses: (1) application of the systemic fungicide Benlate to leatherleaf fern results in long-term increases in populations of deleterious fluorescent pseudomonads that endophytically colonize rhizomes; (2) such endophytic colonization, resulting from Benlate treatment, is associated with damage to leatherleaf fern and development of symptoms of fern distortion syndrome (FDS). Mean populations of fluorescent pseudomonads and total aerobic bacteria in the rhizosphere were significantly increased 3 months after the application of Benlate 50 WP and Benlate 50 DF as foliar sprays and drenches, compared to populations in the non-treated control. In another test, at 5 months after treatment, endophytic populations of fluorescent pseudomonads inside rhizomes of plants treated with Benlate 50 WP and Benlate 50 DF (both from DuPont) were significantly higher than populations in rhizomes of plants treated with three different generic sources of Benlate 50 WP, the active ingredient benomyl, and MBC. In the same test, treatments with the two DuPont sources of Benlate, but not the generic sources, benomyl, or MBC resulted in reduced overall growth of plants, including reductions weight of fronds, roots and rhizomes and caliper of rhizomes. Three long-term experiments examined the effect of Benlate 50 WP and Benlate 50 DF on populations of fluorescent pseudomonads and development of distortions of frond growth and other symptoms associated with FDS. In two tests at 24 months after application, treatment with both formulations of Benlate increased endophytic populations of fluorescent pseudomonads inside rhizomes, compared to controls, in both experiments, and endophytic populations inside petioles of fronds arising from rhizomes were significantly greater with both Benlate treatments in one test where this parameter was measured. Also at 24 months after application, the Benlate treatments were associated with significant increases in severity of FDS, using a previously reported rating scale and by measuring several other parameters associated with FDS. In the third long-term test, the first crop of fronds was cut 24 months after treatment. Four weeks later, the numbers of actively growing rhizomes (that had produced new fronds) were significantly reduced on Benlate-treated plants, compared to the controls. At 6 months after cutting (30 months after Benlate treatments), weights of fronds were lower on Benlate-treated plants than on controls. Characterization of fluorescent pseudomonads isolated from inside rhizomes and petioles at 24 months after treatment revealed that applications of Benlate resulted in a marked increase in pectinolytic activity. DNA sequencing of 150 strains of fluorescent pseudomonads from inside rhizomes and 60 from inside petioles at 24 months after application of Benlate indicated low matches to type strains in the ribosomal data base. Phylogenetic characterization of the strains indicated the existence of 10 clusters. Pronounced shifts in frequency of strains in the various clusters were noted with Benlate treatment: over 70% of strains from rhizomes of Benlate-treated plants belonged to cluster E3, compared to only 2% of the strains from control rhizomes, and 98% of the control strains belonged to clusters C, D2, F, and G, while none of the strains from petioles or rhizomes of Benlate-treated plants belonged to these four clusters. In summary, treatment of leatherleaf fern with Benlate 50 WP and Benlate 50 DF from DuPont led to a progression of long-term deleterious effects on leatherleaf fern that were associated with increased populations of fluorescent pseudomonads that were functionally and phylogenetically different from pseudomonads from control plants.

Fern Distortion Syndrome of Leatherleaf Fern in Costa Rica: Symptoms, Incidence, and Severity.

A syndrome has been recognized on leatherleaf fern (Rumohra adiantiformis) in Costa Rica for many years that causes widespread damage but has not been described in the literature. A full description of the syndrome, termed fern distortion syndrome (FDS), is reported here, along with evidence that FDS is a new disease and that it is associated with endophytic fluorescent pseudomonads but not with any other major groups of pathogens or pests. The main aboveground symptoms of FDS are twisting and distortions of fronds, which make the fronds unmarketable. In advanced cases of FDS, fronds are often thickened, new frond growth ceases or slows dramatically, and uneven sporulation is apparent on the underside of fronds. Symptoms of FDS belowground are reduced diameter of rhizomes and reduced overall root mass. The incidence of FDS in Costa Rica was typically over 80%, and severity typically ranged from 1.26 to 2.48 using a 0 to 3 rating scale in fields propagated vegetatively with rhizomes from fields with FDS. In contrast, in three fields planted 1.5 to 4 years previously with rhizomes derived from tissue culture, incidence and severity were markedly lower: 23 to 34% and 0.24 to 0.36, respectively. Paired sampling of symptomatic and asymptomatic plants revealed significantly greater populations of fluorescent pseudomonads inside rhizomes of symptomatic plants.

Effect of long-term vineyard monoculture on rhizosphere populations of pseudomonads carrying the antimicrobial biosynthetic genes phlD and/or hcnAB

The impact of repeated culture of perennial plants (i.e. in long-term monoculture) on the ecology of plant-beneficial bacteria is unknown. Here, the influence of extremely long-term monocultures of grapevine (up to 1603 years) on rhizosphere populations of fluorescent pseudomonads carrying the biosynthetic genes phlD for 2,4-diacetylphloroglucinol and/or hcnAB for hydrogen cyanide was determined. Soils from long-term and adjacent short-term monoculture vineyards (or brushland) in four regions of Switzerland were baited with grapevine or tobacco plantlets, and rhizosphere pseudomonads were studied by most probable number (MPN)-PCR. Higher numbers and percentages of phlD(+) and of hcnAB(+) rhizosphere pseudomonads were detected on using soil from long-term vineyards. On focusing on phlD, restriction fragment length polymorphism profiling of the last phlD-positive MPN wells revealed seven phlD alleles (three exclusively on tobacco, thereof two new ones). Higher numbers of phlD alleles coincided with a lower prevalence of the allele displayed by the well-studied biocontrol strain Pseudomonas fluorescens F113. The prevalence of this allele was 35% for tobacco in long-term monoculture soils vs. >60% in the other three cases. We conclude that soils from long-term grapevine monocultures represent an untapped resource for isolating novel biocontrol Pseudomonas strains when tobacco is used as bait.

Characterization of biosurfactant-producing strains of fluorescent pseudomonads in a soilless cultivation system

The use of biosurfactants is a promising alternative in biological control of zoospore-producing plant pathogens. In the present study, biosurfactant production by the indigenous population of fluorescent pseudomonads in a soilless plant cultivation system was studied during the growing season. A total of 600 strains was screened and of these 18.5% were observed to produce biosurfactants. Production of both antibiotics and biosurfactant was uncommon among the isolated strains. A selective effect of the cultivation system filter was observed on the biosurfactant-producing strains and these strains were only occasionally observed after the filter, despite having a significantly higher motility than the nonbiosurfactant-producing strains. The majority of biosurfactant-producing strains were isolated from the filter skin, which suggests that this is a suitable surface for inoculation with biocontrol strains.

Selective Enhancement of the Fluorescent Pseudomonad Population After Amending the Recirculating Nutrient Solution of Hydroponically Grown Plants with a Nitrogen Stabilizer

Fluorescent pseudomonads have been associated, via diverse mechanisms, with suppression of root disease caused by numerous fungal and fungal-like pathogens. However, inconsistent performance in disease abatement, after their employment, has been a problem. This has been attributed, in part, to the inability of the biocontrol bacterium to maintain a critical threshold population necessary for sustained biocontrol activity. Our results indicate that a nitrogen stabilizer (N-Serve, Dow Agrosciences) selectively and significantly enhanced, by two to three orders of magnitude, the resident population of fluorescent pseudomonads in the amended (i.e., 25 microg ml(-1) nitrapyrin, the active ingredient) and recycled nutrient solution used in the cultivation of hydroponically grown gerbera and pepper plants. Pseudomonas putida was confirmed as the predominant bacterium selectively enhanced. Terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rDNA suggested that N-Serve selectively increased P. putida and reduced bacterial diversity 72 h after application. In vitro tests revealed that the observed population increases of fluorescent pseudomonads were preceded by an early growth suppression of indigenous aerobic heterotrophic bacteria (AHB) population. Interestingly, the fluorescent pseudomonad population did not undergo this decrease, as shown in competition assays. Xylene and 1,2,4-trimethylbenzene (i.e., the inert ingredients in N-Serve) were responsible for a significant percentage of the fluorescent pseudomonad population increase. Furthermore, those increases were significantly higher when the active ingredient (i.e., nitrapyrin) and the inert ingredients were combined, which suggests a synergistic response. P. putida strains were screened for the ability to produce antifungal compounds and for the antifungal activity against Pythium aphanidermatum and Phytophthora capsici. The results of this study suggest the presence of diverse mechanisms with disease-suppressing potential. This study demonstrates the possibility of using a specific substrate to selectively enhance and maintain desired populations of a natural-occurring bacterium such as P. putida, a trait considered to have great potential in biocontrol applications for plant protection.

Chemo-biological suppression of root-infecting zoosporic pathogens in recirculating hydroponic systems

Zoosporic root-infecting pathogens are the most destructive organisms in enclosed environment agricultural systems which recycle irrigation water. Numerous strategies have been proposed for reducing or eliminating these pathogens from recycled irrigation water. These strategies include the following: filtration, sedimentation, chlorination, ozonation, heat, ultraviolet light, application of antimicrobial chemicals, suppressive potting substrates, and biological control agents. The latter strategy has been the focus of numerous investigations but consistency in performance in disease abatement following their application has not been realized. This has been attributed, in part, to the inability of these potential biocontrol bacteria to maintain a critical threshold population necessary for sustained biocontrol activity. That threshold population has been estimated at not less than 106 CFU g−1 of soil or root. Recently, we demonstrated that amending recycled irrigation water with specific carbon substrates (i.e., N-Serve® or Truban®) resulted in the selective enhancement of the indigenous fluorescent pseudomonad populations to levels at or above the presumed threshold levels necessary for disease control. In our current study, we verified the ubiquity of that response in different cultural systems involving two different host plants (i.e., cucumber and pepper) and, most significantly, we document, for the first time, significant and sustained disease abatement. Additionally, we demonstrated that nitrapyrin, the active ingredients in N-Serve® and Truban®, exerted direct antifungal activity whereas the inert ingredients had an indirect role in disease suppression. Specifically, the inert ingredients were responsible for the increase in the fluorescent pseudomonad population. Amending the recirculating nutrient solution with a representative fluorescent pseudomonad isolate verified and substantiated their role in disease control. Cumulatively, our results support the hypothesis that it is possible to modify the environment to make it more conducive to the multiplication and survival of indigenous biocontrol bacteria. The resident microflora in many, if not most, habitats already contain prospective biological control members who, if enhanced, could function as antagonists to other microorganisms in the same habitat.

CHARACTERIZATION OF ANTAGONISTIC ROOT-ASSOCIATED FLUORESCENT PSEUDOMONADS OF TRANSGENIC AND NON-TRANSGENIC CITRANGE TROYER PLANTS

To study the influence of rolABC Troyer citrange plants on potentially beneficial root-associated bacteria, transgenic and non-transgenic plant lines were monitored over the course of one year. Culturable bacterial populations were not significantly different between rolABC and wild type lines, whereas the population of fluorescent pseudomonads varied according to the plant lines and the date of sampling. A total of 321 representative fluorescent pseudomonads were investigated to determine antagonistic activity against Fusarium solani and Phoma tracheiphila, and two different antagonistic groups were identified. IAA-producing Pseudomonas fluorescens strains were well distributed among transgenic and non-transgenic plant lines. A subset of 42 representative antagonistic P. fluorescens strains, characterized by ARDRA and ERIC-PCR, showed a high degree of DNA heterogeneity. Four main groups were identified. Three groups were heterogeneous and contained isolates from transgenic and non-transgenic plant lines whereas one group was homogeneous and included only strains from transgenic lines belonging to the same antagonistic group. On the whole, the fluorescent populations isolated from the rolABC lines showed some differences from those isolated from wild types, and a partial correlation was observed between grouping of strains based on antagonistic activity and on ERIC-PCR fingerprints and the transgenic character of the plants.

Soil Bacterial Diversity Responses to Root Colonization by an Ectomycorrhizal Fungus are not Root-Growth-Dependent

The hypothesis tested in this present study was that the ectomycorrhizosphere effect on the bacterial community was not root-growth-dependent. The impacts of ectomycorrhizal infection (Pisolithus albus COI007) and a chemical fertilization to reproduce the fungal effect on root growth were examined on (1) the structure of bacterial community and (2) fluorescent pseudomonad and actinomycete populations in the mycorrhizosphere of Acacia auriculiformis using both culture-independent and culture-dependent methods. A. auriculiformis plants were grown in disinfested soil in pots with or without addition of the ectomycorrhizal fungus or N/P/K fertilization (to reproduce the fungal effect on root growth) for 4 months and then transferred to 20-L pots filled with nondisinfested sandy soil. The fungal and fertilizer applications significantly improved the plant growth after 4-month culture in the disinfested soil. In the nondisinfested cultural substrate, these positive effects on plant growth were maintained. The total soil microbiota was significantly different within the treatments as revealed from DNA analysis [denaturing gradient gel electrophoresis (DGGE)]. The structure of fluorescent pseudomonad populations was also affected by fungal and fertilizer applications. In contrast, no qualitative effect was observed for the actinomycete communities within each treatment, but fungal inoculation significantly decreased the number of actinomycetes compared to the fertilizer application treatment. These results show that the mycorrhizosphere effect is not root-growth-dependent but is mainly due to the presence of the ectomycorrhizal fungus and more particularly to the extramatrical mycelium.

Functional diversity of soil microbial community, rock phosphate dissolution and growth of Acacia seyal as influenced by grass-, litter- and soil-feeding termite nest structure amendments

We tested termite mound materials belonging to different feeding groups: Cubitermes (soil-feeder), Trinervitermes (grass-feeder) and Macrotermes (litter-feeder), as natural microbial inoculum to promote plant growth and increase nutrient supplies from soil organic matter and inorganic amendments (rock phosphate), through their effects on soil microorganisms (functional diversity of soil microflora, arbuscular mycorrhizal fungi, rhizobia, fluorescent pseudomonads, actinomycetes and saprophytic fungi). Experiments were made in a pot experiment with Acacia seyal, a leguminous tree abundant in West Africa, with a sandy soil amended or not with rock phosphate. Results indicated a stimulation of plant growth with Cubitermes and Trinervitermes mound powder (plant height and shoot biomass), similar to what was obtained with rock phosphate alone. Leaf content in N was also increased in the termite treatments (except in Macrotermes soil), whereas mycorrhizal colonization was inhibited as compared to the control. The development of saprophytic fungi was significantly higher in the soils amended with rock phosphate and this effect was hypothesized to be related to the production of large quantities of oxalic acid by fungal populations. The fluorescent pseudomonad populations notably increased in the soils dually amended with mound powders and rock phosphate, and this could be due to the fact that some species of this bacterial group are able to dissolve rock phosphate. The organic and inorganic amendments decreased the soil catabolic evenness in all the mound powder treatments. Among the mound materials tested, Cubitermes mound powder had the most promising effect, especially on SIR response to oxalate. It is concluded that soils amended both with rock phosphate and Cubitermes mound soil could promote the development of microbial communities, which could help to metabolize this compound and consequently enhance plant growth.

Characterization of soil microbial communities under different potato cropping systems by microbial population dynamics, substrate utilization, and fatty acid profiles

The effects of 11 different 2- and 3-yr potato crop rotations on soil microbial communities were characterized over three field seasons using several techniques. Assessments included microbial populations determined by soil dilution plate counts on various general and selective culture media, microbial activity by fluorescein diacetate (FDA) hydrolysis, single carbon source substrate utilization (SU) profiles, and fatty acid methyl ester (FAME) profiles. Potato rotation crops evaluated in research plots at Newport, ME, included barley/clover, canola, green bean, millet, soybean, sweet corn, and a continuous potato control. Soil populations of culturable bacteria and overall microbial activity tended to be highest following barley, canola, and sweet corn rotations, and lowest with continuous potato. Differences among rotations were less apparent during the potato phase of the rotations. Populations of actinomycetes and fluorescent pseudomonads tended to be greater in barley rotations than in most other rotations. SU profiles derived from BIOLOG GN2 plates indicated that certain rotations, including barley, canola, and sweet corn tended to have higher overall microbial activity, and barley and sweet corn rotations averaged higher substrate richness and diversity. Soybean and potato rotations tended to have lower substrate richness and diversity. Principal component analyses of SU data revealed differences among rotation soil communities in their utilization of individual carbon sources and substrate guilds, including carbohydrates, carboxylic acids, amines/amides, and amino acids. Analyses of soil FAME profiles demonstrated distinct differences among all the rotation soils in their relative composition of fatty acids, indicating differences in their microbial community structure. Fatty acids most responsible for differentiation among rotation soils included 16:1 ω5c, 16:1 ω7c, 18:2 ω6c, 18:1 ω9c, 12:0, and 13:0 anteiso, with 16:1 ω5c being the single greatest determinant. Overall, monounsaturated fatty acids, particularly 16:1 ω5c, were most prevalent in sweet corn rotations and polyunsaturates were highest in barley and millet rotations. Straight chain saturated fatty acids comprised the greatest proportion of fatty acids in soils under continuous potato. FAME biomarkers for microorganism groups indicated barley and millet rotations had the highest ratio of fungi to bacteria, and soybean and continuous potato had the lowest ratio. This research has demonstrated that different crop rotations have distinctive effects on soil microbial communities that are detectable using a variety of techniques. Further studies will identify more specific changes associated with particular rotations and relate these changes to potential effects on disease management, crop health, and crop productivity.

Modification of fluorescent pseudomonad community and control of apple replant disease induced in a wheat cultivar-specific manner

Apple orchard soils were cropped for three successive 28-day growth cycles with a perennial ryegrass/annual ryegrass mixture or 1 of 11 wheat cultivars prior to planting with Gala apple seedlings. Prior cropping of soils with ryegrass had no significant impact on growth of apple, but cultivation with some, but not all, wheat cultivars substantially enhanced apple seedling growth. Enhanced growth of apple was associated with a reduction in root infection by the dominant fungal pathogens resident to the respective orchard soils. Wheat cultivars that enhanced growth of apple modified the genetic and species composition of the fluorescent pseudomonad population resident to orchard soil. The fluorescent pseudomonad population from soils cropped with wheat cultivars that enhanced apple seedling growth possessed a higher capacity to inhibit in vitro growth of multiple elements of the fungal complex that incites apple replant disease than did the population from non-treated orchard soil or soil cropped with a wheat cultivar that did not enhance growth of apple. Application of wheat root exudates to soil also modified the antagonistic potential and composition of the fluorescent pseudomonad population in a cultivar-specific manner. This study demonstrates the significant role of plant genotype in determining composition of the saprophytic soil microbial community. It also establishes the need to consider not only plant species but also plant genotype when evaluating the potential benefit of specific cover crops as a component within a systems approach for the management of soilborne plant diseases.

Phylogeny of HCN synthase-encoding hcnBC genes in biocontrol fluorescent pseudomonads and its relationship with host plant species and HCN synthesis ability.

Hydrogen cyanide (HCN) is a broad-spectrum antimicrobial compound involved in biological control of root diseases by many plant-associated fluorescent pseudomonads. The HCN synthase is encoded by three biosynthetic genes (hcnA, hcnB, and hcnC), but little is known about the diversity of these genes in fluorescent Pseudomonas spp. and in other bacteria. Here, the partial hcnBC sequence was determined for a worldwide collection of biocontrol fluorescent Pseudomonas spp. Phylogenies based on hcnBC and deduced protein sequences revealed four main bacterial groups, but topological incongruences were found between hcnBC and rrs-based phylogenies, suggesting past lateral transfer of hcnBC among saprophytic root-colonizing pseudomonads. Three of the four groups included isolates from different countries and host plants. Yet, these groups corresponded to distinct, ecologically-adapted populations of HCN-producing biocontrol fluorescent pseudomonads, as indicated by high hcnBC distinctness ratio values and the differences in production levels of HCN in vitro found between groups. This is in accordance with previous results on catabolic properties and biocontrol abilities of these strains. HCN synthase gene diversity may thus reflect the adaptive radiation of HCN+ biocontrol fluorescent pseudomonads. Positive correlations were found between HCN production in vitro and plant protection in the cucumber/Pythium ultimum and tomato/Fusarium oxysporum f. sp. radicis-lycopersici pathosystems.

Fluorescent pseudomonad population sizes baited from soils under pure birch, pure Douglas-fir, and mixed forest stands and their antagonism towardArmillaria ostoyaein vitro

The relationship between forest stand composition in southern interior British Columbia and fluorescent pseudomonad bacteria populations was investigated using seedling bioassays. The objectives of this study were to (i) compare the relative population sizes of fluorescent pseudomonads baited from soils in pure paper birch (Betula papyrifera Marsh.), pure Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and mixed stands of the two species and (ii) determine if fluorescent pseudomonads from these soils have inhibitory effects against the root pathogen Armillaria ostoyae (Romagn.) Herink in vitro. Soil from birch stands supported four times more pseudomonads on seedling baits than soil from Douglas-fir stands, with the mixed stands intermediate. Soil from young stands yielded twice as many rhizosphere pseudomonads as soil from mature stands. Pseudomonad population size was positively correlated with percent cover and density of birch, and negatively correlated with basal area of Douglas-fir, percent cover of Douglas-fir, and carbon/nitrogen ratio of the soil. Greater than 50% of the fluorescent isolates reduced radial growth of A. ostoyae by more than 20% and greater than 90% reduced biomass of the fungus in dual culture tests. Cell-free bacterial culture filtrates added to the growth medium also reduced growth of A. ostoyae. This study provides evidence that paper birch provides a more favorable environment for fluorescent pseudomonads than Douglas-fir and suggests a mechanism by which paper birch can positively influence the susceptibility of managed forest stands to Armillaria root disease.

Interactions between ectomycorrhizal symbiosis and fluorescent pseudomonads on Acacia holosericea: isolation of mycorrhiza helper bacteria (MHB) from a Soudano-Sahelian soil

Acacia holosericea seedlings were planted in 1-l pots filled with a soil collected from an Australian Acacia plantation in Southern Senegal. After 6 months of culture, mycorrhizosphere soil, roots, galls induced by root-knot nematodes and Rhizobium nodules were sampled from each pot. The diversity of this bacterial group was characterized by siderotyping (pyoverdine IsoElectric Focusing (IEF) analysis) and by restriction fragment length polymorphism (RFLP). The effect of these isolates on the establishment of the ectomycorrhizal symbiosis between an Australian Acacia ( A. holosericea) and Pisolithus sp. strain IR100 was studied. In the mycorrhizosphere soil, the population of fluorescent pseudomonads was represented by strains of two different siderovars (groups of bacterial strains presenting an identical pyoverdine–IEF pattern): siderovar 1 (74%) and siderovar 2 (26%). The siderotyping of the isolates around galls of the root-knot nematodes revealed three siderovars (40% from siderovar 1, 40% from siderovar 2 and about 15% from siderovar 3). RFLP of 16S rDNA divided the isolates into four different groups with MspI, two with HhaI and two with HaeIII endonucleases. The establishment of the ectomycorrhizal symbiosis with A. holosericea was promoted by 14 bacterial strains isolated from the mycorrhizosphere soil, three isolates from the roots and four from the galls. Shoot biomass of A. holosericea seedlings was stimulated by eight bacterial isolates from soil, six isolates from galls and seven from roots. These mycorrhiza helper bacteria could have a great ecological importance in tropical areas through the reforestation programs.

Interactions between ectomycorrhizal symbiosis and fluorescent pseudomonads on Acacia holosericea: isolation of mycorrhiza helper bacteria (MHB) from a Soudano-Sahelian soil

Abstract Acacia holosericea seedlings were planted in 1-l pots filled with a soil collected from an Australian Acacia plantation in Southern Senegal. After 6 months of culture, mycorrhizosphere soil, roots, galls induced by root-knot nematodes and Rhizobium nodules were sampled from each pot. The diversity of this bacterial group was characterized by siderotyping (pyoverdine IsoElectric Focusing (IEF) analysis) and by restriction fragment length polymorphism (RFLP). The effect of these isolates on the establishment of the ectomycorrhizal symbiosis between an Australian Acacia (A. holosericea) and Pisolithus sp. strain IR100 was studied. In the mycorrhizosphere soil, the population of fluorescent pseudomonads was represented by strains of two different siderovars (groups of bacterial strains presenting an identical pyoverdine-IEF pattern): siderovar 1 (74%) and siderovar 2 (26%). The siderotyping of the isolates around galls of the root-knot nematodes revealed three siderovars (40% from siderovar 1, 40% from siderovar 2 and about 15% from siderovar 3). RFLP of 16S rDNA divided the isolates into four different groups with MspI, two with HhaI and two with HaeIII endonucleases. The establishment of the ectomycorrhizal symbiosis with A. holosericea was promoted by 14 bacterial strains isolated from the mycorrhizosphere soil, three isolates from the roots and four from the galls. Shoot biomass of A. holosericea seedlings was stimulated by eight bacterial isolates from soil, six isolates from galls and seven from roots. These mycorrhiza helper bacteria could have a great ecological importance in tropical areas through the reforestation programs.