Biological Control Of Soil-borne Pathogens Research Articles

Biological control of soil-borne pathogens in arid lands: a review

Biological control of soil-borne pathogens in arid lands: a review

Hydrogen cyanide production by soil bacteria: Biological control of pests and promotion of plant growth in sustainable agriculture

Currently, plant diseases and insect infestations are mainly controlled by the extraneous application of pesticides. Unfortunately, the indiscriminate use of such agrochemicals can cause ecological and environmental problems, as well as human health hazards. To obviate the potential pollution arising from the application of agrochemicals, biological control of soilborne pathogens or insect pests using antagonistic microorganisms may be employed. Certain soil bacteria, algae, fungi, plants and insects possess the unique ability to produce hydrogen cyanide (HCN), which plays an important role in the biotic interactions of those organisms. In particular, cyanogenic bacteria have been found to inhibit the growth of various pathogenic fungi, weeds, insects, termites and nematodes. Thus, the use of HCN-producing bacteria as biopesticides offers an ecofriendly approach for sustainable agriculture. The enzyme, HCN synthase, involved in the synthesis of HCN, is encoded by the hcnABC gene cluster. The biosynthetic regulation of HCN, antibiotics and fluorescent insecticidal toxins through the conserved global regulatory GacS/GacA system is elaborated in this review, including approaches that may optimize cyanogenesis for enhanced pest control. In addition, the effects of bacterially synthesized HCN on the production of indole acetic acid, antibiotics and fluorescent insecticidal toxins, 1-aminocyclopropane-1-carboxylate deaminase utilization and phosphate solubilization may result in the stimulation of plant growth. A more detailed understanding of HCN biosynthesis and regulation may help to elaborate the precise role of this compound in biotic interactions and sustainable agriculture.

Glucanolytic rhizobacteria associated with wheat- maize cropping system suppress the Fusarium wilt of tomato (Lycopersicum esculentum L)

Tomato wilt caused by F. oxysporum (Fox) is one of the most destructive diseases, which causes qualitative and quantitative losses. In the present study, the bio-efficacy of glucanolytic bacteria was assessed on tomato against Fusarium wilt under net house conditions. A 178 rhizobacteria were isolated from the wheat-maize cropping system of arid and semi-arid regions. Only ten out of 178 rhizobacteria produced glucanases. The glucanolytic bacteria exhibited variable ability to produce β-1, 3/4-glucanases (0.11 – 0.30 U/mL). The glucanase production was higher at pH 7 (SZ =7.89 mm - 14.11 mm) followed by pH 9 (SZ=5.78 mm - 10.56 mm) and pH 5 (SZ=5.55 mm - 10.00 mm). The glucanolytic bacteria MAZ 10SR, 13, 18, 32, 48, 51, 73, 86, 117, 151 & 165 suppressed F. oxysporum (43–68%) causing tomato wilt and other phytopathogens viz F. moniliforme (44–63%), M. phaseolina (42–65%) and P. oryzae (42–67%). The glucanolytic bacteria also produced different antifungal metabolites/compounds viz siderophores, protease, and lipopeptide antibiotic surfactin. In planta assays indicated the ability of glucanolytic bacteria Bacillus subtilis MAZ 10SR, 51, 117 & 165 to colonize the tomato rhizosphere by 7.4 – 8.9 log CFU/g of soil. They decreased the Fusarium wilt incidence with disease severity of 22–31% as compared to that of control (disease severity = 56%). A significant quantity of glucanases (0.33–0.70 U/mg of soil) was recovered from the tomato rhizosphere inoculated with the effective glucanolytic bacteria. The glucanolytic rhizobacteria were identified as Achromobacter mucicolens, Enterobacter cloacae, Serratia marcescens, and Bacillus spp. by 16S rRNA gene analysis. The utilization of glucanolytic bacteria could be a worthy strategy in the biological control of soil-borne pathogens.

Whole-Genome Sequence of Bacillus cereus AR156, a Potential Biocontrol Agent with High Soilborne Disease Biocontrol Efficacy and Plant Growth Promotion.

ABSTRACTBacillus cereus AR156 was originally isolated from the forest soil of Zhenjiang, a city in China. To shed new light on the molecular mechanisms underlying the biological control of soilborne pathogens, the whole genome of this strain was sequenced. Here, we report the draft genome sequence of this strain, consisting of a single circularized contig measuring 5.66 Mb, with an average GC content of 35.5% and 5,367 open reading frames.

Biopesticides: Use of Rhizosphere Bacteria for Biological Control of Plant Pathogens

The pesticides used to control pests and diseases are also implicated in ecological, environmental and human health hazards. To reduce the deleterious effects of these agrochemicals, certain antagonistic microorganisms have been characterised from rhizosphere of different crop plants that suppress various plant diseases and thus, minimize the use of pesticides. The application of these specific antagonistic microorganisms in biological control of soilborne pathogens has been studied intensively in the last two decades. These beneficial rhizosphere microorganisms inhibit the pathogenic bacteria and fungi by producing antibiotics, bacteriocins, siderophores, hydrolytic enzymes and other secondary metabolites. The efficiency of these biocontrol products can be improved by manipulation of the environment, using mixtures of beneficial organisms, physiological and genetic enhancement of the biocontrol mechanisms, manipulation of formulations and integration of biocontrol with other alternative methods that provide additive effects. These biocontrol agents could be effectively utilised in sustainable agriculture for improving growth of crop plants.

Unearthing the genomes of plant-beneficial Pseudomonas model strains WCS358, WCS374 and WCS417.

BackgroundPlant growth-promoting rhizobacteria (PGPR) can protect plants against pathogenic microbes through a diversity of mechanisms including competition for nutrients, production of antibiotics, and stimulation of the host immune system, a phenomenon called induced systemic resistance (ISR). In the past 30 years, the Pseudomonas spp. PGPR strains WCS358, WCS374 and WCS417 of the Willie Commelin Scholten (WCS) collection have been studied in detail in pioneering papers on the molecular basis of PGPR-mediated ISR and mechanisms of biological control of soil-borne pathogens via siderophore-mediated competition for iron.ResultsThe genomes of the model WCS PGPR strains were sequenced and analyzed to unearth genetic cues related to biological questions that surfaced during the past 30 years of functional studies on these plant-beneficial microbes. Whole genome comparisons revealed important novel insights into iron acquisition strategies with consequences for both bacterial ecology and plant protection, specifics of bacterial determinants involved in plant-PGPR recognition, and diversity of protein secretion systems involved in microbe-microbe and microbe-plant communication. Furthermore, multi-locus sequence alignment and whole genome comparison revealed the taxonomic position of the WCS model strains within the Pseudomonas genus. Despite the enormous diversity of Pseudomonas spp. in soils, several plant-associated Pseudomonas spp. strains that have been isolated from different hosts at different geographic regions appear to be nearly isogenic to WCS358, WCS374, or WCS417. Interestingly, all these WCS look-a-likes have been selected because of their plant protective or plant growth-promoting properties.ConclusionsThe genome sequences of the model WCS strains revealed that they can be considered representatives of universally-present plant-beneficial Pseudomonas spp. With their well-characterized functions in the promotion of plant growth and health, the fully sequenced genomes of the WCS strains provide a genetic framework that allows for detailed analysis of the biological mechanisms of the plant-beneficial traits of these PGPR. Considering the increasing focus on the role of the root microbiome in plant health, functional genomics of the WCS strains will enhance our understanding of the diversity of functions of the root microbiome.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1632-z) contains supplementary material, which is available to authorized users.

Powder Coating Formulations with Pseudomonas fluorescens M45 Enhance the Emergence Rate and Healthy Stand Establishment of Sesame in Disease-Conducive Soil

Sesame is a major cooking oil crop in Korea. One of the primary problems in sesame cultivation is low healthy stand establishment due to the occurrence of seedling rot and damping-off resulting from a complex of soil-borne pathogens in the field. To address the problem, the bioformulation of Pseudomonas fluorescens M45 was prepared in powder form using clay and vermiculite, and was evaluated for its effect on biological control of soil-borne pathogens in sesame cultivation. In the petri dish trial, the emergence rate was overall good (> 92%) regardless of seeds being pelleted and/or M45-treated. In both pot and field trials containing disease-conducive soils, seed-pelleting substantially reduced emergence rate, whereas seed-pelleting with M45 significantly improved the emergence rate (> 26%). The emergence rate of sesame seeds treated with the strain M45 was greater than 30% regardless of seed pelletization. We also found that M45r colonized in the roots at the density of 1.6×10 5 cfu/g. With aid of the bioformulation, however, root colonization of the strain was significantly increased to 4.0×10 6 cfu/g. The powder formulation with strain M45 enhanced the rate of healthy stand establishment in disease-conducive soil. Therefore, bioformulation with strain M45 is a promising method to overcome problems associated with the successive cultivation of sesame.

Mycorrhiza induced resistance in potato plantlets challenged by Phytophthora infestans

Biological control of soil-borne pathogens by arbuscular mycorrhizal (AM) fungi has been repeatedly demonstrated. However, their role in the control of above-ground hemibiotrophic pathogens is less conclusive. Here, we investigated in vitro the impact of an AM fungus on Phytophthora infestans in potato plants. The leaf infection index was decreased in mycorrhizal potato plants. Real-Time Quantitative PCR revealed the induction of two pathogenesis related genes ( PR1 and PR2) in the leaves of mycorrhizal plants shortly after infection with P. infestans. These results suggested a systemic resistance in mycorrhizal plants, related to the priming of the two PR genes in potato.

A Simple and Rapid Method for Functional Analysis of Plant Growth-promoting Rhizobacteria Using the Development of Cucumber Adventitious Root System

Many plant growth-promoting rhizobacteria (PGPRs) have been known for beneficial effects on plants including biological control of soilborne pathogens, induced systemic resistance to plant pathogens, phytohormone production, and improvement of nutrient and water uptake of plants. We developed a simple and rapid method for screening potential PGPR, especially phytohormone producing rhizobacteria, or for analyzing their functions in plant growth using cucumber seedling cuttings. Surface-sterilized cucumber seeds were grown in a plastic pot containing steamed vermiculite. After 7 days of cultivation, the upper part 2 cm in length of cucumber seedling, was cut and used as cucumber cuttings. The base of cutting stem was then dipped in a microcentrifuge tube containing 1.5ml of a bacterial suspension and incubated at <TEX>$25^{\circ}C$</TEX> with a fluorescent light for 10 days. Number and length of developed adventitious roots from cucumber cuttings were examined. The seedling cuttings showed various responses to the isolates tested. Some isolates resulted in withering at the day of examination or in reduced number of roots developed. Several isolates stimulated initial development of adventitious roots showing more adventitious root hair number than that of untreated cuttings, while some isolate had more adventitious root hair number and longer adventitious roots than that of untreated control. Similar results were obtained from the trial with rose cuttings. Our results suggest that this bioassay method may provide a useful way for differentiating PGPR's functions involved in the development of root system.

Selection for biocontrol bacteria antagonistic toward Rosellinia necatrix by enrichment of competitive avocado root tip colonizers

Biological control of soil-borne pathogens is frequently based on the application of antagonistic microorganisms selected solely for their ability to produce in vitro antifungal factors. The aim of this work was to select bacteria that efficiently colonize the roots of avocado plants and display antagonism towards Rosellinia necatrix, the causal agent of avocado white root rot. A high frequency of antagonistic strains (ten isolates, 24.4%) was obtained using a novel procedure based on the selection of competitive avocado root tip colonizers. Amplification and sequencing of the 16S rRNA gene, in combination with biochemical characterization, showed that eight and two of the selected isolates belonged to the genera Pseudomonas and Stenotrophomonas, respectively. Characterization of antifungal compounds produced by the antagonistic strains showed variable production of exoenzymes and HCN. Only one of these strains, Pseudomonas sp. AVO94, produced a compound that could be related to antifungal antibiotics. All of the ten selected strains showed twitching motility, a cell movement involved in competitive colonization of root tips. Production of N-acyl-homoserine lactones and indole-3-acetic acid was also reported for some of these isolates. Resistance to several bacterial antibiotics was tested, and three strains showing resistance to only one of them were selected for biocontrol assays. The three selected strains persisted in the rhizosphere of avocado plants at levels considered crucial for efficient biocontrol, 10 5–10 6 colony forming units/g of root; two of them, Pseudomonas putida AVO102 and Pseudomonas pseudoalcaligenes AVO110, demonstrated significant protection of avocado plants against white root rot.

Biological control of soil-borne pathogens by fluorescent pseudomonads

Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

Effect of Population Density of Pseudomonas fluorescens on Production of 2,4-Diacetylphloroglucinol in the Rhizosphere of Wheat

ABSTRACT The role of antibiotics in biological control of soilborne pathogens, and more generally in microbial antagonism in natural disease-suppressive soils, often has been questioned because of the indirect nature of the supporting evidence. In this study, a protocol for high pressure liquid chromatography/mass spectrometry is described that allowed specific identification and quantitation of the antibiotic 2,4-diacetylphloroglucinol (Phl) produced by naturally occurring fluorescent Pseudomonas spp. on roots of wheat grown in a soil suppressive to take-all of wheat. These results provide, for the first time, biochemical support for the conclusion of previous work that Phl-producing fluorescent Pseudomonas spp. are key components of the natural biological control that operates in take-all-suppressive soils in Washington State. This study also demonstrates that the total amount of Phl produced on roots of wheat by P. fluorescens strain Q2-87, at densities ranging from approximately 10(5) to 10(7) CFU/g of root, is proportional to its rhizosphere population density and that Phl production per population unit is a constant (0.62 ng/10(5) CFU). Thus, Phl production in the rhizosphere of wheat is strongly related to the ability of the introduced strain to colonize the roots.

John Rishbeth, 10 July 1918 - 1 June 1991

Plant pathology is about diseases of plants caused by infectious agents such as viroids, mycoplasmas, viruses, bacteria and fungi and, to a far less extent, by non-infectious agents such as aerial pollutants. Almost always, the diseases studied have been and are of economically important higher plants, and in contrast to medical and veterinary pathology, plant pathology covers all aspects of disease including control. Its scope, therefore, is unusually large, even for a biological discipline, ranging from the molecular, biochemical and physiological analyses of interactions between plants and the disease-causing agents, at sub-cellular and cellular levels through intermediate stages to the study of interactions at whole organism levels in populations of crop or wild plants. These latter were John Rishbeth's main interests whereas mine were at the other end of the scale. However, I did have an interest, albeit secondary, in biological control of plant diseases, a field in which John became an international authority. So I followed closely his work on biological control of soil-borne pathogens and also his studies of the biology and ecology of these pathogens upon which his methods of control were so firmly based. Thus I accepted with great pleasure the invitation to write about John Rishbeth despite the fact that I am far removed from forest pathology and thus, perhaps, less well placed than others to pay full tribute to his outstanding research in this field.

Changes in populations of soil microorganisms, nematodes, and enzyme activity associated with application of powdered pine bark

Evaluation of enzyme activities in combination with taxonomic analyses may help define the mechanisms involved in microbial decomposition of orgaic amendments and biological control of soilborne pathogens. In this study, powdered pine bark was added to nematode-infested soil at rates of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 g kg−1. Total fungal populations did not differ among treatments immediately after application of pine bark. After 7 days, fungal populations were positively correlated with increasing levels of pine bark. This increase was sustained through 14 and 21 days.Penicillium chrysogenum andPaecilomves variotii were the predominant fungal species isolated from soil amended with pine bark. Total bacterial populations did not change with addition of pine bark at 0, 7, and 14 days after treatment. At 21 and 63 days, total bacterial populations declined in soil receiving the highest rates of pine bark. Addition of pine bark powder to soil caused a shift in predominant bacterial genera fromBacillus spp. in nonamended soil, toPseudomonas spp. in amended soil. Soil enzyme activities were positively correlated with pine bark rate at all sampling times. Trehalase activity was positively correlated with total fungal populations and with predominant fungal species, but was not related to bacterial populations. The number of non-parasitic (non-stylet bearing) nematodes andMeloidogyne arenaria in soil and roots were not correlated with pine bark rate. However,Heterodera glycines juveniles in roots, and the number of cysts g−1 root, declined with increasing levels of pine bark.

INFLUENCE OF TRICHODERMA ON THE GROWTH OF BEDDING PLANTS

Trichoderm a spp. are currently being investigated for biological control of soil-borne pathogens and their potential to enhance plant growth and development. The influence of T. harzianum and T. hamatum on growth of 7 bedding plant species was Investigated. Trichoderm a formulated in peat moss and wheat bran, was mixed into germination and growing media at 1 × 106 cfu per gram of medium. Seeds were germinated in plugs and later grown in cellpacks containing a treated and non-treated medium until market stage. Plants were evaluated by measuring height, fresh and dry weight, and number and timing of flowering. Growth enhancement was found in marigold (14.8% dw), petunia (15.5% dw) and tomato (38.2% dw), however, no significant differences were seen in celosia, impatiens, salvi a and vinca. Results suggest that growth enhancement by Trichoderm a is species dependent and that Trichoderm a applied in the plug mix remains-effective through marketing stage.

Biological Control of Soilborne Plant Pathogens in the Rhizosphere with Bacteria

Biological control of soilborne pathogens by introduced microorganisms has been studied for over 65 years (9, 49), but during most of that time it has not been considered commercially feasible. Since about 1 965, however, interest and research in this area have increased steadily (9), as reflected by the number of books (10, 47,49, 152) and reviews about it (11,26,30, 106, 143, 153, 173, 174, 183) that have appeared . Concurrently, there has been a shift to the opinion that biological control can have an important role in agriculture in the future, and it is encouraging that several companies now have programs to develop biocontrol agents as commercial products. This renewed interest in biocontrol is in part a response to public concern about hazards associated with chemical pesticides. Microorganisms that can grow in the rhizosphere are ideal for use as biocontrol agents, since the rhizosphere provides the front-line defense for roots against attack by pathogens. Pathogens encounter antagonism from rhizosphere microorganisms before and during primary infection and also during secondary spread on the root. In some soils described as microbiologi­ cally suppressive to pathogens (172), microbial antagonism of the pathogen is especially great, leading to substantial disease control. Although pathogen­ suppressive soils are rare, those identified are excellent examples of the full potential of biological control of soilborne pathogens.

The mechanisms of biological control of plant diseases

Examples of biological control of plant diseases are analyzed to identify the mechanisms by which such control is effected. The evidence supporting the hypothesis that metabolites produced by one organism may inhibit another is presented. The fact that the formation of such a toxic material is an adaptive phenomenon in response to the presence of the inhibited organism is of considerable significance. Failure to compete successfully for suitable substrates is a form of biological control, that probably ranks second in importance. Lysis appears not to be a primary mechanism of microbial antagonism in the soil. Biological control of soil-borne pathogens is an important factor in disease control in nature. Its exploitation requires sustained and imaginative effort.

Biological control of soil-borne pathogens of wheat.

Biological control of soil-borne pathogens of wheat.