Effect of foliar application of antibiotics and gibberellic acid on the rhizosphere microflora of pea,infected withVerticillium dahliae

A study was made of the effects of foliar spray of bacitracin, chloramphenicol and gibberellic acid on the rhizosphere microflora of pea seedlings (Pisum sativum L.) infected withVerticillium dahliae. The antibiotics increased fungus and actinomycete counts and reduced the bacterial populations in the rhizosphere. Gibberellic acid at 10 ppm concentration reduced all three groups of microorganisms while at 100 ppm fungi and actinomycetes increased slightly. Invariably the rhizosphere effect was as follows: bacteria > fungi > actinomycetes. Foliar sprays also affected the percentage occurrence of particular genera of fungi in the rhizosphere; for example,Trichoderma spp. were stimulated by all the treatments, the maximum being with 10 ppm gibberellic acid, even though the total fungus count was reduced. The disease severity was markedly reduced by foliar sprays.     

General microflora,arbuscular mycorrhizal colonization and occurrence of endophytes in the rhizosphere of two age groups of <Emphasis Type="Italic">Ginkgo biloba</Emphasis> L.of Indian Central Himalaya

The populations of the general microflora (bacteria, actinomycetes and fungi) in the rhizosphere and their corresponding non-rhizosphere soil samples of Ginkgo biloba L. of two age groups (Group A, <25 years-young trees; Group B, >60 years-old trees) growing under a temperate location of Indian Himalayan Region (IHR) have been determined. Observations were also made for the diversity, distribution and colonization of arbuscular mycorrhizal (AM) fungi and occurrence of endophytes in roots of G. biloba. The population of general microflora was found to be higher in the rhizosphere of Group B trees, more clearly reflected in terms of rhizosphere: soil (R:S) ratios. Contrary to this, per cent colonization and spore densities of AM fungi were higher in the rhizosphere of Group A trees as compared to the rhizosphere of Group B. AM fungal colonization was observed mostly in form of loose coils. All the spores detected, belonged to the genus Glomus with five different types. Presence of endophytes (both bacteria and fungi) was observed in the cortical cells of G. biloba roots, more profound in case of Group B trees. Data suggest that, while the species of Glomus dominated the rhizosphere of G. biloba, an inverse correlation exist between the colonization of general microflora and the colonization of AM fungi including endophytes.     

Changes of tomato rhizosphere microflora following application of the herbicide diphenamid to soil infested with Fusarium oxyspomm f.sp. lycopersici

Significant variations were detected in species composition between untreated rhizosphere and nonrhizosphere soils of tomato plants. Application of different concentrations of active ingredient of the herbicide diphenamid (5–250 ppm) to these soils caused significant alterations in species assemblages as compared with untreated soils. Also variations in species composition were denoted between treated rhizosphere and non-rhizosphere soils.Diphenamid concentrations of 10–100 ppm significantly affected microbial counts in soil and rhizosphere of tomato plants. Counts have been stimulated at diphenamid concentrations ranging from 10–50 ppm for fungi and 10–100 ppm for bacteria. At concentrations higher than the upper limits of these ranges, R/S values were not significantly affected.The results also indicated that Fusarium oxyspomm f.sp. lycopersici populations were unaffected by diphenamid at the recommended field rate (10 ppm). Above this concentration and within the conditions of the experiment, the pathogen maintained its population at detectable inocula. Population counts of Aspergillus candidus, a species reported to be able to degrade diphenamid, were high in both treated rhizosphere and non-rhizosphere soils.     

Improvements in the physical properties of soil under the influence of the rhizosphere microflora of four different plant species

Summary Finger millet or locally known asragi (Eleusine coracana Gaertn.), sorghum (Sorghum vulgare Pers.), greengram (Phaseolus aureus Roxb.) and soybean (Glycine max L.) plants were raised on sterilized, sterilized and reinoculated with soil microflora and unsterile sandy loam soil in pots for 45 days. Qualitative studies on the edaphosphere microflora indicated the continuation of rhizosphere effect beyond the root surface (rhizosphere) region. Increased microbial population in the sterilized soil was attributed to the effect of sterilization in favour of faster establishment of added microorganisms. In general, steam sterilization had detrimental effects, whereas crop growth had beneficial effects on the soil physical properties. Ragi and greengram were found superior to sorghum and soybean in improving soil structure as evidenced by increased aggregate stability and hydraulic conductivity and decreased dispersion. Soil aggregates of less than 2.00 mm size were found to be increased due to crop growth. The rhizosphere microflora in association with roots of the growing plants is suggested to play a pivotal role in improving soil structure.     

Respiration and microflora of the rhizosphere soil of wheat influenced by foliar application of urea

The numbers of micromycetes and bacteria were investigated with respect to oxygen consumption in the rhizosphere soil of wheat and in non-rhizosphere soil. Plants after foliar application of urea (2 % solution) and non-treated plants were cultivated in degraded chernozem and garden soil in a green-house. Changes in oxygen consumption by the suspensions of rhizosphere and non-rhizosphere soils corresponded to changes in the number of bacteria designated as the rhizosphere effect (R/S). Values of R/S depended on the presence of organic substrates. Changes in oxygen consumption by the soil suspension from the rhizosphere of wheat occurring due to foliar application of urea corresponded to changes in the amount of microflora. The results obtained are discussed with respect to a possible utilization of the data to follow metabolic activity of soils in a natural environment (in situ) determined according to oxygen consumption by a soil suspension, and to assess changes in the microflora of rhizosphere and non-rhizosphere soil.     

Investigations into rhizosphere microflora

Summary Effect of light and dark treatments on rhizosphere and rhizoplane microflora of two crops,viz Triticum aestivum. Linn. andHordeum vulgare Linn. has been investigated. Higher microbial population was recorded from the rhizosphere of plants exposed to continuous light while continuous dark set harboured lesser microflora. The microflora in 12 hours alternate light and dark set exhibited an intermediate condition. Amino acids (both qualitatively and quantitatively) in root extracts were the least in continuous dark and the maximum in continuous light set. The variation in root leakage, physiological and morphological condition of the plants was thought to be the cause for change in microbial activity. A part of the Ph. D. thesis approved by University of Gorakhpur, Gorakhpur, India (1969).     

Biochemical weathering of calcium-bearing minerals by rhizosphere micro-organisms,and its influence on calcium accumulation in trees

Summary The rhizosphere microflora of redcedar was found to have a higher percentage of calcium silicate-solubilizing rhizosphere bacteria than the rhizosphere microflora of white pine of similar age growing in the same locality under identical conditions. Though the species composition of the silicatesolubilizing microbial population varied considerably from winter to spring, the redcedar roots had more silicate-solubilizers than the white pine roots inboth seasons. Since redcedar is known to have a higher calcium content than white pine, these results indicate that calcium accumulation by redcedar and white pine is strongly influenced by the rate at which calcium is released from soil minerals by the weathering action of the trees' rhizosphere micro-organisms. A symbiotic relationship between the trees and the microflora is suggested.     

Comparative identification by fatty acid analysis of soil,rhizosphere, and geocarposphere bacteria of peanut (Arachis hypogaea L.)

Bacterial isolates were collected from the geocarposphere, rhizosphere, and root-free soil of field grown peanut (Arachis hypogaea L.) at three sample dates, and the isolates were identified by analysis of fatty acid methyl-esters to determine if qualitative differences exist among the bacterial microflora of these zones. Five bacterial genera were associated with isolates from soil, while pod and root isolates constituted 16 and 13 genera, respectively, indicating that bacterial diversity was higher in the rhizosphere and geocarposphere than in soil. The dominant (most frequently identified) genus across all three samples dates was Flavobacterium, for pods, Pseudomonas for roots, and Bacillus, for root-free soil. Sixteen bacterial taxa were only isolated from the geocarposphere, 7 only from the rhizosphere, and 5 only from soil. These results show that specific bacterial taxa are preferentially adapted to colonization of the geocarposphere and suggest that the soil, rhizosphere, and geocarposphere constitute three distinct ecological niches. Bacteria which colonize the geocarposphere should be examined as potential biological control agents for pod-invading fungi such as the toxigenic strains of Aspergillus flavus and A. parasiticus.     

Effects of Pseudomonas putida modified to produce phenazine-1-carboxylic acid and 2,4-diacetylphloroglucinol on the microflora of field grown wheat

Pseudomonas putida WCS358r, genetically modified to have improved activity against soil-borne pathogens, was released into the rhizosphere of wheat. Two genetically modified derivatives carried the phzor the phl biosynthetic gene loci and constitutively produced either the antifungal compound phenazine-1-carboxylic acid (PCA) or the antifungal and antibacterial compound 2,4-diacetylphloroglucinol (DAPG). In 1997 and 1998, effects of single introductions of PCA producing derivatives on the indigenous microflora were studied. A transient shift in the composition of the total fungal microflora, determined by amplified ribosomal DNA restiction analysis (ARDRA), was detected. Starting in 1999, effects of repeated introduction of genetically modified microorganisms (GMMs) were studied. Wheat seeds coated with the PCA producer, the DAPG producer, a mixture of the PCA and DAPG producers, or WCS358r, were sown and the densities, composition and activities of the rhizosphere microbial populations were measured. All introduced strains decreased from 10⁷CFU per gram of rhizosphere sample to below the detection limit after harvest of the wheat plants. The phz genes were stably maintained in the PCA producers, and PCA was detected in rhizosphere extracts of plants treated with this strain or with the mixture of the PCA and DAPG producers. The phl genes were also stably maintained in the DAPG producing derivative of WCS358r. Effects of the genetically modified bacteria on the rhizosphere fungi and bacteria were analyzed by using amplified ribosomal DNA restriction analysis. Introduction of the genetically modified bacterial strains caused a transient change in the composition of the rhizosphere microflora. However, introduction of the GMMs did not affect the several soil microbial activities that were investigated in this study. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phyllosphere and rhizosphere microflora of pearl millet with reference to downy mildew incited bySclerospora graminicola (Sacc.) Schroet

Summary Analysis of phyllosphere microflora showed that in the resistant cultivar (PHB-14) there was a significantly higher population of fungi, gram positive and gram negative bacteria, compared to susceptible cultivar (NHB-3) under healthy and diseased situations. The cultivars during earhead stage supported maximum phyllosphere fungal and gram negative bacterial populations.The rhizosphere of the susceptible cultivar under downy mildew influence supported maximum fungal and gram negative bacterial populations and the rhizosphere effect was the highest in downy mildew colonized susceptible NHB-3 at the earhead stage (95 days after planting).Among the fungi isolated Cladosporium dominated the phyllosphere of resistant, susceptible and susceptible cultivar under the influence of downy mildew.Among the rhizosphere microflora the population ofAspergillus sp. was found to be greatest in resistant, susceptible and the susceptible cultivar under the influence of downy mildew.     

Effect of thiram on plant growth and rhizosphere microflora of barley and nodulation in cowpea

Summary Thiram applied as a seed dresser fungicide had no inhibitory effect on seedling height and rhizosphere microflora of barley plants (Hordeum vulgare) and on nodulation of cowpea plants (Vigna catjang).     

Rhizosphere microflora ofCajanus cajan in relation to Fusarium wilt resistance

Summary Studies were made to find out the factors responsible for resistance inCajanus cajan (L.) Millsp. variety C-11-6 against wilt caused byFusarium udum Butl. It was thought to be either due to associated antagonistic microflora in the rhizosphere or due to biochemical constituents in the plant itself. Screening of the rhizosphere isolates revealed the absence of any potent antagonists againstF. udum suggesting that the resistance in C-11-6 variety is not due to antagonistic organisms in the rhizosphere.     

Microbial changes in clover rhizosphere after foliar and soil application of cobalt

Application of cobalt(II) nitrate to the leaves of red clover (Trifolium pratense L.) resulted in a pronounced increase of dry weight and the number of root nodules. Counts of bacteria in the rhizosphere, content of ammonia and production of carbon dioxide m rhizosphere soil were also higher, whereas the content of nitrates decreased Differences m the counts of bacteria, aotmomycetes, Azotobaoter, anaerobic bacteria and cellulose decomposing bacteria m the rhizosphere of control and treated plants were not directly related to the way of application of cobalt. GeneraPenicillium, Fusarium andTrichoderma predominated among fungi. The relative occurrence of pemcillia was higher after the application of cobalt, the incidence of fusaria was lower. The effects of foliar and soil application of cobalt on rhizosphere microflora were not identical.     

Effects of organic amendments to soil on the rhizosphere microflora of antirrhinum infected withVerticillium dahliae Kleb.

Summary Organic (e.g. chitin, green manure, cellulose) amendments to soil induced quantitative and qualitative changes in the rhizosphere microflora of antirrhinum plants infected withVerticillium dahliae Kleb. Whereas reduction in disease severity occurred with chitin and green manure amendments, an increase in disease severity was observed with the application of cellulose. The reduction of the disease severity with chitin and green manure may be correlated with the increased population of actinomycetes in the antirrhinum rhizosphere.     

Development of rhizosphere and rhizoplane microflora of Aristida coerulescens in the Libyan desert

Summary Microflora of rhizosphere soil, rhizoplane and macerated root-portions of Aristida coerulescens, naturally occurring in the Libyan desert, were different in count and isolates, in the different root zones. A rhizosphere effect characteristic of each zone is shown. The root base contained the lowest numbers of microflora (bacteria and fungi) whilst the root tip included the highest counts. Distribution of most of the individual fungal species in the different root zones and root-surfaces is given in text.     

Effects of inorganic amendments (N,P and K) to soil on the rhizosphere microflora of antirrhinum plants infected withVerticillium dahliae Kleb

Summary A study of the inorganic amendments (N, P and K) to soil, and their effect on the rhizosphere microflora, as well as their relation to the control of wilt of antirrhinum plants caused byVerticillium dahliae Kleb. was done. Ammonium sulphate was the only chemical found to be significantly inhibitory toV. dahliae in vitro. Soil amendments (NPK) affected the rhizosphere microorganisms of the antirrhinum plants. Higher concentration of the chemicals were phytotoxic. It was further observed that ammonium sulphate, and the combined chemicals (NPK 25%) in soil delayed the senescence in healthy plants, suggests that chemical fertilisers affected the host plants directly. Addition of ammonium sulphate (0.25%), calcium nitrate (0.25%, 0.5%) combined NPK (0.25%) to soil caused considerable reduction in disease severity. It is assumed that this reduction may be caused by the (1) fungitoxic nature of the chemicali.e. ammonium sulphate, (2) antagonistic environment for the pathogen in the rhizosphere was boostedi.e. where calcium nitrate was added as soil amendments and (3) reduction in disease severity in soil-amended with combined NPK, may be due to the fact that antagonistic actinomycete population was boosted in the rhizosphere.     

Influence of seed and root exudations on the rhizosphere effect inSorghum vulgare andCrotalaria juncea

The influence of seed and root exudations on the rhizosphere effect ofSorghum vulgare andCrotalaria juncea was examined. While the quantitative occurrence of certain morphological and physiological groups of bacteria on the seeds and rhizospheres of the two plant species differed, there were similarities in the occurrence of certain physiological and nutritional groups of bacteria on the seed as well as on the rhizosphere. The occurrence of some genera of fungi on the seeds and in the rhizosphere indicated the influence of both seeds and root exudations on the rhizosphere effect. Increase in root exudations of amino acids and sugars correlated with a concomitant increase in microbial activity in the rhizosphere on the 15th day of growth in both plant species but there was a delayed effect of the exuded chemicals on the rhizosphere microflora after 30 d of plant growth. Based on the Ph.D. thesis of the first author submitted to the University of Agricultural Sciences, Bangalore.     

Studies of the Efficacy of Alfalfa and Reed in the Phytoremediation of Hydrocarbon-Polluted Soil

The efficacy of plants as means of decontaminating hydrocarbon-polluted soil has been studied. Ditch reed (Phragmites australis) and alfalfa (Medicago sativa) markedly intensified processes of pollutant destruction, the effect being particularly pronounced in the case of polycyclic aromatic hydrocarbons. Comparative analysis of microflora in soils (including those devoid of plants and rhizosphere) demonstrated that, in addition to preventing a pollutant-induced decrease in the amount of heterotrophic microorganisms, the plants stimulated their development, significantly increasing the population of degraders. Effects of plants on major physiological groups of soil microorganisms under conditions of pollution were ambiguous. The rhizosphere microbial consortium of alfalfa was less susceptible to effects of pollutants than that of reed.     

Changes in the rhizosphere microflora of maize seedlings by pretreatment of roots with hormones and urea

Summary Effect of pretreatment of root of maize seedlings with gibberellic acid, maleic hydrazide and urea was investigated. Gibberellic acid at 1 ppm stimulated the growth of fungi, bacteria, and actinomycetes in the rhizosphere region of maize seedlings while at 5 ppm concentration population of bacteria and actinomycetes were suppressed. Maleic hydrazide treatment stimulated all the three groups at 5 ppm concentration but at 1 ppm there was increase in population of only fungi and actinomycetes. Urea stimulated the growth of fungi, bacteria, and actinomycetes. The order of predominance of different groups also changed with treatments. It was fungi > bacteria > actinomycetes in case of gibberellic acid at 1 ppm and 5 ppm and maleic hydrazide at 1 ppm; and bacteria > fungi > actinomycetes in case of maleic hydrazide at 5 ppm, urea at 0.1M and in control.     

白马雪山云南黄芪与灰毛康定黄芪根际微生物物种多样性及抗生物膜活性菌株筛选

【背景】细菌生物膜是造成病原菌耐药性增强和持续感染的主要因素,但目前尚无针对抗菌膜的特效药物。特境植物根际微生物可产生大量具有提高宿主免疫功能的活性成分,极具抗生物膜药源开发潜力。【目的】了解滇西北高寒特境白马雪山分布的云南黄芪与灰毛康定黄芪植物根际微生物的物种多样性,并对可培养菌株进行抑菌与抗生物膜活性筛选。【方法】采用宏基因组技术结合传统微生物培养方法,对采自我国云南迪庆藏族自治州德钦县白马雪山的云南黄芪与灰毛康定黄芪的根际微生物进行物种多样性研究,并通过“孔板法”测定其可培养菌株发酵液乙酸乙酯粗浸膏的抗菌、抗生物膜活性。【结果】宏基因组测序结果显示,云南黄芪根际土壤样本中的微生物来自6门7纲8目8科9属10种,其中栖热菌属为优势菌群;灰毛康定黄芪根际土壤样本中的微生物来自6门8纲10目11科14属15种,其中慢生根瘤菌属为优势菌群。通过纯培养共获得145株可培养菌株,包括112株细菌和33株真菌。其中,云南黄芪根际细菌59株,共计16属35种,优势属为假单胞菌属和链霉菌属;根际真菌19株,共计4属5种,优势属为曲霉属;灰毛康定黄芪根际细菌53株,归属于16属29种,优势属为芽孢杆菌属与寡养单胞菌属;根际真菌14株,归属于3属4种,优势属为曲霉属。从不同种水平上选择51株细菌和7株真菌为代表菌株进行抗生素药源评估,发现5株细菌及1株真菌发酵液的乙酸乙酯粗浸膏具有中等至较强的抗革兰阳性菌活性,而且其中4株具有抗MRSA生物膜活性,最终确定了链霉属放线菌Streptomyces fulvissimus KTA1和曲霉属真菌Aspergillus fumigatus YNF5为潜力活性菌株。【结论】首次报道了滇西北地区高寒特境黄芪属植物根际微生物具有较好的物种多样性,而且具有一定的抗生素药用资源开发潜力。本研究对滇西北高寒特境特色植物来源的微生物资源开发利用与保护具有重要的借鉴意义。