Types, structure and potential for axial water flow in the deepest roots of field-grown cereals

Deep root systems that extend into moist soil can significantly increase plant productivity. Here, the components of soil-grown root systems of wheat, barley and triticale are characterized, and types and water conducting potential of deep roots in the field are assessed. Root system components were characterized in plants grown in soil in PVC tubes, based on their origin and number and the arrangement of xylem tracheary elements (XTE) viewed using fluorescence microscopy. A new nomenclature is proposed. Deep roots were harvested in the field, and root types of the current crop and remnant roots from previous crops were identified by fluorescence and cryo-scanning electron microscopy. Four types of axile (framework) and five types of branch root were distinguished in the three cereals. Six per cent of deep roots were axile roots that originated from the base of the embryo; 94% were branch roots, of which 48% had only two XTE (10 microm diameter), and thus potentially low axial flow. Only 30% of roots in the cores were from the current crop, the remainder being remnants. Selection for more deep-penetrating axile roots and increased vascular capacity of deep branches is of potential benefit. Conventional root-length density measurements should be interpreted and applied cautiously.     

Fluorescent pseudomonads in the rhizosphere of plants and their relation to root exudates

Fluorescent pseudomonads were present in chernozem soil not influenced by plant roots (10(3)-10(4) per g dry soil) in the rhizosphere soil of various plants (10(4)-10(5) per g soil) and on roots (10(3) to 10(7) per g fresh roots), depending on the species and age of the plant. Relative species representation of fluorescent pseudomonads changed on the roots and in the plant rhizosphere as compared with free soil. Pseudomonas fluorescens, representing 60-93% of the population of fluorescent pseudomonads predominated on the roots of all plants investigated. Somewhat different results were obtained in rhizosphere soil. Relatively higher numbers of P. fluorescens were detected in the rhizosphere soil of cucumber and maize, numbers in the rhizosphere soil of wheat were practically the same as in free soil and higher numbers of P. putida were found in the rhizosphere soil of barley. Almost all components contained in the root exudates of the plants studied, including beta-pyrazolylalanine from the root exudates of cucumbers were utilized as carbon and energy sources. Root exudates of wheat and maize were utilized by the strain P. putida K2 with an efficiency of 73-91%, depending on species and age of the plant.     

Root cap removal increases root penetration resistance in maize (Zea mays L)

The root cap assists the passage of the root through soil by means of its slimy mucilage secretion and by the sloughing of its outer cells. The root penetration resistance of decapped primary roots of maize (Zea mays L. cv. Mephisto) was compared with that of intact roots in loose (dry bulk density 1.0 g cm-3; penetration resistance 0.06 MPa) and compact soil (1.4 g cm-3; penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the impedance to root growth. Root elongation rate and diameter were the same for decapped and intact roots when the plants were grown in loose soil. In compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots, whilst the diameter was 30% larger. Root penetration resistances of intact and decapped seminal axis were 0.31 and 0.52 MPa, respectively, when the roots were grown in compacted soil. These results indicated that the presence of a root cap alleviates much of the mechanical impedance to root penetration, and enables roots to grow faster in compacted soils.     

Irrigation differentially impacts populations of indigenous antibiotic-producing pseudomonas spp. in the rhizosphere of wheat

This work determined the impact of irrigation on the seasonal dynamics of populations of Pseudomonas spp. producing the antibiotics phenazine-1-carboxylic acid (Phz(+)) and 2,4-diacetylphloroglucinol (Phl(+)) in the rhizosphere of wheat grown in the low-precipitation zone (150 to 300 mm annually) of the Columbia Plateau of the Inland Pacific Northwest. Population sizes and plant colonization frequencies of Phz(+) and Phl(+) Pseudomonas spp. were determined in winter and spring wheat collected during the growing seasons from 2008 to 2009 from selected commercial dryland and irrigated fields in central Washington State. Only Phz(+) bacteria were detected on dryland winter wheat, with populations ranging from 4.8 to 6.3 log CFU g(-1) of root and rhizosphere colonization frequencies of 67 to 100%. The ranges of population densities of Phl(+) and Phz(+) Pseudomonas spp. recovered from wheat grown under irrigation were similar, but 58 to 100% of root systems were colonized by Phl(+) bacteria whereas only 8 to 50% of plants harbored Phz(+) bacteria. In addition, Phz(+) Pseudomonas spp. were abundant in the rhizosphere of native plant species growing in nonirrigated areas adjacent to the sampled dryland wheat fields. This is the first report that documents the impact of irrigation on indigenous populations of two closely related groups of antibiotic-producing pseudomonads that coinhabit the rhizosphere of an economically important cereal crop. These results demonstrate how crop management practices can influence indigenous populations of antibiotic-producing pseudomonads with the capacity to suppress soilborne diseases of wheat.     

Survival of genetically modified Pseudomonas fluorescens introduced into subtropical soil microcosms

Abstract A genetically modified strain of Pseudomonas fluorescens and its parent showed grossly similar decline rates following introduction into subtropical clay and sandy soils. In unplanted clay soit at pH 6.9 and 25°C, population densities declined progressively from about 10⁸ to 10³ colony forming units (cfu) g⁻¹ dry soil over 75 days, but in unplanted sandy soil the introduced populations could not be detected after 25 days. In clay soil at pH 8.7 or 4.7, or at environmental temperature, decay rates were enhanced as compared to those at pH 6.9 and 25°C. Counts of introduced strains in clay bulk soil and in rhizosphere and rhizoplane of maize suggested that the introduced bacteria competed well with the native bacteria, and colonized the roots at about 10⁶ cfu g⁻¹ dry root at 25°C, over 20 days. However, rhizoplane colonization was lower at environmental temperature. The decay rate of both strains was slower in planted than in unplanted sandy soil. The population densities in the rhizosphere and rhizoplane in the sandy soil were significantly lower than those in the clay soil. Both introduced strains colonized the maize roots in both soils, using seeds coated with bacteria in 1% carboxymethyl cellulose. Introduced cells were localized at different sites along the roots of plants developing in clay soil, with higher densities in the original (near the seeds) and root hair zones as compared to the intermediate zones. No significant difference was observed between the extent of root colonization of the genetically modified strain and its parent.     

Production of the Antibiotic Phenazine-1-Carboxylic Acid by Fluorescent Pseudomonas Species in the Rhizosphere of Wheat

Pseudomonas fluorescens 2-79 and P. aureofaciens 30-84 produce the antibiotic phenazine-1-carboxylic acid and suppress take-all, an important root disease of wheat caused by Gaeumannomyces graminis var. tritici. To determine whether the antibiotic is produced in situ, wheat seeds were treated with strain 2-79 or 30-84 or with phenazine-nonproducing mutants or were left untreated and then were sown in natural or steamed soil in the field or growth chamber. The antibiotic was isolated only from roots of wheat colonized by strain 2-79 or 30-84 in both growth chamber and field studies. No antibiotic was recovered from the roots of seedlings grown from seeds treated with phenazine-nonproducing mutants or left untreated. In natural soils, comparable amounts of antibiotic (27 to 43 ng/g of root with adhering soil) were recovered from roots colonized by strain 2-79 whether or not the pathogen was present. Roots of plants grown in steamed soil yielded larger bacterial populations and more antibiotic than roots from natural soils. In steamed and natural soils, roots from which the antibiotic was recovered had significantly less disease than roots with no antibiotic, indicating that suppression of take-all is related directly to the presence of the antibiotic in the rhizosphere.     

Effect of bacterial polysaccharides on the growth ofGaeumannomyces graminis var.tritici and wheat roots

Agrobacterium sp. and related species which in the soil and in the rhizosphere of wheat accompany the fungus Gaemannomyces graminis var. tritici and cause take-all of the wheat roots produced polysaccharides in pure cultures (glucans, mannoglucans and galactomannoglucans). These polysaccharides were utilized better by the mycelium of G. graminis than glucose and polysaccharides of plant origin that occurred on the surface of wheat roots (the so-called mucigel). At lower concentrations these bacterial polysaccharides stimulated growth of wheat roots, higher concentrations (more than 0.1%) were inhibitory. Bacteria inoculated on the surface of wheat first inhibited and then stimulated the development of the plants and their growth. Changes in the growth rate of wheat, the rhizosphere of which was colonized by bacteria simultaneously with the fungus G. graminis and also some changes in the course of the disease of wheat roots caused by the fungus can be explained by the inhibitory or stimulatory effect of polysaccharides of accompanying bacteria.     

Induced Reporter Gene Activity, Enhanced Stress Resistance, and Competitive Ability of a Genetically Modified Pseudomonas fluorescens Strain Released into a Field Plot Planted with Wheat

The fates of Pseudomonas fluorescens R2fR and its mutant derivative RIWE8, which contains a lacZ reporter gene responsive to wheat root exudate, were compared in a field microplot. Inoculant survival, root colonization, translocation, resistance to stress factors, and reporter gene activity were assessed in bulk and wheat rhizosphere soils. Populations of both strains declined gradually in bulk and wheat rhizosphere soils and on the wheat rhizoplane as determined by specific CFU and immunofluorescence (IF). In samples from both bulk soil and wheat rhizosphere, IF cell counts were up to 3 orders of magnitude greater than the corresponding numbers of CFU after 120 days, indicating the presence of nonculturable inoculant cells. Estimates of RIWE8-specific target DNA molecule numbers in bulk soil samples 3 and 120 days after inoculation by most-probable-number PCR coincided with the corresponding CFU values. Transport of both strains to deeper soil layers was observed by 3 days after introduction into the microplot. Both strains colonized wheat roots similarly, and cells were seen scattered on the surface of 1-month-old wheat seedling roots by immunogold labelling-scanning electron microscopy. On average, reporter gene activity was significantly higher in wheat rhizosphere soil containing RIWE8 cells than in bulk soil or in soils containing R2fR cells. For both strains, resistance to the four stress factors ethanol, high temperature, high osmotic tension, and oxidative stress increased progressively with residence in soil. Cells from the rhizosphere of 11-day-old seedlings showed similar levels of resistance to osmotic and oxidative stresses and enhanced resistance to ethanol and heat as compared to cells from bulk soil. By 37 days, populations of R2fR and RIWE8 in the rhizosphere were significantly more sensitive to osmotic stress than were populations in bulk soil, whereas differences in response to the other stress factors were less evident. Hence, except for the induction of reporter gene expression in strain RIWE8 in the wheat rhizosphere, the data indicated that there were no great differences in the ecological properties in soil between the lacZ-modified and parental strains.     

Microorganisms in the rhizosphere of wheat colonized by the fungusGaeumannomyces graminis var.tritici

The population of microorganisms in wheat rhizosphere changed in the presence of the fungus Gaeumannomyces graminis var. tritici causing the take-all of wheat. In the majority of cases when the soil was artificially contaminated by the fungus, both the number of bacteria in the rhizosphere and the bacteria/fungi ratio temporarily increased. At the beginning bacteria growing in the presence of NH4+ predominated, later bacteria utilizing organic N-substances prevailed. Pseudomonas fluorescens and the related species colonized the rhizosphere and the soil to a greater extent in the presence of G. graminis. The wheat rhizosphere with G. graminis was found to contain a higher level of the slime-producing bacterium Agrobacterium spp.; this microorganism occurred on hyphal surfaces (in hyphosphere) of both G. graminis growing in soil and Mucor spp. Changes in microbial populations in the wheat rhizosphere during the first stage of colonization by G. graminis can be partly explained by a simultaneous rhizosphere colonization by microorganisms which accompany this fungus in soil. In the period of increase in the number of bacteria in rhizosphere a temporary stimulation of wheat growth was observed.     

Detection of recombinant Pseudomonas putida in the wheat rhizosphere by fluorescence in situ hybridization targeting mRNA and rRNA

A method was developed to detect a specific strain of bacteria in wheat root rhizoplane using fluorescence in situ hybridization and confocal microscopy. Probes targeting both 23S rRNA and messenger RNA were used simultaneously to achieve detection of recombinant Pseudomonas putida (TOM20) expressing toluene o-monooxygenase (tom) genes and synthetic phytochelatin (EC20). The probe specific to P. putida 23S rRNA sequences was labeled with Cy3 fluor, and the probe specific to the tom genes was labeled with Alexa647 fluor. Probe specificity was first determined, and hybridization temperature was optimized using three rhizosphere bacteria pure cultures as controls, along with the P. putida TOM20 strain. The probes were highly specific to the respective targets, with minimal non-specific binding. The recombinant strain was inoculated into wheat seedling rhizosphere. Colonization of P. putida TOM20 was confirmed by extraction of root biofilm and growth of colonies on selective agar medium. Confocal microscopy of hybridized root biofilm detected P. putida TOM20 cells emitting both Cy3 and Alexa647 fluorescence signals.     

麦棉套作棉花根际非根际土壤微生物和土壤养分

在麦棉套作栽培模式下,设置不隔根、纱网隔根和塑膜隔根3种麦棉套种方式,研究麦棉套作对棉花根际和非根际土壤微生物数量、活性和土壤养分（全氮、有效磷和速效钾）含量的影响,结果表明：麦棉套作有利于棉花根际与非根际土壤细菌的增殖,盛蕾期不隔根处理棉花根际土壤与非根际土壤细菌数量分别是塑膜隔根处理的2.57和2.81倍.但麦棉套作不利于土壤真菌和放线菌的增殖.细菌在土壤微生物区系中占99.9%.所以,麦棉套作显著提高了棉花土壤微生物数量,同时也增强了微生物活性.麦棉共处期纱网隔根处理棉花土壤全氮、有效磷、速效钾含量显著高于不隔根处理和塑膜隔根处理,证明麦棉套作系统中小麦根系分泌物与脱落物的存在对棉花土壤养分含量的增加有明显的促进作用,即存在种间营养补偿效应.而共处期不隔根处理套作棉土壤养分含量总体上显著低于隔根处理的现象则反映出小麦根系对棉花土壤养分的竞争作用大于其对棉花土壤养分的促进作用.小麦收获后,小麦根系对棉花养分的竞争作用解除,不隔根处理棉花土壤养分含量显著高于塑膜隔根和纱网隔根处理.     

In Situ Localization of Azospirillum brasilense in the Rhizosphere of Wheat with Fluorescently Labeled, rRNA-Targeted Oligonucleotide Probes and Scanning Confocal Laser Microscopy

The colonization of wheat roots by Azospirillum brasilense was used as a model system to evaluate the utility of whole-cell hybridization with fluorescently labeled, rRNA-targeted oligonucleotide probes for the in situ monitoring of rhizosphere microbial communities. Root samples of agar- or soil-grown 10- and 30-day-old wheat seedlings inoculated with different strains of A. brasilense were hybridized with a species-specific probe for A. brasilense, a probe hybridizing to alpha subclass proteobacteria, and a probe specific for the domain Bacteria to identify and localize the target bacteria. After hybridization, about 10 to 25% of the rhizosphere bacteria as visualized with 4(prm1),6-diamidino-2-phenylindole (DAPI) gave sufficient fluorescence signals to be detected with rRNA-targeted probes. Scanning confocal laser microscopy was used to overcome disturbing effects arising from autofluorescence of the object or narrow depth of focus in thick specimens. This technique also allowed high-resolution analysis of the spatial distribution of bacteria in the rhizosphere. Occurrence of cells of A. brasilense Sp7 and Wa3 was restricted to the rhizosphere soil, mainly to the root hair zone. C-forms of A. brasilense were demonstrated to be physiologically active forms in the rhizosphere. Strain Sp245 also was found repeatedly at high density in the interior of root hair cells. In general, the combination of fluorescently labeled oligonucleotide probes and scanning confocal laser microscopy provided a very suitable strategy for detailed studies of rhizosphere microbial ecology.     

Soil physical strength rather than excess ethylene reduces root elongation of Eucalyptus seedlings in mechanically impeded sandy soils

Seedling establishment in heavily compact soils is hampered by poor root growth caused by soil chemical or physical factors. This study aims to determine the role of ethylene in regulating root elongation through mechanically impeded sandy soils using Eucalyptus todtiana F. Muell seedlings. Concentrations of ethephon (1, 10, and 100???M) were added to non-compact soils, and endogenous ethylene production from seedling roots was compared to ethylene production of roots grown in physically compacted field soils (98.6?% sand). The ethylene-inhibitor 3,5-diiodo-4-hydroxybenzoic acid (DIHB) (0.1???M) was included for each treatment to counteract the negative effects of excess ethylene or compact soils on root elongation. Root elongation was reduced in high ethylene soils by 49?% and high bulk density soils by 44?%. Root ethylene production increased ninefold in roots grown in the high ethylene environment (100???M), but decreased 80?% in compact soils. The use of DIHB did not alter root length and produced varying results with respect to ethylene production, suggesting an interaction effect involving high amounts of soil ethylene. While ethylene regulates root growth, the physical strength of sandy soils is the major factor limiting root elongation in mechanically impeded soils.     

External and Internal Root Colonization of Maize by Two Pseudomonas Strains: Enumeration by Enzyme-Linked Immunosorbent Assay (ELISA)

Maize root colonization by two fluorescent Pseudomonas strains M.3.1. and TR335, isolated respectively from maize and tomato roots, were studied in hydroponic conditions. Each bacterium was inoculated separately, and three different colonization areas were studied: nutrient solution, rhizoplane, and endorhizosphere. The two Pseudomonas strains established large rhizosphere populations, and rhizoplane colonization of the entire root system was similar for both strains. However, strain M.3.1. colonized the endorhizosphere more efficiently than strain TR335. Seminal root cuttings from the tip to the seed allowed the assessment of colonization of three different root areas (i.e., root cap and elongation area, root-hair zone, and mature zone). Rhizoplane colonizations of all these three areas by M.3.1. were significantly the same, whereas strain TR335 colonized the rhizoplane of the root cap and elongation area more actively than the root-hair zone and mature zone. Population size of the strain M.3.1. in the internal tissue of these areas was greater than that of strain TR335. Co-inoculations of the two strains indicated a stimulation of the population size of strain M.3.1. regardless of root area studied, whereas population size of strain TR335 remained unchanged. These results demonstrated that external and internal maize root tissues were colonized to a greater extent by a strain isolated from a maize rhizosphere than by one isolated from another rhizosphere. Received: 26 September 1996 / Accepted: 1 November 1996     

Evolution of bacterial communities in the wheat crop rhizosphere

The gap between current average global wheat yields and that achievable through best agronomic management and crop genetics is large. This is notable in intensive wheat rotations which are widely used. Expectations are that this gap can be reduced by manipulating soil processes, especially those that involve microbial ecology. Cross‐year analysis of the soil microbiome in an intensive wheat cropping system revealed that rhizosphere bacteria changed much more than the bulk soil community. Dominant factors influencing populations included binding to roots, plant age, site and planting sequence. We demonstrated evolution of bacterial communities within the field rhizosphere. Early in the season, communities tightly bound to the root were simplest. These increased in diversity with plant age and senescence. Loosely bound communities also increased in diversity from vegetative to reproductive plant stages but were more stable than those tightly bound to roots. Planting sequence and, to a lesser extent, wheat genotype also significantly affected rhizosphere bacteria. Plasticity in the rhizosphere generated from crop root system management and genetics offers promise for manipulating the soil ecology of intense cereal systems. Analyses of soil microbiomes for the purpose of developing agronomic benefit should include roots as well as soil loosely adhered to the roots, and the bulk soil.     

Selection of a plant-bacterium pair as a novel tool for rhizostimulation of polycyclic aromatic hydrocarbon-degrading bacteria

We developed a novel procedure for the selection of a microbe-plant pair for the stable and efficient degradation of naphthalene. Based on the rationale that root exudate is the best nutrient source available in soil, the grass (Lolium multiflorum) cultivar Barmultra was selected because of its abilities to produce a highly branched root system, root deeply, and carry a high population of Pseudomonas spp. bacteria on its roots. Starting with a mixture of total rhizobacteria from grass-like vegetation collected from a heavily polluted site and selecting for stable naphthalene degradation as well as for efficient root colonization, Pseudomonas putida strain PCL1444 was isolated. The strain's ability to degrade naphthalene was shown to be stable in the rhizosphere. Moreover, it had superior root-colonizing properties because, after the inoculation of grass seedlings, it appeared to colonize the root tip up to 100-fold better than the efficient root colonizer Pseudomonas fluorescens WCS365. Strain PCL1444 uses root exudate as the dominant nutrient source because the presence of grass seedlings in soil results in up to a 10-fold increase of PCL1444 cells. Moreover, the root colonized by strain PCL1444 was able to penetrate through an agar layer, resulting in the degradation of naphthalene underneath this layer. In addition, the inoculation of grass seeds or seedlings with PCL1444 protected them against naphthalene phytotoxicity. Finally, this plant-microbe combination appeared able to degrade naphthalene from soil that was heavily polluted with a complex mixture of polycyclic aromatic hydrocarbons. To our knowledge, this is the first time that a naturally occurring bacterium has been selected for the combination of the abilities to degrade a pollutant and colonize plant roots. We suggest that the principle described here, to select a bacterium which combines efficient root colonization with a beneficial activity, also can be used to improve the selection of other more efficient plant-bacterium pairs for beneficial purposes such as biocontrol, biofertilization, and phytostimulation.     

Rhizosphere competitiveness of trichloroethylene-degrading, poplar-colonizing recombinant bacteria

Indigenous bacteria from poplar tree (Populus canadensis var. eugenei 'Imperial Carolina') and southern California shrub rhizospheres, as well as two tree-colonizing Rhizobium strains (ATCC 10320 and ATCC 35645), were engineered to express constitutively and stably toluene o-monooxygenase (TOM) from Burkholderia cepacia G4 by integrating the tom locus into the chromosome. The poplar and Rhizobium recombinant bacteria degraded trichloroethylene at a rate of 0.8 to 2.1 nmol/min/mg of protein and were competitive against the unengineered hosts in wheat and barley rhizospheres for 1 month (colonization occurred at a level of 1.0 x 10(5) to 23 x 10(5) CFU/cm of root). In addition, six of these recombinants colonized poplar roots stably and competitively with populations as large as 79% +/- 12% of all rhizosphere bacteria after 28 days (0.2 x 10(5) to 31 x 10(5) CFU/cm of root). Furthermore, five of the most competitive poplar recombinants (e.g., Pb3-1 and Pb5-1, which were identified as Pseudomonas sp. strain PsK recombinants) retained the ability to express TOM for 29 days as 100% +/- 0% of the recombinants detected in the poplar rhizosphere expressed TOM constitutively.     

Association of the Plant-beneficial Fungus Verticillium lecanii with Soybean Roots and Rhizosphere

Colonization of soybean roots by the biocontrol fungus Verticillium lecanii was studied in vitro and in situ. For in vitro experiments, V. lecanii was applied to soybean root tip explant cultures. Four weeks after inoculation, the fungus grew externally on at least half of the roots (all treatments combined), colonizing 31% to 71% of root length (treatment means). However, when a potato dextrose agar plug was available as a nutrient source for the fungus, root tips inoculated soon after transfer were not colonized by V. lecanii unless Heterodera glycines was present. Scanning electron microscopy of colonized roots from in vitro cultures revealed a close fungus-root association, including fungal penetration of root cells in some specimens. In the greenhouse, soybeans in sandy soil and in loamy sand soil were treated with V. lecanii applied in alginate prills. The fungus was detected at greater depths from the sandy soil than from the loamy sand soil treatment, but fungus population numbers were small and variable in both soils. Root box studies coupled with image processing analysis of the spatial distribution of V. lecanii in sandy soil supported these findings. Verticillium lecanii was detected randomly in the rhizosphere and soil of root boxes, and was rarely extensively distributed. These in vitro and in situ experiments indicate that V. lecanii can grow in association with soybean roots but is a poor colonizer of soybean rhizosphere in the soil environment.     

Effects of Fungal Root Pathogens on the Population Dynamics of Biocontrol Strains of Fluorescent Pseudomonads in the Wheat Rhizosphere

The influences of Gaeumannomyces graminis var. tritici (which causes take-all of wheat), Rhizoctonia solani AG-8 (which causes rhizoctonia root rot of wheat), Pythium irregulare, P. aristosporum, and P. ultimum var. sporangiiferum (which cause pythium root rot of wheat) on the population dynamics of Pseudomonas fluorescens 2-79 and Q72a-80 (bicontrol strains active against take-all and pythium root rot of wheat, respectively) in the wheat rhizosphere were examined. Root infection by either G. graminis var. tritici or R. solani resulted in populations of both bacterial strains that were equal to or significantly larger than their respective populations maintained on roots in the absence of these pathogens. In contrast, the population of strain 2-79 was significantly smaller on roots in the presence of any of the three Pythium species than on noninfected roots and was often below the limits of detection (50 CFU/cm of root) on Pythium-infected roots after 40 days of plant growth. In the presence of either P. aristosporum or P. ultimum var. sporangiiferum, the decline in the population of Q72a-80 was similar to that observed on noninfected roots; however, the population of this strain declined more rapidly on roots infected by P. irregulare than on noninfected roots. Application of metalaxyl (which is selectively inhibitory to Pythium spp.) to soil naturally infestated with Pythium spp. resulted in significantly larger rhizosphere populations of the introduced bacteria over time than on plants grown in the same soil without metalaxyl. It is apparent that root infections by fungal pathogens may either enhance or depress the population of fluorescent pseudomonads introduced for their control, with different strains of pseudomonads reacting differentially to different genera and species of the root pathogens.     

Microbial complexes from apogeotropic roots and from rhizosphere of cycad plants

The microbial complexes of soil, the rhizosphere, and the rhizoplane of the apogeotropic (coralloid) roots of cycad plants were comparatively studied. The aseptically prepared homogenates of the surface-sterilized coralloid roots did not contain bacterial microsymbiont, indicating that it was absent in the root tissues. At the same time, associated bacteria belonging to different taxonomic groups were detected in increasing amounts in the cycad rhizoplane, rhizosphere, and the surrounding soil. The bacterial communities found in the cycad rhizoplane and the surrounding soil were dominated by bacteria from the genus Bacillus. The saprotrophic bacteria and fungi colonizing the cycad rhizosphere and rhizoplane were dominated by microorganisms capable of degrading the plant cell walls. The local degradation of the cell wall was actually observed on the micrographs of the thin sections of cycad roots in the form of channels, through which symbiotic cyanobacterial filaments can penetrate into the cortical parenchyma.