Two novel bacterial biosensors for detection of nitrate availability in the rhizosphere

The nitrate-regulated promoter of narG in Escherichia coli was fused to promoterless ice nucleation (inaZ) and green fluorescent protein (GFP) reporter genes to yield the nitrate-responsive gene fusions in plasmids pNice and pNgfp, respectively. While the promoter of narG is normally nitrate responsive only under anaerobic conditions, the L28H-fnr gene was provided in trans to enable nitrate-dependent expression of these reporter gene fusions even under aerobic conditions in both E. coli DH5alpha and Enterobacter cloacae EcCT501R. E. cloacae and E. coli cells containing the fusion plasmid pNice exhibited more than 100-fold-higher ice nucleation activity in cultures amended with 10 mM sodium nitrate than in nitrate-free media. The GFP fluorescence of E. cloacae cells harboring pNgfp was uniform at a given concentration of nitrate and increased about 1,000-fold when nitrate increased from 0 to 1 mM. Measurable induction of ice nucleation in E. cloacae EcCT501R harboring pNice occurred at nitrate concentrations of as low as 0.1 microM, while GFP fluorescence was detected in cells harboring pNgfp at about 10 microM. In the rhizosphere of wild oat (Avena fatua), the whole-cell bioreporter E.cloacae(pNgfp) or E. cloacae(pNice) expressed significantly higher GFP fluorescence or ice nucleation activity when the plants were grown in natural soils amended with nitrate than in unamended natural soils. Significantly lower nitrate abundance was detected by the E. cloacae(pNgfp) reporter in the A. fatua rhizosphere compared to in bulk soil, indicating plant competition for nitrate. Ice- and GFP-based bacterial sensors thus are useful for estimating nitrate availability in relevant microbial niches in natural environments.     

Lettuce cultivar mediates both phyllosphere and rhizosphere activity of Escherichia coli O157:H7

Plant roots and leaves can be colonized by human pathogenic bacteria, and accordingly some of the largest outbreaks of foodborne illness have been associated with salad leaves contaminated by E. coli O157. Integrated disease management strategies often exploit cultivar resistance to provide a level of protection from economically important plant pathogens; however, there is limited evidence of whether the genotype of the plant can also influence the extent of E. coli O157 colonization. To determine cultivar-specific effects on colonization by E. coli O157, we used 12 different cultivars of lettuce inoculated with a chromosomally lux-marked strain of E. coli O157:H7. Lettuce seedlings grown gnotobiotically in vitro did exhibit a differential cultivar-specific response to E. coli O157 colonization, although importantly there was no relationship between metabolic activity (measured as bioluminescence) and cell numbers. Metabolic activity was highest and lowest on the cultivars Vaila-winter gem and Dazzle respectively, and much higher in endophytic and tightly bound cells than in epiphytic and loosely bound cells. The cultivar effect was also evident in the rhizosphere of plants grown in compost, which suggests that cultivar-specific root exudate influences E. coli O157 activity. However, the influence of cultivar in the rhizosphere was the opposite to that in the phyllosphere, and the higher number and activity of E. coli O157 cells in the rhizosphere may be a consequence of them not being able to gain entry to the plant as effectively. If metabolic activity in the phyllosphere corresponds to a more prepared state of infectivity during human consumption, leaf internalization of E. coli O157 may pose more of a public health risk than leaf surface contamination alone.     

Genetic and Phenotypic Diversity of Bacillus polymyxa in Soil and in the Wheat Rhizosphere

Diversity among 130 strains of Bacillus polymyxa was studied; the bacteria were isolated by immunotrapping from nonrhizosphere soil (32 strains), rhizosphere soil (38 strains), and the rhizoplane (60 strains) of wheat plantlets growing in a growth chamber. The strains were characterized phenotypically by 63 auxanographic (API 50 CHB and API 20B strips) and morphological features, serologically by an enzyme-linked immunosorbent assay, and genetically by restriction fragment length polymorphism (RFLP) profiles of total DNA in combination with hybridization patterns obtained with an rRNA gene probe. Cluster analysis of phenotypic characters by the unweighted pair group method with averages indicated four groups at a similarity level of 93%. Clustering of B. polymyxa strains from the various fractions showed that the strains isolated from nonrhizosphere soil fell into two groups (I and II), while the third group (III) mainly comprised strains isolated from rhizosphere soil. The last group (IV) included strains isolated exclusively from the rhizoplane. Strains belonging to a particular group exhibited a similarity level of 96%. Serological properties revealed a higher variability among strains isolated from nonrhizosphere and rhizosphere soil than among rhizoplane strains. RFLP patterns also revealed a greater genetic diversity among strains isolated from nonrhizosphere and rhizosphere soil and therefore could not be clearly grouped. The RFLP patterns of sorbitol-positive strains isolated from the rhizoplane were identical. These results indicate that diversity within populations of B. polymyxa isolated from nonrhizosphere and rhizosphere soil is higher than that of B. polymyxa isolated from the rhizoplane. It therefore appears that wheat roots may select a specific subpopulation from the soil B. polymyxa population.     

玉米根际与非根际解磷细菌的分布特点

植物光合作用产物约有 12 %～ 5 0 %通过根系进入根际土壤中 ,不同的植物 ,同一植物不同的生长发育时期 ,不仅根际分泌物的数量有差异 ,而且分泌物的种类也不同[4 ] 。这些分泌物不仅是微生物很好的培养基 ,而且一些分泌物可能抑制或有利于甚至刺激某些微生物的繁殖 ,从而导致根际微生物种群结构的变化。根际微生物的数量、活性和群落结构及其变化 ,直接影响到植物吸收水分、养分 ,也影响植物对恶劣环境的抵抗能力 ,尤其是与病菌的侵入和感染关系非常密切[6] 。Ｐ是植物最重要的营养元素之一 ,大多数土壤都具有很强的固定Ｐ的能力 ,Ｐ肥的利…     

The cloning and characterization of phage promoters, directing high expression of luciferase in Pseudomonas syringae pv. phaseolicola, allowing single cell and microcolony detection

Regions of DNA containing promoter sequences from a Pseudomonas syringae pv. phaseolicola -specific phage (φ11P) were identified by shotgun cloning into a broad-host-range promoter-probe vector (pQF70). When used in conjunction with the luciferase reporter genes, one of these DNA fragments, 19H, directed gene expression at a level which enabled the subsequent light output (bioluminescence) of single cells of P. syringae pv. phaseolicola to be detected and visualized using a charge-coupled device (CCD). The P. syringae pv. phaseolicola φ11P, 19H and P. aeruginosa φPLS27, HcM promoters gave a 50-fold increase in bioluminescence (maximum relative light output) compared to similar constructs containing other well-characterized promoters, for example, tetracycline. Similar bioluminescent characteristics of the transformed bacterium, were observed during growth with and without antibiotic-selection. When lux ⁺ bacteria were inoculated onto French bean leaf ( Phaseolus vulgaris L.), the resultant secondary halo blight lesions were bioluminescent and during phylloplane colonization by the lux ⁺ bacterium, bioluminescence on leaf surfaces was detected and imaged by the CCD. Use of these newly identified promoters, combined with the greatly increased sensitivity of bioluminescence detection by the CCD, thus provided a new dimension for the study of natural ecological populations during the bacterial colonization of plants.     

Competition between Pseudomonas fluorescens Ag1 and Alcaligenes eutrophus JMP134 (pJP4) during colonization of barley roots

Abstract: To use deliberately released beneficial microorganisms in the rhizosphere, we need a better understanding of the process of root colonization by seed-borne or soil-borne inocula. In this study, we determine the survival of Pseudomonas fluorescens Ag1 and Alcaligenes eutrophus JMP134, their colonization ability as affected by substrates, and the relative importance of migration versus competition for colonization of the root. Ag1 and the 2,4-dichlorophenoxy-acetic acid (2,4-D) degrader JMP134 were inoculated in sterile barley rhizosphere systems. After inoculation of seeds with individual strains, comparable population sizes were established in the rhizosphere as determined by immunofluorescence microscopic total cell counts. Both strains were motile and able to colonize the entire root system without percolating water to stimulate passive transport. Comparing immunofluorescence microscopic cell counts with colony-forming units demonstrated that a subpopulation of A. eutrophus JMP134 closely associated with the root was non-culturable in contrast to the population in rhizosphere soil. Hence, the sole use of culture-dependent methods may give misleading information about the distribution of bacteria in the rhizosphere. Colonization studies with both strains showed that co-inoculation of Ag1 and JMP134 caused a decrease of the population size of JMP134 if 2,4-D was not added to the soil as a specific carbon source for this strain. Thus, competition for limited carbon sources might influence the composition of the bacterial community in the rhizosphere. We also found that the presence of an established inoculum in the soil reduced subsequent root colonization by a seed-inoculated strain, probably by filling available niches, also indicating that competition from other bacteria may be an important factor determining the distribution of seed-borne inocula. This factor may be just as important for the distribution of bacteria as migration.     

A Study of the Breakdown of Organic Phosphates by Micro-organisms from the Root Region of Certain Pasture Grasses

A survey of micro-organisms isolated from the root surface, rhizosphere soil and nonrhizosphere soil of pasture grasses (Ryegrass, S23; Timothy, S50; Cocksfoot, S143) was made to determine their ability to attack phenolphthalein diphosphate, sodium glycerophosphate, sodium phytate, lecithin, ribonucleic and deoxyribonucleic acids. The total numbers of organisms capable of decomposing these compounds were higher on the root surface and in the rhizosphere soil of the grasses when compared with the numbers in nonrhizosphere soil. Occasionally, preferential stimulation of organisms hydrolysing some of the compounds was observed in the root regions. The addition of deoxyribonucleic acid to medium inhibited the numbers of colonies developing in dilution plate counts. This inhibition was generally more pronounced on root surface organisms than on those from nonrhizosphere soil.     

Survival ofPseudomonas syringae pv.lachrymans with cucumber roots

Summary Survival ofPseudomonas syringae pv.lachrymans with seedling cucumber roots, root washings, rhizosphere soil, and nonrhizosphere soil was determined 7–8 days after the soil surface was watered with a cell suspension of the bacterium. Plants were in pots in the green-house and soil was not sterilized. Survival was best with roots and root washings, next best in rhizosphere soil, and poor in nonrhizosphere soil.     

Serological and Ecological Characteristics of a Nodule-Dominant Serotype from an Indigenous Soil Population of Rhizobium leguminosarum bv. trifolii

Although at least 13 antigenically distinct serotypes of Rhizobium leguminosarum bv. trifolii exist in an Abiqua silty clay loam soil, one serotype, AS6, occupies ≥50% of the root nodules formed on field-grown subclover and between 33 and 78% of the nodules formed on five annual clover species grown in the same soil under laboratory conditions. The dominance of subclover nodules by serotype AS6 was reproducible over a 4-year sampling period and throughout the entire 200- by 100-m pasture examined. Serotype AS6 was composed of three antigenically distinct subtypes (AS6-a, AS6-b, and AS6-c). Each subtype contributed about one-third of the AS6 isolates recovered from nodules of field-grown subclover plants and maintained similar population densities in nonrhizosphere and rhizosphere soil. Rhizobia with the AS6 antigenic signature accounted for from 20 to 100% of the soil populations of R. leguminosarum in arable and pasture soils under legumes throughout the state of Oregon. Over a 12-month period, the population densities of the serotype AS6 complex and three minor nodule-occupying serotypes (AG4, AP17, and AS21) were measured in the rhizospheres of field-grown subclover and orchard grass and in nonrhizosphere Abiqua soil. Regardless of season or serotype, the orchard grass rhizosphere effect was minimal, with the ratio between rhizosphere and nonrhizosphere serotype population densities ranging between 2.5 (midsummer) and 10.5 (spring). In contrast, the magnitude of the subclover rhizosphere effect varied seasonally and among serotypes. Between October and December the ratios for all serotypes were similar (12.5 to 25.5). However, in the spring (April and May), the magnitude of the rhizosphere effect varied among the indigenous serotypes (ratios, 10.5 to 442) and for minor nodule-occupying serotypes AS21 (ratio, 442) and AP17 (ratio, 47) was as great as, or even greater than, the magnitude of the rhizosphere effect observed with the AS6 complex (ratio, 65.5).     

Survival and Activity of lux-Marked Aeromonas salmonicida in Seawater

The fish pathogen Aeromonas salmonicida was chromosomally marked with genes encoding bacterial luciferase, luxAB, isolated from Vibrio fischeri, resulting in constitutive luciferase production. During exponential growth in liquid batch culture, luminescence was directly proportional to biomass concentration, and luminometry provided a lower detection limit of approximately 10(sup3) cells ml(sup-1), 1 order of magnitude more sensitive than enzyme-linked immunosorbent assay detection. In sterile seawater at 4(deg)C, lux-marked A. salmonicida entered a dormant, nonculturable state and population activity decreased rapidly. The activity per viable cell, however, increased by day 4, indicating that a proportion of the population remained active and culturable. Putative dormant cells were not resuscitated after the addition of a range of substrates.     

Enhancement of Population Densities of Pseudomonas putida PpG7 in Agricultural Ecosystems by Selective Feeding with the Carbon Source Salicylate

Sodium salicylate (1,000 μg/ml) was delivered through a drip irrigation system to agricultural field soils planted to tomato and infested with Pseudomonas putida PpG7, the host of the salicylate catabolic plasmid NAH7. In nonfumigated soils infested with approximately 10³ CFU of PpG7 per g in the top 30 cm, population densities were increased up to 112-fold within 14 days of the initial application of salicylate compared with the densities in the respective nonamended soils. Mean season-long population densities of PpG7 in the top 30 cm of soil were significantly increased (P < 0.01) from 216 CFU/g in nonamended soils to 1,370 CFU/g in salicylate-amended soils. In the respective rhizosphere soils, mean population densities of PpG7 were significantly increased (P < 0.01) from 92 to 2,066 CFU/cm of root. Soil fumigation interacted (P < 0.01) with salicylate amendment and further increased the mean population densities of PpG7 in nonrhizosphere soil by an additional 5,689 CFU/g of soil. This fumigation effect was not detected in rhizosphere soils. The effect of salicylate in increasing population densities of PpG7 in soil also was affected by inoculum level, field site, and soil depth. Proportionate differences were greater in soils infested with approximately 10³ CFU of PpG7 per g than in comparable soils infested with 10⁵ CFU/g. In low-inoculum soils, increases from salicylate amendments were 26- and 29-fold in rhizosphere and nonrhizosphere soils, respectively, and in high-inoculum soils, the respective increases were 5.6- and 5-fold. No increases of fungi able to utilize salicylate were detected in soils amended with salicylate. However, soil fumigation with metham-sodium significantly reduced (P < 0.01) population densities of fungal salicylate utilizers in rhizosphere and nonrhizosphere soils.     

Mutation of rpiA in Enterobacter cloacae decreases seed and root colonization and biocontrol of damping-off caused by Pythium ultimum on cucumber

Strains of Enterobacter cloacae show promise as biocontrol agents for Pythium ultimum-induced damping-off on cucumber and other crops. E. cloacae A145 is a mini-Tn5 Km transposon mutant of strain 501R3 that was significantly reduced in suppression of damping-off on cucumber caused by P. ultimum. Strain A145 was deficient in colonization of cucumber, sunflower, and wheat seeds and significantly reduced in colonization of corn and cowpea seeds relative to strain 501R3. Populations of strain A145 were also significantly lower than those of strain 501R3 at all sampling times in cucumber, wheat, and sunflower rhizosphere. Populations of strain A145 were not detectable in any rhizosphere after 42 days, while populations of strain 501R3 remained at substantial levels throughout all experiments. Molecular characterization of strain A145 indicated mini-Tn5 Km was inserted in a region of the E. cloacae genome with a high degree of DNA and amino acid sequence similarity to rpiA, which encodes ribose-5-phosphate isomerase. In Escherichia coli, RpiA catalyzes the interconversion of ribose-5-phosphate and ribulose-5-phosphate and is a key enzyme in the pentose phosphate pathway. Ribose-5-phosphate isomerase activity in cell lysates from strain A145 was approximately 3.5% of that from strain 501R3. In addition, strain A145 was a ribose auxotroph, as expected for an rpiA mutant. Introduction of a 1.0-kb DNA fragment containing only the rpiA homologue into strain A145 restored ribose phosphate isomerase activity, prototrophy, seedling colonization, and disease suppression to levels similar to those associated with strain 501R3. Experiments reported here indicate a key role for rpiA and possibly the pentose phosphate pathway in suppression of damping-off and colonization of subterranean portions of plants by E. cloacae.     

Relationship between the number of bacteria added to soil or seeds and their abundance and distribution in the rhizosphere of alfalfa

A study was conducted of the relationship between the density of several bacterial strains introduced into soil or onto seeds and their abundance in the rhizosphere of alfalfa. The abundance of six species in the rhizosphere was directly correlated with the density of bacteria initially added to soil. The density of six species in the rhizosphere of 15-day-old plants also was directly correlated with the density of each strain in nonrhizosphere soil. Tests of seven species added to soil at four inoculum densities showed that bacteria that survived well in the soil attained the highest densities in the rhizosphere and those that survived poorly in the soil were present at the lowest densities in the rhizosphere. Sixteen of 19 bacterial strains added to alfalfa seeds at 10⁷ or 10⁸ cells per g colonized the rhizosphere of 15-day-old plants, but nearly all of the cells were localized in the upper third of the rhizosphere. A study of 12 bacterial strains that failed to colonize the lower part of the rhizosphere if inoculated onto seeds showed that the bacteria colonized the entire rhizosphere of 15-day-old alfalfa plants if initially inoculated throughout the soil. The data suggest that the density of individual bacterial strains in the rhizosphere is dependent on their density in the soil and that seed inoculation only has an effect on the population in the proximal portion of the alfalfa root system.     

Metabolic Status of Bacteria and Fungi in the Rhizosphere of Ponderosa Pine Seedlings

We determined the quantity and metabolic status of bacteria and fungi in rhizosphere and nonrhizosphere soil from microcosms containing ponderosa pine seedlings. Rhizosphere soil was sampled adjacent to coarse, fine, or young roots. The biovolume and metabolic status of bacterial and fungal cells was determined microscopically and converted to total and active biomass values. Cells were considered active if they possessed the ability to reduce the artificial electron acceptor 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride (INT) to visible intracellular deposits of INT formazan. A colorimetric assay of INT formazan production was also used to assess dehydrogenase activity. INT-active microorganisms made up 44 to 55% of the microbial biomass in the soils studied. The proportion of fungal biomass that exhibited INT-reducing activity (40 to 50%) was higher than previous estimates of the active proportion of soil fungi determined by using fluorescein diacetate. Comparison between soils from different root zones revealed that the highest total and INT-active fungal biomass was adjacent to fine mycorrhizal roots, whereas the highest total and active bacterial biomass was adjacent to the young growing root tips. These observations suggest that fungi are enhanced adjacent to the fine roots compared with the nonrhizosphere soil, whereas bacteria are more responsive than fungi to labile carbon inputs in the young root zone. Colorimetric dehydrogenase assays detected gross differences between bulk and rhizosphere soil activity but were unable to detect more subtle differences due to root types. Determination of total and INT-active biomass has increased our understanding of the role of spatial compartmentalization of bacteria and fungi in rhizosphere carbon flow.     

A tripartite microbial reporter gene system for real-time assays of soil nutrient status

Plant-derived carbon is the substrate which drives the rate of microbial assimilation and turnover of nutrients, in particular N and P, within the rhizosphere. To develop a better understanding of rhizosphere dynamics, a tripartite reporter gene system has been developed. We used three lux-marked Pseudomonas fluorescens strains to report on soil (1) assimilable carbon, (2) N-status, and (3) P-status. In vivo studies using soil water, spiked with C, N and P to simulate rhizosphere conditions, showed that the tripartite reporter system can provide real-time assessment of carbon and nutrient status. Good quantitative agreement for bioluminescence output between reference material and soil water samples was found for the C and P reporters. With regard to soil nitrate, the minimum bioavailable concentration was found to be greater than that analytically detectable in soil water. This is the first time that bioavailable soil C, N and P have been quantified using a tripartite reporter gene system.     

Effect of Root Agglutinin on Microbial Activities in the Rhizosphere

A total of 220 bacterial isolates were obtained from pea rhizosphere and nonrhizosphere samples. Of these samples, 100 isolates were chosen randomly to test for their agglutinative reaction against pea root exudate. The percentage of positive agglutination of bacteria isolated from the nonrhizosphere sample was significantly lower than that of bacteria isolated from the rhizosphere sample. Moreover, this agglutinative reaction could not be blocked either by treating the bacterial cells or root exudate with different carbohydrates before they were mixed or by boiling the root exudate first. Bacteria that could be agglutinated by pea root exudate followed the downward growth of the pea root through the soil profile. The greater abilities of such bacteria to colonize the pea rhizosphere were indicated by their higher rhizosphere-colonizing (rhizosphere/nonrhizosphere) ratios, whether the bacteria were added alone or together with nonagglutinating bacteria. However, bacteria did show different agglutinative reactions toward root exudates obtained from different plants.     

本溪山樱根际与非根际解磷细菌群落结构及动态变化

利用选择性培养基,对不同基质中的本溪山樱(Cerasus sachalinensis) 根际与非根际解磷细菌进行了分离、鉴定和分类,分析了3种不同基质中根际与非根际解磷细菌数量和类群的变化.结果表明,从3种不同配比的基质中分离纯化获得的解磷细菌分别属于13个属,以芽孢杆菌属(Bacillus)、假单胞菌属(Pseudomonas)和沙雷铁氏菌属(Serratia)为主.其中添加炉渣的基质最适于解磷细菌的生长与繁殖,其种群数量最高,但类群的多样性指数低于另外两种土壤.本溪山樱不同生育期根际与非根际解磷细菌种群数量不同,新梢停长期根际中定殖的解磷细菌种群最多（共分离到6个菌属）,新梢迅速生长期和落叶期较少,萌芽期最少.根际土壤解磷细菌多样性亦随生育期发生变化,新梢迅速生长期最高,落叶期次之,新梢停长期最低.非根际土壤则有随生育期逐渐减小的趋势.解磷细菌的根际效应较明显     

Fatty acid methyl ester (FAME) profiles as a tool to investigate community structure of two agricultural soils

Soil microbiological parameters may be the earliest predictors of soil quality changes. Recently, molecular techniques such as fatty acid methyl ester (FAME) profiles have been used to characterize soil microbial communities. Fatty acid methyl ester (FAME) from whole soil may be derived from live cells, dead cells, humic materials, as well as plant and root exudates. Our objective was to verify differences in FAME profiles from two agricultural soils with different plants. Soil samples were collected from Ritzville and Palouse silt loams for fatty acid analysis. Soil samples from wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), pea (Pisum sativum L.), jointed goatgrass ( Aegilops cylindrica L.) and downy brome (Bromus tectorum L.) rhizospheres were also collected for fatty acid analysis. Principal component analysis (PCA) of the two soils explained 42% of the variance on PC1, which accounted for Palouse soil. Ritzville soil accounted for 19% of the variance on PC2. Factor analysis showed that rhizosphere microbial communities from various plant species may differ depending on the plant species. Presence of Gram-positive bacteria as identified by a15:0, i15:0, a17:0 and i17:0 peaks were similar between rhizosphere and nonrhizosphere soils. Gram-negative bacteria characterized by short chain hydroxy acids (10:03OH and 12:03OH) as well as cyclopropane acids (cy17:0) were higher in rhizosphere soil than nonrhizosphere. This indicates a possible shift in the bacterial community to more Gram-negative bacteria and fewer Gram-positive bacteria in the rhizospheres of the plants species studied.     

Influence of the zinc hyperaccumulator Thlaspi caerulescens J. & C. Presl. and the nonmetal accumulator Trifolium pratense L. on soil microbial populations

Metal hyperaccumulator plants like Thlaspi caerulescens J. & C. Presl. are used for phytoremediation of contaminated soils. Since little is known about the rhizosphere of hyperaccumulators, the influence of T. caerulescens was compared with the effects of Trifolium pratense L. on soil microbes. High- and low-metal soils were collected near a zinc smelter in Palmerton, Penn. Soil pH was adjusted to 5.8 and 6.8 by the addition of Ca(OH)2. Liming increased bacterial populations and decreased metal toxicity to levels allowing growth of both plants. The effects of the plants on total (culturable) bacteria, total fungi, as well as cadmium- and zinc-resistant populations were assessed in nonrhizosphere and rhizosphere soil. Both plants increased microbial populations in rhizosphere soil compared with nonrhizosphere soil. Microbial populations were higher in soils planted with T. pratense, but higher ratios of metal-resistant bacteria were found in the presence of T. caerulescens. We hypothesize that T. caerutescens acidifies its rhizosphere. Soil acidification in the rhizosphere of T. caerulescens would affect metal uptake by increasing available metals around the roots and consequently, increase the selection for metal-resistant bacteria. Soil acidification may be part of the hyperaccumulation process enhancing metal uptake from soil.     

异丙甲草胺对芹菜根际与非根际生物活性的影响

通过根际袋法土培试验,研究了异丙甲草胺对芹菜根际与非根际土壤酶活性、土壤微生物数量的影响以及异丙甲草胺在根际与非根际土壤中的降解特性.结果表明,异丙甲草胺对土壤过氧化氢酶活性有一定的抑制作用,对土壤脱氢酶活性有激活作用.一般情况下根际土壤酶活性均要大于非根际土壤.异丙甲草胺作用45 d后,芹菜根际土壤细菌、真菌数量大于非根际土壤,根际效应R/S在1.76～2.51之间;异丙甲草胺对土壤放线菌数量影响不大,根际效应不明显.异丙甲草胺在根际土壤与非根际土壤中的降解速率分别为0.0217和0.0176,相应的半衰期分别为31.9和39.4 d.在根际土壤中异丙甲草胺更易降解.