Grazing of protozoa on rhizosphere bacteria alters growth and reproduction of Arabidopsis thaliana

Plant roots are densely colonized by bacteria which form the basis of the rhizosphere bacterial food web with protozoa as most effective predators. We established a well defined laboratory system with Arabidopsis thaliana as model plant allowing to investigate in detail the effect of rhizosphere interactions on plant performance. We used this system to analyse separate and combined effects of natural rhizobacteria and the protozoa Acanthamoeba castellanii on plants.Protozoa and bacteria increased plant growth with the effect of protozoa markedly exceeding that of bacteria only. Arabidopsis immediately responded to the presence of protozoa by increasing carbon but not nitrogen uptake. Later protozoa enhanced plant uptake of nitrogen from organic material and prolonged vegetative growth of Arabidopsis resulting in strongly increased seed production. It is concluded that the immediate plant response was based on changes in rhizosphere signalling inducing increased plant carbon fixation rather than on protozoa-mediated increase in nitrogen availability. The subsequently increased plant nitrogen uptake presumably originated from nitrogen fixed in bacterial biomass made available by protozoan grazing, i.e. the microbial loop in soil. The results suggest that Arabidopsis prepared for the upcoming mobilization of nitrogen by increasing carbon fixation and root carbon allocation which paid-off later by increased nutrient capture and strongly increased plant reproduction.     

Root-induced changes in the rhizosphere: Importance for the mineral nutrition of plants

Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Examples for this are changes in rhizosphere pH in response to the source of nitrogen (NH₄-N versus NO₃-N), and iron and phosphorus deficiency. These pH changes can readily be demonstrated by infiltration of the soil with agar containing a pH indicator. The rhizosphere pH may be as much as 2 units higher or lower than the pH of the bulk soil. Also along the roots distinct differences in rhizosphere pH exist. In response to iron deficiency most plant species in their apical root zones increase the rate of H⁺ net excretion (acidification), the reducing capacity, the rate of Fe^III reduction and iron uptake. Also manganese reduction and uptake is increased several-fold, leading to high manganese concentrations in iron deficient plants. Low-molecular-weight root exudates may enhance mobilization of mineral nutrients in the rhizosphere. In response to iron deficiency, roots of grass species release non-proteinogenic amino acids (?phytosiderophores”?) which dissolve inorganic iron compounds by chelation of Fe^III and also mediate the plasma membrane transport of this chelated iron into the roots. A particular mechanism of mobilization of phosphorus in the rhizosphere exists in white lupin (Lupinus albus L.). In this species, phosphorus deficiency induces the formation of so-called proteoid roots. In these root zones sparingly soluble iron and aluminium phosphates are mobilized by the exudation of chelating substances (probably citrate), net excretion of H⁺ and increase in the reducing capacity. In mixed culture with white lupin, phosphorus uptake per unit root length of wheat (Triticum aestivum L.) plants from a soil low in available P is increased, indicating that wheat can take up phosphorus mobilized in the proteoid root zones of lupin. At the rhizoplane and in the root (root homogenates) of several plant species grown in different soils, of the total number of bacteria less than 1 % are N₂-fixing (diazotrophe) bacteria, mainly Enterobacter and Klebsiella. The proportion of the diazotroph bacteria is higher in the rhizosphere soil. This discrimination of diazotroph bacteria in the rhizosphere is increased with foliar application of combined nitrogen. Inoculation with the diazotroph bacteria Azospirillum increases root length and enhances formation of lateral roots and root hairs similarly as does application of auxin (IAA). Thus rhizosphere bacteria such as Azospirillum may affect mineral nutrition and plant growth indirectly rather than by supply of nitrogen.     

Enhancement in Nutrient Accumulation and Growth of Oil Palm Seedlings Caused by PGPR Under Field Nursery Conditions

Abstract

Plant growth promoting rhizobacteria (PGPR) (e.g., Azospirillum and Bacillus spp.) have been reported to enhance growth and fix N₂ with several nonleguminous crops. These rhizobacteria have the potential to be applied to oil palm seedlings and, consequently, reduce the cost of nitrogenous fertilizer. The rhizobacteria are also known as a bioenhancer for the ability to increase root growth and enhanced water and nutrient absorption by the host plants. An experiment was carried out in the field nursery station, Federal Land Development Authorities (FELDA), Bukit Mendi, Pahang, Malaysia, to observe the effects of PGPR inoculation on enhanced nutrient accumulation and plant growth (tops and roots) of oil palm seedlings under field nursery conditions. The inoculation process showed positive response in enhancing higher accumulation of nitrogen (N), phosphorus (P), and potassium (K) in the plant tissues, enhanced root dry weight and top growth (dry matter and leaf chlorophyll content) of the host plants under field nursery conditions.     

Interactions of Azospirillum spp. in soils: a review

 This review summarizes and discusses the current knowledge and the, as yet, unanswered questions on the interactions of Azospirillum spp. in bulk soil (but not in the rhizosphere). It contains sections on the isolation of these bacteria from tropical to temperate soils, and on their short- and long-term persistence in bulk soil. The interactions of these bacteria with soil particles and minerals such as clay, sand and Ca, and the effect of soil pH, soil redox potential, and the cation exchange capacity of the soil on them is demonstrated. Data is presented on the distribution of Azospirillum spp. in soils, on their production of fibrillar material essential for anchoring the cells to soil particles, on the effects of soil irrigation, and of external soil treatments, and on the effect of soil C and C used in bacterial inoculants on the cells. It shows that root exudates possibly govern bacterial motility in the soil. Finally, the effect of pesticide applications, the relationships with other soil microorganisms such as Bdelovibrio spp., Bradyrhizobium spp., and phages, and the potential use of a community-control model of Azospirillum spp. in soil and in the rhizosphere is suggested. Received: 11 November 1998     

Response of alfalfa (Medicago sativa L.) to single and mixed inoculation with phosphate-solubilizing bacteria and Sinorhizobium meliloti

The objectives of this work were to phenotypically and genetically characterize alfalfa rhizosphere bacteria and to evaluate the effect of single or mixed inoculation upon nodulation and biological nitrogen fixation. Thirty-two strains showed tricalcium phosphate solubilization ability, and two of them caused bigger or equal solubilization halos than the control strain P. putida SP22. The comparison of the 16S ribosomal DNA sequences indicated that these strains are phylogenetically related to Bacillus spp. and Pseudomonas spp. A beneficial effect of both isolates on alfalfa growth was observed in coinoculation assays. Pseudomonas sp. FM7d caused a significant increase in root and shoot dry weight, length, and surface area of roots, number, and symbiotic properties of alfalfa plants. The plants coinoculated with Sinorhizobium meliloti B399 and the Bacillus sp. M7c showed significant increases in the measured parameters. Our results indicating that strains Pseudomonas sp. FM7d and Bacillus sp. M7c can be considered for the formulation of new inoculants.     

Association of Azospirillum brasilense with pearl millet (Pennisetum americanum (L.) Leeke)

Summary Microscopic observations of the root system of pearl millet (Pennisetum americanum (L.) Leeke) var. BJ 104 after surface sterilization and incubation in phosphate malate triphenyl tetrazolium chloride (TTC) revealed extensive colonization by Azospirillum spp. when plants were grown in sterile, partially sterile and field conditions as evidenced by the TTC-reducing property of active cells of the bacterium. Quantitative studies showed the need to standardize the techniques further to ensure more precise monitoring of the bacteria in the rhizosphere, as large numbers of soil bacteria were found capable of growth on specific media, thus interfering with the plate counts. Seed inoculation with A. brasilense increased the mean grain yield of pearl millet under different agroclimatic conditions in India. The mean increase in grain yield due to inoculation over uninoculated controls was also noticed with graded levels of fertilizer nitrogen (urea). Inoculation alone contributed to increased nitrogen uptake of plants with varying levels of fertilizer nitrogen application under sandy loam soil conditions (pH 7.3). The effects of inoculation were more prominent under lower levels of nitrogen than at the higher levels. The root biomass under field conditions was increased with Azospirillum spp. inoculation at 10 and 20 kg N/ha than their corresponding uninoculated controls.     

N2-fixing bacteria in the rhizosphere: Quantification and hormonal effects on root development

The paper summarizes the results of a series of experiments on enumeration of N₂-fixing bacteria (diazotrophs) and hormonal effects of Azospirillum on root development. Numbers of N₂-fixing and N-heterotrophic bacteria were determined on the root (rhizoplane plus “inner” root surface) and in the rhizosphere soil (0–3 mm from the root surface) of Arrhenatherum elatius, other forage grasses and some herbaceous plant species. Pot experiments involved freshly collected soil from an unfertilized grassland area containing its natural population of N₂-fixing bacteria. The MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial population was always lower on the root than in the rhizosphere soil, suggesting that diazotrophs were not selectively advantaged at the root surface. Supply of mineral nitrogen (NH₄NO₃) decreased the proportion of N₂-fixing bacteria at the rhizoplane as well as in the rhizosphere soil. Similar results were obtained when N was supplied via the leaves. The data suggest that N₂-fixing bacteria in the rhizosphere are poor competitors once they loose their competitive advantage of binding dinitrogen. Correspondingly, the increase in the MPN of the diazotrophs found during plant development was interpreted as a result of decreased available combined N in the rhizosphere. The proportion of N₂-fixing bacteria relative to the total number of bacteria was generally below 1%. Considering the potential amount of substrate released from the roots and the substrate requirement of the bacterial population, N₂-fixation was considered insignificant for plant growth under the given conditions. For the investigations on possible beneficial effects on plant development by bacterial hormones, Azospirillum brasilense was chosen because evidence suggests that amongst the soil bacteria releasing hormones, especially IAA, certain strains of this species are more important than other bacteria. Application of A. brasilense Cd (ATCC 29710) onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs. Similar results were obtained after application of IAA. This suggests that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense. The increase in root surface may improve acquisition of nutrients and enhance growth of plants. Another hormonal effect of A. brasilense was an increase in nodulation of Medicago sativa grown on agar. Again pure IAA resulted in a similar increase in nodule number. Increases in nodule number were only in part associated with a change in root morphology. Therefore an effect of IAA on the plant immanent regulation system for nodulation is likely.     

Dual inoculation of pretransplant stage Oryza sativa L. plants with indigenous vesicular-arbuscular mycorrhizal fungi and fluorescent Pseudomonas spp.

Summary This study examined the response of rice (Oryza sativa L.) plants at the pretransplant/nursery stage to inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi and fluorescent Pseudomonas spp., singly or in combination. The VAM fungi and fluorescent Pseudomonas spp. were isolated from the rhizosphere of rice plants. In the plants grown in soil inoculated with fluorescent Pseudomonas spp. alone, I found increases in shoot growth, and in root length and fine roots, and decreases in root growth, and P and N concentrations. In contrast, in the plants colonized by VAM fungi alone, the results were the reverse of those of the pseudomonad treatment. Dual inoculation of soil with VAM fungi and fluorescent Pseudomonas spp. yielded plants with the highest biomass and nutrient acquisition. In contrast, the plants of the control treatment had the lowest biomass and nutrient levels. The dual-inoculated plants had intermediate root and specific root lengths. The precentages of mycorrhizal colonization and colonized root lengths were significantly lower in the dual-inoculated treatment than the VAM fungal treatment. Inoculation of plants with fluorescent Pseudomonas spp. suppressed VAM fungal colonization and apparently reduced photosynthate loss to the mycorrhizal associates, which led to greater biomass and nutrient levels in dual-inoculated plants compared with plants inoculated with VAM fungi alone. Dual inoculation of seedlings with fluorescent Pseudomonas spp. and VAM fungi may be preferable to inoculation with VAM alone and may contribute to the successful establishment of these plants in the field.     

The influence of the root-knot nematode Meloidogyne incognita, the nematicide aldicarb and the nematophagous fungus Pochonia chlamydosporia on heterotrophic bacteria in soil and the rhizosphere

Plant‐pathogenic nematodes are a major cause of crop damage worldwide, the current chemical nematicides cause environmental damage, but alternatives such as biological control are less effective, so further understanding of the relationship between nematodes, nematicides, biological control agents and soil and rhizosphere microorganisms is needed. Microbial populations from roots of cabbage and tomato plants infested with the root‐knot nematode Meloidogyne incognita were compared with those from plants where the nematode was controlled by the nematicide aldicarb, or a nematophagous fungus with biological control potential, Pochonia chlamydosporia. The total numbers of culturable bacteria and fungi in rhizosphere soil were similar in all three treatments for both plants, around 100‐fold more than in control soil in which there were no plants. However, there were clear differences in the catabolic diversity, assessed by Biolog EcoPlate? carbon substrate utilization assays, between microbial populations from unplanted soil and the rhizosphere. In cabbage, a poor host for M. incognita, the rhizosphere population from P. chlamydosporia‐treated plants was distinct from the population from untreated and aldicarb‐treated plants. In tomato, a host susceptible to the nematode, the catabolic diversity of populations from aldicarb‐ and P. chlamydosporia‐treated plants was similar and differed from the untreated, nematode‐infested plants. The genetic diversity of the fast‐growing heterotrophic bacteria in the tomato rhizosphere, indicated by PCR fingerprinting with ERIC primers, was very different in the infested roots, whereas the profiles of isolates from both aldicarb‐ and P. chlamydosporia‐treated roots were similar. Evidently, nematodes have a greater impact on the rhizosphere population of a susceptible host, tomato, than a poor one, cabbage, and nematode‐infested roots are colonized by a different subpopulation of soil microbes from that on plants where infection is controlled, illustrating differences in root morphology and physiology.     

Effects of interactions between arbuscular mycorrhizal fungi and bacteria on the growth of Lotus corniculatus L.: From the perspective of regulating rhizosphere fungal community

Arbuscular mycorrhizal fungi (AMF) provide essential nutrients to crops and are affected by fertilizers. Phosphate-solubilizing bacteria (PSB), nitrogen-fixing bacteria (NFB), and AMF have mutually beneficial relationships with plants, but the effects of their interactions on plant growth by regulating rhizosphere fungal community have not been sufficiently studied. In this study, a greenhouse pot experiment was conducted to investigate the interactions between AMF and bacteria (PSB and NFB) on the growth of Lotus corniculatus L. Specifically, the role of rhizosphere fungal community in the growth of Lotus corniculatus L. was explored using Illumina MiSeq high-throughput sequencing. The results showed that combined inoculation of AMF with PSB and NFB increased plant biomass, plant height, and fungal colonization rate. The richness, complexity, and stability of rhizosphere fungal community also increased after combined inoculation of AMF with PSB and/or NFB, particularly with PSB. In addition, combined inoculation of AMF with PSB and NFB enriched the abundance of beneficial microorganisms, with Chaetomium and Humicola showing the greatest alterations. The structural equation model showed that the interactions of AMF with PSB and NFB promoted plant growth by affecting fungal network structure and soil enzyme activities involved in carbon, nitrogen, and phosphorus cycling. These findings provide evidence for the effects of interactions of AMF with PSB and NFB on rhizosphere fungal community and plant growth.     

Influence of long-term conservation tillage on soil and rhizosphere microorganisms

 In long-term field experiments on sandy loam and loamy sand soils, the influence of conservation and conventional tillage on soil and rhizosphere microorganisms was studied. Conservation tillage stimulated rhizosphere bacteria on winter wheat, winter barley, winter rye and maize in different soil layers. Particularly the populations of Agrobacterium spp. and Pseudomonas spp. were increased. On the sandy loam, N₂ fixation and nodulation of pea plants were significantly increased. No influence of different soil tillage was determined on the colonization of the rhizosphere by mycorrhiza and saprophytic fungi. Stubble residues infected with Gaeumanomyces graminis were infectious for a longer time on the soil surface than after incorporation into the soil. Received: 10 March 1998     

Responses by iron-efficient and inefficient oat cultivars to inoculation with siderophore-producing bacteria in a calcareous soil

Rhizosphere bacteria may enhance plant uptake of Fe by producing siderophores that chelate sparingly soluble Fe³⁺ in calcareous soils. To evaluate the extent to which plants benefit from colonization of the roots by prolific siderophore-producing bacteria, we inoculated two oat cultivars with six strains of bacteria that produced high concentrations of siderophores under Felimiting conditions in vitro. Oat cv Coker 227, an Fe-efficient cultivar, which produces the phytosiderophore avenic acid, and cv TAM 0-312, and Fe-inefficient cultivar, which does not produce the phytosiderophore, were grown in a calcareous soil (Weswood silt loam) on a light bench in the laboratory. Half of the plants were fertilized with a nutrient solution containing 5 mM Fe and half with a nutrient solution containing no Fe. After 6 weeks of growth, we compared colonization of the roots by the inoculant bacteria and the dry weight and Fe content of roots and shoots. Three species of Pseudomonas colonized the roots of both oat cultivars in high numbers (10⁶ cells g^-1 root dry weight), whereas the remaining bacteria colonized the roots in substantially lower numbers (10⁴ cells g^-1 root dry weight). Plants fertilized with 5 mM Fe were larger and supported greater numbers or rhizosphere bacteria per gram of root than plants not supplied with Fe. Comparisons of the Fe content and dry weight of roots and shoots revealed few significant differences between inoculated and uninoculated plants, or among the plants inoculated with the different strains of siderophore-producing bacteria. The differences that were observed revealed no consistent response to inoculation. We conclude that inoculation of the roots of the two oat cultivars with bacteria that produce high concentrations of siderophores in response to an Fe deficiency had little or no effect on Fe acquisition by the plants.     

Effects on yield and nutrition of mycorrhizal and nodulated Pueraria phaseoloides exerted by P-solubilizing rhizobacteria

We studied the effect of bacteria involved in rock phosphate (four isolates), iron phosphate (two isolates), and aluminium phosphate (two isolates) solubilization, and two phytate-mineralizing bacteria in terms of their interaction with two Glomus spp. on Pueraria phaseoloides growth and nutrition. The plant —Rhizobium sp. — mucorrhiza symbiosis system may increase in yield and nutrition in association with specific rhizosphere bacteria that solubilize calcium, iron, and aluminium phosphates. No benefit from phytate-mineralizing bacteria was found under these experimental conditions. P. phaseloides growth responses were influenced in different ways by specific combinations of the selected bacteria and arbuscular mycorrhizal fungi. Considerable stimulation of nutrient uptake was observed with fungus-bacteria combinations of Azospirillum sp. 1, Bacillus sp. 1 or Enterobacter (spp. 1 or 2) associated with G. mosseae. The fact that Bacillus sp. 1, a calcium-phosphate solubilizing isolate, positively interacted with G. mosseae and negatively with G. fasciculatum is an indication of specific functional compatibility between the biotic components integrated in the system. From our results, the interactions between bacterial groups able to solubilize specific phosphate and mycorrhizal fungi cannot be interpreted as occurring only via P solubilization mechanisms since no generalized effect was obtained. Iron-phosphate solubilizing microorganisms were more active alone than in dual associations with Glomus sp., but the aluminium-phosphate dissolving isolates positively interacted in mycorrhizal plants. Further work is needed in this area in order to elucidate the mechanisms that affect rhizosphere microorganism interactions. G. mosseae was more effective but less infective than G. fasciculatum in most of the combined treatments.     

Selective influence on populations of rhizosphere or rhizoplane bacteria and actinomycetes by mycorrhizas formed by Glomus fasciculatum

The influence of infection by the vesicular-arbuscular (VA) mycorrhizal fungus Glomus fasciculatum on populations of general taxonomic and functional groups of naturally-occurring rhizosphere and rhizoplane bacteria and actinomycetes associated with roots of sweet corn (Zea mays var. rugosa) and subterranean clover (Trifolium subterraneum L.) was assayed on selective media. Total numbers of bacteria, but not actinomycetes, on the rhizoplane increased on plants with VA mycorrhizas (VAM) compared to plants without VAM. Bacteria and actinomycete populations were not affected quantitatively in the rhizosphere soil of VAM plants. However, VAM affected specific groups of bacteria and actinomycetes in both the rhizosphere soil and rhizosplane. Rhizosphere soil of mycorrhizal plants contained more facultative anaerobic bacteria, had fewer fluorescent pseudomonads, but had the same number of Gram-negative bacteria as non-mycorrhizal plants. Of the actinomycetes assayed, populations of both Streptomyces spp and chitinase-producing actinomycetes decreased in the rhizosphere, but not in the rhizoplane of mycorrhizal plants.Leachates of VAM and non-VAM rhizosphere soil were also compared for the presence or activity of bacteria that could influence sporulation by the root pathogen Phytophthora cinnamomi Rands. Fewer sporangia and zoospores were produced by P. cinnamomi in leachates of rhizosphere soil from VAM plants than from non-VAM plants, suggesting that sporangium-inducing microorganisms had declined or sporangium-inhibitors had increased.Since assays for specific functional groups of microorganisms revealed changes even when total numbers seemed the same, we conclude that the microbial equilibrium had been altered by formation of VA mycorrhizas.     

Nodulation competition among Bradyrhizobium japonicum strains as influenced by rhizosphere bacteria and iron availability

Summary Bacteria isolated from the root zones of field-grown soybean plants [Glycine max (L.) Merr.] were examined in a series of glasshouse experiments for an ability to affect nodulation competition among three strains of Bradyrhizobium japonicum (USDA 31, USDA 110, and USDA 123). Inocula applied at planting contained competing strains of B. japonicum with or without one of eleven isolates of rhizosphere bacteria. Tap-root nodules were harvested 28 days after planting, and nodule occupancies were determined for the bradyrhizobia strains originally applied. Under conditions of low iron availability, five isolates (four Pseudomonas spp. plus one Serratia sp.) caused significant changes in nodule occupancy relative to the corresponding control which was not inoculated with rhizosphere bacteria. During subsequent glasshouse experiments designed to verify and further characterize these effects, three fluorescent Pseudomonas spp. consistently altered nodulation competition among certain combinations of bradyrhizobia strains when the rooting medium did not contain added iron. This alteration typically reflected enhanced nodulation by USDA 110. Two of these isolates produced similar, although less pronounced, effects when ferric hydroxide was added to the rooting medium. The results suggest that certain rhizosphere bacteria, particularly fluorescent Pseudomonas spp., can affect nodulation competition among strains of R. japonicum. An additional implication is that iron availability may be an important factor modifying interactions involving the soybean plant, B. japonicum, and associated microorganisms in the host rhizosphere.Paper No. 10648 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601, USA     

Effect of the introduction of the nitrogen-fixing bacteria <Emphasis Type="Italic">Pseudomonas putida</Emphasis> 23 on the nitrogen balance in soil

The inoculation of red beets with the nitrogen-fixing bacteria Pseudomonas putida 23 increased the activity of the nitrogen fixation in the rhizosphere of the plants grown on meadow soil in the central part of the Oka River floodplain. The yield of the red beets and the uptake by plants of nitrogen from the soil and from the ¹⁵N-labeled nitrogen fertilizer applied on the trial microplot increased significantly. A statistically significant additional fixation of nitrogen from the atmosphere and a positive balance of nitrogen in the soil-plant system without significant changes in the bulk content of the soil nitrogen after the plant growing were found in a greenhouse experiment with the application of P. putida. It can be supposed that the excessive nitrogen determined in this system is related to the incorporation into plants of atmospheric nitrogen fixed in the rhizosphere of the inoculated plants. The application of P. putida 23 makes it possible to decrease the rates of NPK fertilizer by two times without losses in the yield of red beets.     

Improvement of simazine degradation by inoculation of corn and soybean plants with rhizobacteria

We isolated the strains of aerobic bacteria, that were able to use the simazine (2-chloro4,6-bis(ethylamino)-s-triazine) herbicide as a sole source of nitrogen, from the roots of corn plants and the nodules of soybean plants. Partial base sequences of 16S rRNA genes and physiological characteristics of the representative isolates revealed that they belonged to Agrobacterium radiobacter and Bradyrhizobium japonicum, respectively. Inoculation of corn plants with the isolates of A. radiobacter resulted in a faster decrease in the amount of simazine in vermiculite soil. Neither bacteria nor corn plants alone were able to promote the decrease in the amount of simazine. Soybean plants modulated by the isolates of B. japonicum were more sensitive to simazine in a hydroponic medium compared to non-nodulated soybeans, though a significant decrease in the amount of simazine in the media was observed. The results obtained in the present study indicate that the rhizosphere of corn and soybean plants containing root nodules is an appropriate habitat for simazine-degrading bacteria to degrade the herbicide.     

Nitrogen-fixing Bacteria Associated with Graminaceous Roots with Special Reference to Spirillum lipoferum Beijerinck

Methods and results for field estimation of nitrogen fixation in the rhizosphere of grasses and cereals and the problem of choosing a reliable method for estimates based on acetylene reduction tests are reviewed. Growth and nitrogen fixation of Azotobacter spp. and of Spirillum lipoferum in plant rhizospheres are discussed on the basis of their physiology. Special consideration is given to the oxygen sensitivity of the nitrogenase reaction and adaptation to microaerophilic conditions, plant root exudates, plant specificity and genotype, mineral nitrogen and to microbial synergism, antagonism and competition. A better understanding of factors essential for a successful saprophytic competition of S. lipoferum in the root environment could allow utilization of its nitrogen fixing potential in crop production. Examples of growth promoting and ineffective associations of this organism with various graminaceous host plants are given.     

利用微生物防除根寄生杂草列当

根寄生杂草列当(Orobanche spp.)已经严重制约全球许多地区的农业发展,寻找有效防除措施迫在眉睫。由于列当具有特殊生活史且与寄主关系密切,常规防除杂草措施难以达到理想防效。目前,尚无既能有效防除列当又不对寄主造成危害且便于大规模推广应用的列当防除措施。在众多列当防除措施中,微生物防除越来越引起关注和重视。本文对微生物防除列当的国内外研究进展及防除机理进行了综述。目前,列当生防微生物的研究主要集中在镰刀菌(Fusarium spp.)等列当病原菌和根瘤菌(Rhizobium spp.)等列当寄主植物共生菌上。微生物防除列当的机制主要包括两方面:一是通过产生代谢产物直接影响列当的萌发和生长,或通过降解列当种子萌发诱导物质间接影响列当的萌发;二是通过提高寄主植物自身对列当的抗性间接影响列当的寄生和生长。此外,本文还重点介绍了植物土传病害的土壤拮抗微生物防除列当杂草的可行性及研究进展。植物土传病害病原菌和列当均首先通过在地下侵染作物的根系进而危害作物正常生长,而作物抗土传病害的机理也与抗列当的机理类似。因此,存在于土壤中具有防治植物土传病害能力的微生物可能也具有防除根寄生杂草列当的功能。本团队前期试验从植物土传病害的土壤拮抗微生物中筛选到在盆栽试验中能够有效防除向日葵列当(O.cumana Wallr.)和瓜列当(O.aegyptiaca Pers.)的放线菌各1株,分别为淡紫褐链霉菌(Streptomyces enissocaesilis Sveshnikova)和密旋链霉菌(Streptomyces pactum Bhuyan B.K)。其中,密旋链霉菌的菌剂在田间试验中既降低了瓜列当的出土数量又增加了番茄的产量。总之,微生物是防除根寄生杂草列当的一条有效途径。     

Bacterial communities associated with nodal roots of rice plants along with the growth stages: Estimation by PCR-DGGE and sequence analyses

Bacterial communities in rice roots that developed from different nodes and at different growth stages were compared by using polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) analysis of 16S rDNA. Rice root samples were collected at three stages, namely tillering (July 2), maximum tillering (July 21), and ripening (September 12). The bacterial diversity in rice roots was found to increase along with the growth stages of the rice plants as well as the root age from the numbers of DGGE bands. The community structure of the bacteria was also found to change with the growth stages and root age from cluster analysis. Sequence analysis of the DGGE bands indicated that the dominant bacteria associated with rice roots were Gram-negative bacteria, especially β-Proteobacteria irrespective of the growth stages and root age. DGGE bands related to Janthinobacterium agaricidamnosum W1r3T and Clostridium sp. FCB90-3 were ubiquitous in many roots irrespective to the sampling date. Principal component analysis enabled to characterize the DGGE bands related to nitrogen-fixing Azoarcus spp., and Azovibrio sp. BS20-3 in the samples collected on July 2 and on July 21, and the myxobacteria collected on September 12, respectively, as representative bacteria in the bacterial communities. The habitat around older rice roots at every sampling date was more reductive than that around younger rice roots, and the DGGE bands related to Spirochaeta spp. were specific in older roots at every sampling date. Some specific bacteria that were most closely related to the DGGE bands were found from principal component analysis to characterize young and old. roots at each growth stage as follows: aerobes Flavobacterium sp. 90 clone 2 and Janthinobacterium agaricidamnosus W1r3T in young roots and facultative anaerobes Dechloromonas sp. MissR and Anaeromyxobacter dehalogenans 2CP-3 in old nodal roots on July 2, strict anaerobe Geobacter pelophilus Dfr2 and aerobes Nitrosospira sp. Nsp17 and uncultured Nitrospira sp. clone 4-1 in old roots on July 21, and different Clostridium spp. in both young and old roots and Desulfovibrio magneticus RS-1 in old roots on September 12, respectively. A larger number of the closest relatives of anaerobic bacteria grew at the late stage than at the early stages, and in old roots than in younger roots. Thus, the environment of paddy roots was remarkably heterogeneous as a bacterial habitat, where not only the whole root system but also a root may create oxic and anoxic environments.