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.     

Lignification in wheat roots parasitised by Gaeuntannomyces graminis and Phialophora radicicola

Preliminary results suggest a possible relationship between lignin synthesis in wheat roots and the observed interaction between Gaeumannomyces graminis (Sacc.) Arx & Olivier and Phialophora radicicola Cain var. graminicola Deacon when they parasitise wheat roots. It was found that colonisation of wheat roots by P. radicicola resulted in a qualitative change in the lignin of the root, such that the content of the p-hydroxy type of aromatic nucleus was reduced almost to zero. It was also found that some of the metabolic precursors of lignin were inhibitory to the growth of G. graminis in Petri-dish culture. Most inhibitory of these precursors was caffeic acid, which reduced the growth rate of G. graminis by half at a concentration of 37 ppm. It is tentatively suggested that colonisation by P. radicicola results in an increased activity of polyphenol oxidase in the root tissues. This would lead to a more rapid synthesis of caffeic acid, with a depletion of the level of p-coumaric acid, and probably an increase in the levels of ferulic acid and sinapic acid. As well as bringing about a change in the composition of the lignin of the root, as the results show, the possible accumulation of caffeic acid in the root tissues might explain the greater resistance to infection by G. graminis observed in roots colonised first by P. radicicola.     

小麦内生细菌及其对根茎部主要病原真菌的抑制作用

对小麦植株不同生育期、不同器官的内生细菌进行了分离和数量变化分析.结果表明,根、茎、叶及未成熟籽粒等器官中存在大量的内生细菌,鲜组织中平均约含内生细菌5.0×10⁵ CFU·g^-1,其中根系中内生细菌数量达7.8×10⁵ CFU·g^-1,而茎秆、叶片和未成熟籽粒中内生细菌数量分别为4.8×10⁵、3.2×10⁵和2.8×10⁵ CFU·g^-1.内生细菌数量在不同生育期也存在差异,幼苗期平均约为3.1×10⁵ CFU·g^-1、拔节期和灌浆期分别为5.7×10⁵和7.0×10⁵ CFU·g^-1.不同小麦田块之间存在明显差异,长武县一田块植物鲜组织中内生细菌的数量为6.1×10⁵ CFU·g^-1,而大荔县一田块约为3.9×10⁵ CFU·g^-1.试验结果发现,对小麦全蚀病菌具有拮抗作用的内生细菌有51株、对小麦纹枯病菌具有抑制作用的内生细菌有45株.用平板对峙法测定,有71株对两种病原真菌均有拮抗作用,对小麦全蚀病菌抑菌圈直径大于10 mm的有23株,其中来源于根系、茎秆、叶片和籽粒的分别为6株、7株、9株和1株;对小麦纹枯病菌抑菌圈超过10 mm的有20株,其中来源于根系、茎秆、叶片和籽粒的分别为7株、5株、7株和1株,说明从小麦叶片诱捕分离的内生细菌中,对小麦全蚀病菌和纹枯病菌抑菌作用较强的分离株比率最高,其次为茎秆,而根部和未成熟籽粒中比例明显较低.     

禾谷多粘菌( Polymyxa graminis )侵染及传毒体系的研究

在人工气候箱内,以小麦为寄 主建立了专性寄生禾谷多粘菌（Ｐｏｌｙｍｙｘａ ｇｒａｍｉｎｉｓ）的侵染体系,ｇｒａｍｉｎｉｓ能够快速大量繁殖,生活史缩短为１３￣１５ｄ。简化了单孢子堆分离以及病根表面消毒等分离纯化方法,对接种的菌源材料、寄主苗龄、温度、ｐＨ值及营养液成分等影响因素进行了测定,优化完善了Ｐ．ｇｒａｍｉｎｉｓ的砂培条件。建立了针对小麦黄花叶病毒（ｗｈｅａ４ｔ ｙｅｌｌｏｗ ｍｏｓａｉｃ ｖ     

Contribution of phenazine antibiotic biosynthesis to the ecological competence of fluorescent pseudomonads in soil habitats.

Phenazine antibiotics produced by Pseudomonas fluorescens 2-79 and Pseudomonas aureofaciens 30-84, previously shown to be the principal factors enabling these bacteria to suppress take-all of wheat caused by Gaeumannomyces graminis var. tritici, also contribute to the ecological competence of these strains in soil and in the rhizosphere of wheat. Strains 2-79 and 30-84, their Tn5 mutants defective in phenazine production (Phz-), or the mutant strains genetically restored for phenazine production (Phz+) were introduced into Thatuna silt loam (TSL) or TSL amended with G. graminis var. tritici. Soils were planted with three or five successive 20-day plant-harvest cycles of wheat. Population sizes of Phz- derivatives declined more rapidly than did population sizes of the corresponding parental or restored Phz+ strains. Antibiotic biosynthesis was particularly critical to survival of these strains during the fourth and fifth cycles of wheat in the presence of G. graminis var. tritici and during all five cycles of wheat in the absence of take-all. In pasteurized TSL, a Phz- derivative of strain 30-84 colonized the rhizosphere of wheat to the same extent that the parental strain did. The results indicate that production of phenazine antibiotics by strains 2-79 and 30-84 can contribute to the ecological competence of these strains and that the reduced survival of the Phz- strains is due to a diminished ability to compete with the resident microflora.     

Contribution of phenazine antibiotic biosynthesis to the ecological competence of fluorescent pseudomonads in soil habitats.

Phenazine antibiotics produced by Pseudomonas fluorescens 2-79 and Pseudomonas aureofaciens 30-84, previously shown to be the principal factors enabling these bacteria to suppress take-all of wheat caused by Gaeumannomyces graminis var. tritici, also contribute to the ecological competence of these strains in soil and in the rhizosphere of wheat. Strains 2-79 and 30-84, their Tn5 mutants defective in phenazine production (Phz-), or the mutant strains genetically restored for phenazine production (Phz+) were introduced into Thatuna silt loam (TSL) or TSL amended with G. graminis var. tritici. Soils were planted with three or five successive 20-day plant-harvest cycles of wheat. Population sizes of Phz- derivatives declined more rapidly than did population sizes of the corresponding parental or restored Phz+ strains. Antibiotic biosynthesis was particularly critical to survival of these strains during the fourth and fifth cycles of wheat in the presence of G. graminis var. tritici and during all five cycles of wheat in the absence of take-all. In pasteurized TSL, a Phz- derivative of strain 30-84 colonized the rhizosphere of wheat to the same extent that the parental strain did. The results indicate that production of phenazine antibiotics by strains 2-79 and 30-84 can contribute to the ecological competence of these strains and that the reduced survival of the Phz- strains is due to a diminished ability to compete with the resident microflora.     

Micro-organisms in the rhizosphere of plants inoculated withAzotobacter chroococcum

Summary The hypothesis that inoculation withAzotobacter chroococcum affects the growth of plants indirectly through changing the rhizosphere microflora was investigated. Inoculated and uninoculated wheat and tomato plants were grown in the glasshouse in two different soils, and total bacteria, chitinolytic bacteria, actinomycetes, glucosefermenting bacteria, aerobic cellulose-decomposing bacteria, and anaerobes were determined in intervals in the rhizosphere and in the soil. Root-surface fungi were studied using the Harley and Waid's root-washing technique¹⁰. Azotobacter became established in the rhizosphere of wheat and tomato plants and stimulated their growth. All the bacterial groups examined were more abundant in the rhizosphere than in the soil. Inoculation with Azotobacter delayed the colonization of roots by bacteria, actinomycetes, and fungi in the rhizosphere, but had no effect on other organisms. Inoculation did not affect the dominant root-surface fungi, and minor changes were not consistent.Part of a thesis accepted by the University of London for the degree of Ph.D. in Microbiology.     

Relationships between plant roots,proteolytic organisms and activity of protease

Proteolytic bacteria represented 18–58% of the bacterial population isolated from the rhizoplane of different crops. The activity of protease was considerably higher on roots of wheat growing in the soil than in the rhizosphere or free soil. However, only a slightly positive rhizosphere effect in the relative occurrence of casein-hydrolyzing bacteria could be observed. An indirect relationship between numbers of bacteria hydrolyzing casein and the activity of the enzyme could be found. The activity of protease related to a unit of culturable proteolytic bacteria was considerably higher on the root than in the rhizosphere and in the soil. A relationship between characteristics of the production of the enzyme by proteolytic bacteria and the protease activity on the surface of roots was demonstrated. The resulting enzyme activity on the surface of roots depended apparently on growth conditions of the plant and nature of root exudates and was influenced both by inactivation and protection due to adsorption of the enzyme by roots.     

Death of the cereal root cortex: its relevance to biological control of take-all

Cell death in the root cortex of cereals was assessed by an inability to detect nuclei, using acridine orangelfluorescence microscopy after fixation and mild acid hydrolysis. Seminal roots were scanned at x 100 magnification and their cortices were considered dead when nuclei were absent from all cell layers except the innermost one, adjacent to the endodermis; this cell layer remains alive long after the rest of the cortex has died. Cortical death of wheat and barley roots occurred in the absence of major pathogens. Cell death started behind the root hair zone of the main root axis, initially in the outermost cell layer of the cortex and then progressively inwards towards the endodermis; however, the cortex remained alive for a distance of c. 800 μm around emerging root laterals. The rate of cortical death was more rapid in wheat than in barley, both under field conditions and in the glasshouse at 20 °C. Thus, field-grown spring wheat (Sicca) showed 50% death of the root cortex in the top 6 cm of first seminal roots after 35 days (growth stage 1–2), whereas spring barley (Julia) showed 50% death of the root cortex after 67 days (growth stage 8). In the glasshouse, the top 9 cm of first seminal roots on 16-day plants showed 55% cortical death in wheat (Cappelle-Desprez) but only 2.5% cortical death in barley (Igri). The same rates of death were found in all subsequent seminal roots. The wheat root cortex died at the same rate in sterile and unsterile conditions, and at the same rate in the presence/absence of Phialophora radicicola Cain var. graminicola Deacon or Aureobasidium bolleyi (Sprague) von Arx. Hence, although P. radicicola and other soil microorganisms may benefit from root cortex death they do not exert biological control of take-all by enhancing or retarding the rate of this process. To study the effects of cortical death on take-all, Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker was point-inoculated at the tips and on older (5 and 15 day) regions of wheat seminal roots. After 17 days at 20 °C the fungus had grown to the same extent as runner-hyphae in all cases, but the severity of disease decreased with increasing age of the root cortex prior to inoculation; thus, G. graminis caused most extensive vascular discoloration and most intense vascular blockage in roots inoculated at their tips. Similar experiments on wheat and barley roots inoculated separately with P. radicicola and G. graminis suggest that at least three factors associated with cortical death influence infection by these fungi: (1) initially, cell death may enhance infection because nutrients are made available to the parasites and host resistance within the cortex is reduced; (2) weak parasites and soil saprophytes may colonise dead and dying cortices in competition with G. graminis and P. radicicola and thereby reduce infection by these fungi; (3) changes in the endodermis and adjacent cell layers may be associated with cortical death and may retard invasion of the stele. Future work will seek to establish the relative importance of these factors and extend this study to other cereal host-fungus combinations.     

Role of a phenazine antibiotic from Pseudomonas fluorescens in biological control of Gaeumannomyces graminis var. tritici.

Pseudomonas fluorescens 2-79 (NRRL B-15132) and its rifampin-resistant derivative 2-79RN10 are suppressive to take-all, a major root disease of wheat caused by Gaeumannomyces graminis var. tritici. Strain 2-79 produces the antibiotic phenazine-1-carboxylate, which is active in vitro against G. graminis var. tritici and other fungal root pathogens. Mutants defective in phenazine synthesis (Phz-) were generated by Tn5 insertion and then compared with the parental strain to determine the importance of the antibiotic in take-all suppression on wheat roots. Six independent, prototrophic Phz- mutants were noninhibitory to G. graminis var. tritici in vitro and provided significantly less control of take-all than strain 2-79 on wheat seedlings. Antibiotic synthesis, fungal inhibition in vitro, and suppression of take-all on wheat were coordinately restored in two mutants complemented with cloned DNA from a 2-79 genomic library. These mutants contained Tn5 insertions in adjacent EcoRI fragments in the 2-79 genome, and the restriction maps of the region flanking the insertions and the complementary DNA were colinear. These results indicate that sequences required for phenazine production were present in the cloned DNA and support the importance of the phenazine antibiotic in disease suppression in the rhizosphere.     

Colonization of barley roots by endophytic fungi and their reduction of take-all caused by Gaeumannomyces graminis var. tritici

Fungal root endophytes obtained from natural vegetation were tested for antifungal activity in dual culture tests against the root pathogen Gaeumannomyces graminis var. tritici. Fifteen isolates, including Acremonium blochii, Acremonium furcatum, Aspergillus fumigatus, Cylindrocarpon sp., Cylindrocarpon destructans, Dactylaria sp., Fusarium equiseti, Phoma herbarum, Phoma leveillei, and a sterile mycelium, selected based on the dual culture test, were inoculated on barley roots in growth tubes under axenic conditions, both in the absence and presence of G. graminis var. tritici. All isolates colonized the rhizosphere and very often the root cortex without causing disease symptoms and without affecting plant growth. Eight isolates significantly reduced the symptoms caused by G. graminis var. tritici, and 6 of them reduced its presence in the roots.     

Root mucilage from pea and its utilization by rhizosphere bacteria as a sole carbon source

Plant roots secrete a complex polysaccharide mucilage that may provide a significant source of carbon for microbes that colonize the rhizosphere. High molecular weight mucilage was separated by high-pressure liquid chromatography gel filtration from low molecular weight components of pea root exudate. Purified pea root mucilage generally was similar in sugar and glycosidic linkage composition to mucilage from cowpea, wheat, rice, and maize, but appeared to contain an unusually high amount of material that was similar to arabinogalactan protein. Purified pea mucilage was used as the sole carbon source for growth of several pea rhizosphere bacteria, including Rhizobium leguminosarum 8401 and 4292, Burkholderia cepacia AMMD, and Pseudomonas fluorescens PRA25. These species grew on mucilage to cell densities of three- to 25-fold higher than controls with no added carbon source, with cell densities of 1 to 15% of those obtained on an equal weight of glucose. Micromolar concentrations of nod gene-inducing flavonoids specifically stimulated mucilage-dependent growth of R. leguminosarum 8401 to levels almost equaling the glucose controls. R. leguminosarum 8401 was able to hydrolyze p-nitrophenyl glycosides of various sugars and partially utilize a number of purified plant polysaccharides as sole carbon sources, indicating that R. leguminosarum 8401 can make an unexpected variety of carbohydrases, in accordance with its ability to extensively utilize pea root mucilage.     

Numbers and locations of native bacteria on field-grown wheat roots quantified by fluorescence in situ hybridization (FISH)

Native bacteria, Pseudomonas and filamentous bacteria were quantified and localized on wheat roots grown in the field using fluorescence in situ hybridization (FISH). Seminal roots were sampled through the season from unploughed soil in a conservation farming system. Such soils are spatially heterogeneous, and many roots grow slowly through hard soil with cracks and pores containing dead roots remnant from previous crops. Root and rhizosphere morphology, and contact with soil particles were preserved, and autofluorescence was avoided by observing sections in the far-red with Cy5 and Cy5.5 fluorochromes. Spatial analyses showed that bacteria were embedded in a stable matrix (biofilm) within 11 microm of the root surface (range 2-30 microm) and were clustered on 40% of roots. Half the clusters co-located with axial grooves between epidermal cells, soil particles, cap cells or root hairs; the other half were not associated with visible features. Across all wheat roots, although variable, bacteria averaged 15.4 x 10(5) cells per mm(3) rhizosphere, and of these, Pseudomonas and filaments comprised 10% and 4%, respectively, with minor effects of sample time, and no effect of plant age. Root caps were most heavily colonized by bacteria along roots, and elongation zones least heavily colonized. Pseudomonas varied little with root development and were 17% of bacteria on the elongation zone. Filamentous bacteria were not found on the elongation zone. The most significant factor to rhizosphere populations along a wheat root, however, was contact with dead root remnants, where Pseudomonas were reduced but filaments increased to 57% of bacteria (P < 0.001). This corresponded with analyses of root remnants showing they were heavily colonized by bacteria, with 48% filaments (P < 0.001) and 1.4%Pseudomonas (P = 0.014). Efforts to manage rhizosphere bacteria for sustainable agricultural systems should continue to focus on root cap and mucilage chemistry, and remnant roots as sources of beneficial bacteria.     

Gluconic acid: an antifungal agent produced by Pseudomonas species in biological control of take-all

Pseudomonas strain AN5 (Ps. str. AN5), a non-fluorescent Australian bacterial isolate, is an effective biological control (biocontrol) agent of the take-all disease of wheat caused by the fungus Gaeumannomyces graminis var. tritici (Ggt). Ps. str. AN5 controls Ggt by producing an antifungal compound which was purified by thin layer and column chromatography, and identified by NMR and mass spectroscopic analysis to be d-gluconic acid. Commercially bought pure gluconic acid strongly inhibited Ggt. Two different transposon mutants of Ps. str. AN5 which had lost take-all biocontrol did not produce d-gluconic acid. Gluconic acid production was restored, along with take-all biocontrol, when one of these transposon mutants was complemented with the corresponding open reading frame from wild-type genomic DNA. Gluconic acid was detected in the rhizosphere of wheat roots treated with the wild-type Ps. str. AN5, but not in untreated wheat or wheat treated with a transposon mutant strain which had lost biocontrol. The antifungal compounds phenazine-1-carboxylic acid and 2,4-diacetylphloroglucinol, produced by other Pseudomonads and previously shown to be effective in suppressing the take-all disease, were not detected in Ps. str. AN5 extracts. These results suggest that d-gluconic acid is the most significant antifungal agent produced by Ps. str. AN5 in biocontrol of take-all on wheat roots.     

Peptidoglycan from Bacillus cereus mediates commensalism with rhizosphere bacteria from the Cytophaga-Flavobacterium group

Previous research in our laboratory revealed that the introduction of Bacillus cereus UW85 can increase the populations of bacteria from the Cytophaga-Flavobacterium (CF) group of the Bacteroidetes phylum in the soybean rhizosphere, suggesting that these rhizosphere microorganisms have a beneficial relationship (G. S. Gilbert, J. L. Parke, M. K. Clayton, and J. Handelsman, Ecology 74:840-854, 1993). In the present study, we determined the frequency at which CF bacteria coisolated with B. cereus strains from the soybean rhizosphere and the mechanism by which B. cereus stimulates the growth of CF rhizosphere strains in root exudate media. In three consecutive years of sampling, CF strains predominated among coisolates obtained with B. cereus isolates from field-grown soybean roots. In root exudate media, the presence of B. cereus was required for CF coisolate strains to reach high population density. However, rhizosphere isolates from the phylum Proteobacteria grew equally well in the presence and absence of B. cereus, and the presence of CF coisolates did not affect the growth of B. cereus. Peptidoglycan isolated from B. cereus cultures stimulated growth of the CF rhizosphere bacterium Flavobacterium johnsoniae, although culture supernatant from B. cereus grown in root exudate media did not. These results suggest B. cereus and CF rhizosphere bacteria have a commensal relationship in which peptidoglycan produced by B. cereus stimulates the growth of CF bacteria.     

Mucoid mutants of the biocontrol strain pseudomonas fluorescens CHA0 show increased ability in biofilm formation on mycorrhizal and nonmycorrhizal carrot roots

Extracellular polysaccharides play an important role in the formation of bacterial biofilms. We tested the biofilm-forming ability of two mutant strains with increased production of acidic extracellular polysaccharides compared with the wild-type biocontrol strain Pseudomonas fluorescens CHA0. The anchoring of bacteria to axenic nonmycorrhizal and mycorrhizal roots as well as on extraradical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices was investigated. The nonmucoid wild-type strain P. fluorescens CHA0 adhered very little on all surfaces, whereas both mucoid strains formed a dense and patchy bacterial layer on the roots and fungal structures. Increased adhesive properties of plant-growth-promoting bacteria may lead to more stable interactions in mixed inocula and the rhizosphere.     

Survival of potentially pathogenic human-associated bacteria in the rhizosphere of hydroponically grown wheat

Plants may serve as reservoirs for human-associated bacteria (H-AB) in long-term space missions containing bioregenerative life support systems. The current study examined the abilities of five human-associated potential pathogens, Pseudomonas aeruginosa, Pseudomonas cepacia, Staphylococcus aureus, Streptococcus pyogenes and Escherichia coli, to colonize and grow in the rhizosphere of hydroponically grown wheat, a candidate crop for life support. All of these bacteria have been recovered from past NASA missions and present potential problems for future missions. The abilities of these organisms to adhere to the roots of axenic five-day-old wheat (Triticum aestivum L. cv. Yecora rojo) were evaluated by enumeration of the attached organisms after a one hour incubation of roots in a suspension (approximately 10(8) cfu ml-1) of the H-AB. Results showed that a greater percentage of P. aeruginosa cells adhered to the wheat roots than the other four H-AB. Similarly incubated seedlings were also grown under attempted axenic conditions for seven days to examine the potential of each organism to proliferate in the rhizosphere (root colonization capacity). P. cepacia and P. aerogiunosa showed considerable growth, E. coli and S. aureus showed no significant growth, and S. pyogenes died off in the wheat rhizosphere. Studies examining the effects of competition on the survival of these microorganisms indicated that P. aeruginosa was the only organism that survived in the rhizosphere of hydroponically grown wheat in the presence of different levels of microbial competition.     

Changes in populations of rhizosphere bacteria associated with take-all disease of wheat

Take-all, caused by Gaeumannomyces graminis var. tritici, is one of the most important fungal diseases of wheat worldwide. Knowing that microbe-based suppression of the disease occurs in monoculture wheat fields following severe outbreaks of take-all, we analyzed the changes in rhizosphere bacterial communities following infection by the take-all pathogen. Several bacterial populations were more abundant on diseased plants than on healthy plants, as indicated by higher counts on a Pseudomonas-selective medium and a higher fluorescence signal in terminal restriction fragment length polymorphism analyses of amplified 16S ribosomal DNA (rDNA). Amplified rDNA restriction analysis (ARDRA) of the most abundant cultured populations showed a shift in dominance from Pseudomonas to Chryseobacterium species in the rhizosphere of diseased plants. Fluorescence-tagged ARDRA of uncultured rhizosphere washes revealed an increase in ribotypes corresponding to several bacterial genera, including those subsequently identified by partial 16S sequencing as belonging to species of alpha-, beta-, and gamma-proteobacteria, sphingobacteria, and flavobacteria. The functional significance of some of these populations was investigated in vitro. Of those isolated, only a small subset of the most abundant Pseudomonas spp. and a phlD(+) Pseudomonas sp. showed any significant ability to inhibit G. graminis var. tritici directly. When cultured strains were mixed with the inhibitory phlD(+) Pseudomonas strain, the Chryseobacterium isolates showed the least capacity to inhibit this antagonist of the pathogen, indicating that increases in Chryseobacterium populations may facilitate the suppression of take-all by 2,4-diacetylphloroglucinol-producing phlD(+) pseudomonads.     

Assessment of arbuscular mycorrhizal fungi diversity in the rhizosphere of <Emphasis Type="Italic">Viola calaminaria</Emphasis> and effect of these fungi on heavy metal uptake by clover

 The ability of arbuscular mycorrhizal (AM) fungi from a metal-tolerant plant (Viola calaminaria, violet) to colonise and reduce metal uptake by a non-tolerant plant (Trifolium subterraneum, subterranean clover) in comparison to a metal-tolerant AM fungus isolated from a non-tolerant plant was studied. AM spores from the violet rhizosphere and from violet roots were characterised by polymerase chain reaction (PCR) amplification of the SSU rDNA, and sequencing. Subterranean clover was grown in pots containing a soil supplemented with Cd and Zn salts and inoculated either with a mixture of spores extracted from the violet rhizosphere or with spores of a Cd-tolerant Glomus mosseae P2 (BEG 69), or non-inoculated. The diversity of fungi, including AM fungi, colonising clover roots was assessed and analysed using terminal-restriction fragment length polymorphism. At least four different Glomus species were found in the violet rhizosphere. After 8 weeks in a growth chamber, colonisation of clover roots with spores from the violet rhizosphere increased Cd and Zn concentrations in clover roots without significantly affecting the concentrations of metals in the shoot and plant growth. G. mosseae P2 reduced plant growth and slightly increased the Cd concentration. Only one AM fungus (Glomus b) from the violet rhizosphere colonised clover roots, but other fungi were present. AM fungi from heavy metal-contaminated soils and associated with metal-tolerant plants may be effective in accumulating heavy metals in roots in a non-toxic form. Accepted: 7 July 2000     

The Importance of the Host Plant on the Interaction Between Root-knot Nematodes Meloidogyne spp. and the Nematophagous Fungus, Verticillium chlamydosporium Goddard

The effect of the host plant on the efficacy of Verticillium chlamydosporium as a biological control agent for root-knot nematodes was investigated in four experiments. The growth of the fungus in the rhizosphere differed significantly with different plant species, the brassicas kale and cabbage supporting the most extensive colonization. The presence of nematodes in roots increased the growth of the fungus on most plants, and this effect was associated with the emergence of egg masses on the root surface; the presence of Meloidogyne incognita did not stimulate growth of the fungus in the rhizosphere until 5 weeks after the addition of infective juveniles to soil. The susceptibility of the plant host to M. incognita attack influenced the numbers of nematode eggs parasitized by the fungus. The control of the nematode was less effective on tomato roots, which produced large galls as a result of nematode infection compared with control on potato roots where galls were smaller, despite the greater abundance of the fungus in the rhizosphere of tomato plants. In large galls, a significant proportion of the egg masses remained embedded in the roots and was isolated from the fungus which was confined to the rhizosphere. Hence, the plant species has a marked effect on the efficacy of V. chlamydosporium as a biological control agent.