Mechanism of induced systemic resistance against anthracnose disease in cucumber by plant growth-promoting fungi

Plant growth-promoting fungi (PGPF) such as Phoma sp. (isolates GS8-1, GS8-2 and GS8-3) and non-sporulating fungus (isolate GU21-2) were tested for their ability to induce systemic resistance against Colletotrichum orbiculare in cucumber. These isolates, used as colonized barley kernels to natural soils, induced systemic resistance in the greenhouse as well as in the field. Different elicitors from PGPF such as cell walls retaining or lacking protein and lipids, cell walls lipid fraction, and three fractions of culture filtrate (CF) with different molecular weight ranges were tested for their ability to elicit the defense response of cucumber plants under controlled conditions. Cell wall fraction lacking protein and lipids and the cell wall lipid fraction from root colonizing isolates GS8-1, GS8-2, and GS8-3 protected plants against C. orbiculare. While, only cell wall lipid fraction and CF fractions of different molecular weight ranges of the isolate GU21-2 protected plants against C. orbiculare infection. Methanol-soluble substances from CF fractions with molecular weight greater than 12,000 and less than 8000 consistently protected plants against the pathogen. Germination of C. orbiculare spores decreased significantly at 72 h of incubation on leaves of PGPF-protected plants. Isolate GU21-2 was the most effective in inhibiting spore germination. On the other hand, isolates GS8-1 and GU21-2 induced lignifications in the hypocotyls of seven-day-old cucumber seedlings after challenge inoculation with C. orbiculare. Three-week-old cucumber plants treated with isolate GU21-2 and challenged with C. orbiculare showed increased activities of exo- and endo-forms of glucanase and chitinase, as well as peroxidase and polyphenol oxidase in the second true leaves. Induction treatment with isolate GS8-1 also increased the activities of these enzymes with the exception of exo-glucanase. This study shows that the inoculation of PGPF or its CF resulted in additive effect on the suppression of anthracnose disease in cucumber.     

Mineralization of forest litter nutrients by heat and combustion

Temperature dependant mineralization dynamics during fire of litter species characteristic of the New Jersey pine barrens was determined. Senescent leaf material of pitch pine (Pinus rigida), white oak (Quercus alba) and black huckleberry (Gaylusssacia baccata) were collected at the time of abscission; sorted, ground and oven-dried at 70 °C. Replicate samples were then heated for 2 h at: 70, 100, 200, 300, 400, and 550 °C. Mass loss and total nitrogen and total phosphorus concentration of the heated material were determined. Additional samples of the residual material were extracted with deionized water, and the filtrate was assayed for the anions: , , ; and cations: , K⁺, Mg⁺⁺, and Ca⁺⁺.By heating leaf litter over a range of temperatures, to simulate the heterogeneous nature of forest litter burning, we identified patterns of nutrient mineralization characteristic of specific temperatures, some of which were common to all three litter species and others unique to individual species. In general, it appears that black huckleberry leaf litter was the most nutrient rich and the most labile. In huckleberry litter, there was a large reserve of soluble nitrogen, sulfur, phosphate, calcium and magnesium that became available upon heating to 200 °C. Pitch pine litter was the most nutrient poor, and the rates of nutrient mineralization were also generally the lowest of the three species studied. White oak litter nutrient concentration and rates of mineralization along the temperature gradient were intermediate. For all three litter species examined organic and inorganic nitrogen losses due to volatilization were >99% upon heating to 550 °C, and soluble magnesium concentrations declined significantly at temperatures of 300 °C, despite having a volatilization temperature greater than 1100 °C. Under the temperature range employed, heating of leaf litter resulted in little volatilization loss of phosphorus; however, the amount of soluble phosphate phosphorus was much lower in all three litter types at temperatures of 300 °C and above. With increasing temperatures, inorganic phosphate ions presumably became bound to cations in the ash, forming insoluble metal phosphates. The dramatic increase of the ratio of total phosphorus to soluble inorganic phosphate at higher temperatures, the loss of soluble magnesium above 300 °C, and the near complete loss of nitrogen at 550 °C suggests that after intense fires availability of these minerals may be dramatically reduced.     

Colonisation of seminal roots of wheat and barley by egg-parasitic nematophagous fungi and their effects on Gaeumannomyces graminis var. tritici and development of root-rot

This study provides evidence that egg-parasitic nematophagous fungi, Pochonia chlamydosporia, Pochonia rubescens and Lecanicillium lecanii, can also reduce root colonisation and root damage by a fungal pathogen. Interactions of nematophagous fungi with the take-all fungus, Gaeumannomyces graminis var. tritici (Ggt), and their influence on severity of the root disease it causes were studied in laboratory and pot experiments. In Petri dish experiments the three nematophagous fungi reduced colonisation of barley roots by Ggt and also reduced necrotic symptoms. On the contrary, root colonisation by nematophagous fungi was unaffected by Ggt. In growth tube experiments, the three nematophagous fungi again reduced Ggt root colonisation and increased effective root length of barley seedlings. This was true for both simultaneous and sequential inoculation of nematophagous fungi versus Ggt. In the pot experiments the inoculum of the tested fungi in soil was applied in the same pot, as a mixture or in layers, or in coupled pots used for wheat grown with a split-root system. The nematophagous fungi P. chlamydosporia (isolate 4624) and L. lecanii (isolate 4629), mixed with Ggt or in split root systems with the pathogen, promoted growth of wheat (i.e. increased shoot weight), although no disease reduction was found. In split root systems, lower levels of peroxidase activity were found in seedlings inoculated with Ggt in combination with the nematophagous isolates 4624 and 4629 than when the take-all fungus was applied alone.Our results show that nematophagous fungi reduce root colonisation by Ggt, root damage and stress induced senescence in Ggt-inoculated plants.     

Mycorrhizal fungi increase biocontrol potential of Pseudomonas fluorescens

Wheat roots are susceptible to colonisation by soil-borne pathogens, such as Gaeumannomyces graminis var. tritici (Ggt), which causes the globally important disease take-all, and mutualistic arbuscular mycorrhizal fungi (AMF). Certain rhizosphere fluorescent Pseudomonas strains have received much attention as potential biocontrol agents given their ability to produce antibiotics, such as 2,4-diacetylphloroglucinol (DAPG), that confer a measure of plant protection. Here we show that Pseudomonas fluorescens only produced DAPG in the presence of soluble carbon from soil containing either Ggt or AMF, and production increased by two orders of magnitude in response to both AMF and Ggt. Encouragement of mycorrhizal colonisation may therefore offer a sustainable strategy for protection against take-all.     

Humic acid bleaching by white-rot fungi isolated from biosolids compost

A screening assay for isolating humic-substances degrading fungi from biosolids compost at the thermophillic phase employed using plates containing 2,2′-azino-bis(3-ethylbenzothiazoline-sulfonic acid) (ABTS), MnCl₂ or the monoazo dye Acid Red 183. Two of the most active fungi out of 70 fungal strains isolated were identified based on rDNA sequences and designated Trametes sp. M23 and Phanerochaete sp. Y6 (accessions no. DQ408582 and DQ438910). These isolates, when compared to a model white-rot fungi T. versicolor and P. chrysosporium, showed the ability to bleach humic acids extracted from biosolids compost while growing under solid-state conditions using perlite as a solid support. T. versicolor and Trametes sp. M23 also exhibited the ability to bleach humic acids from a peat source. Interestingly, only Trametes sp. M23 bleached leonardite humic acid, which is considered to be a highly aromatic and stable type of natural organic matter. To the best of our knowledge, this is the first report of white-rot fungi being isolated and identified from thermophilic composts. Since these fungi are capable of degrading lignin and humic acid and were found active in organic-matter-degradation processes, we suggest that they may play a significant role in the degradation and transformation of these refractory substances during composting.     

Consumption of mycorrhizal and saprophytic fungi by Collembola in grassland soils

Although soil-dwelling Collembola can influence plant growth and nutrient cycling, their specific role in soil food webs is poorly understood. Soil-free microcosm studies suggest that Collembola are primarily fungivores where they feed preferentially on saprophytic fungi (SF) over other fungal types. We directly assessed collembolan consumption of arbuscular mycorrhizal fungi (AMF) and SF using plant-soil mesocosms and natural abundance stable carbon isotope techniques. Mycorrhizal Andropogon gerardii (C₄ grass) seedlings were placed in pots containing Collembola and soil from a C₃ plant dominated site, while mycorrhizal Pascopyrum smithii (C₃ grass) seedlings were placed in pots with Collembola and soil collected at a C₄ plant dominated site. After 6 weeks, collembolans assimilated carbon derived from C₃ and C₄ sources in both A. gerardii and P. smithii treatments. Comparing Collembola isotope values in AMF vs. AMF-suppressed treatments, our data show that both AMF and SF were consumed in these experimental soil environments.     

Comparison of g23 gene sequence diversity between Novosphingobium and Sphingomonas phages and phage communities in the floodwater of a Japanese paddy field

Our previous study indicated that the diversity of the major capsid gene (g23) of T4-type bacteriophages (phages) of Novosphingobium and Sphingomonas strains isolated from the floodwater of a Japanese paddy field is comparable to those of the clones obtained from other Japanese paddy fields. For more strict comparison of the diversity, this study examined g23 sequences between Novosphingobium and Sphingomonas phages and phage communities in the identical floodwater of a Japanese paddy field. The clones were obtained by applying g23-specific primers to DNA extracted from the floodwaters. Many 23 clones in the floodwater were grouped into the same clusters of Paddy Groups I-VI with g23 genes of Novosphingobium/Sphingomonas phages with some clones belonging to an additional cluster. In addition, the remaining clones belonged to the clusters of marine clones and T4-type enterophages. These findings indicate that the g23 genes in the floodwater are more diversified than those of Novosphingobium/Sphingomonas phages including g23 genes closely related to the genes of enterophages and marine origins.     

Population responses of oribatids and enchytraeids to ectomycorrhizal and saprotrophic fungi in plant-soil microcosms

The oribatid mites Oppiella nova, Tectocepheus velatus and Nothrus silvestris and the enchytraeid worm Cognettia sphagnetorum are four common animal species in boreal forest soils. According to the literature, they respond differently to clear-cutting of forest stands. O. nova responds with population decreases, T. velatus and N. silvestris with small changes and C. sphagnetorum with population increases. We hypothesised that the presence/absence of ectomycorrhizal (EM) fungi is a major factor in explaining these reactions. The population responses of these soil animals to inoculation of five species of EM fungi growing in symbiosis with their host tree (Pinus sylvestris L.) and one saprotrophic fungus, Hypholoma capnoides, growing on wood were tested in pot microcosms with artificial soil (peat and vermiculite) for 70-84 days. Additionally, plants without inoculation of EM fungi, plants growing in forest soil (FS) and plant-free peat and vermiculite (PV) were included. O. nova increased significantly in abundance in the treatments with the EM fungi Suillus variegatus and Paxillus involutus, but not in the other treatments. T. velatus increased significantly in abundance in FS, but declined in most of the other treatments, and N. silvestris showed a similar, albeit not significant, response. C. sphagnetorum did not increase in abundance in any of the treatments with EM fungi but increased its abundance four times in both FS and PV. The results show that the fungivore O. nova preferentially feeds on certain EM fungi, especially S. variegatus, whereas the EM fungus Piloderma fallax and the saprotrophic H. capnoides are not preferred. In contrast, C. sphagnetorum is restricted in its population growth by the EM fungi studied, and this dominant microbi-detritivore is clearly favoured by the absence of EM fungi.     

Isolation and characterization of phosphate solubilizing bacteria from the rhizosphere of crop plants of Korea

Whole-cell fatty acids methyl ester (FAME) profile and 16S rDNA sequence analysis were employed to isolate and identify the bacterial groups that actively solubilized phosphates in vitro from rhizosphere soil of various crops of Korea. Out of several hundred colonies that grew on Pikovskaya's medium 13 best isolates were selected based on the solubilization of insoluble phosphates in liquid culture and further characterized and identified. They were clustered under the genera Enterobacter, Pantoea and Klebsiella and the sequences of three representative strains were deposited in the GenBank nucleotide sequence data library under the accession numbers AY335552, AY335553, AY335554.     

Influence of soil type and indigenous pathogenic fungi on bean hypocotyl rot caused by Rhizoctonia solani AG4 HGI in Cuba

Calcisol, ferralsol and vertisol soils, representative of different bean production areas of Villa Clara province in Cuba, were selected to determine the impact of soil type on bean hypocotyl rot severity caused by Rhizoctonia solani AG4 HGI (isolate CuVC-Rs7). In inoculated autoclaved soil, hypocotyl rot was most severe in calcisol soil, followed by ferralsol soils and then vertisol soils. In inoculated natural soils, disease severity was lower in vertisol and calcisol soils and higher in ferralsol soil, indicating that biological factors are suppressing or stimulating the pathogenic efficiency of R. solani. Native binucleate Rhizoctonia AGF, Sclerotium rolfsii and R. solani AG 4 HGI were isolated from bean plants grown in natural calcisol, vertisol and ferralsol soils, respectively. Subsequent studies about the interaction between these fungi and R. solani indicated that they were involved in the variability of disease severity caused by R. solani. The addition of R. solani AG4 HGI (isolate CuVC-Rs7) into each autoclaved soil inoculated with binucleate Rhizoctonia or S. rolfsii resulted in a reduction of disease severity caused by this pathogen while in soils inoculated with native R. solani AG4 HGI, disease severity increased. Irrespective of fungal interactions, calcisol was always the most disease conducive soil and vertisol the most disease repressive soil. The mechanisms by which native pathogenic fungi could influence disease severity caused by R. solani are discussed.     

Endogeic earthworms alter carbon translocation by fungi at the soil-litter interface

The effect of endogeic earthworms (Octolasion tyrtaeum (Savigny)) on the translocation of litter-derived carbon into the upper layer of a mineral soil by fungi was investigated in a microcosm experiment. Arable soil with and without O. tyrtaeum was incubated with ¹³C/¹⁵N-labelled rye leaves placed on plastic rings with gaze (64 μm mesh size) to avoid incorporation of leaves by earthworms. The plastic rings were positioned either on or 3 cm above the soil surface, to distinguish between biotic and chemical/physical translocation of nutrients by fungi and leaching.Contact of leaves to the soil increased ¹³C translocation, whereas presence of O. tyrtaeum reduced the incorporation of ¹³C into the mineral soil in all treatments. Although biomass of O. tyrtaeum decreased during the experiment, more ¹³C and ¹⁵N was incorporated into earthworm tissue in treatments with contact of leaves to the soil. Contact of leaves to the soil and the presence of O. tyrtaeum increased cumulative ¹³CO₂-C production by 18.2% and 14.1%, respectively.The concentration of the fungal bio-indicator ergosterol in the soil tended to be increased and that of the fungal-specific phospholipid fatty acid 18:2ω6 was significantly increased in treatments with contact of leaves to the soil. Earthworms reduced the concentration of ergosterol and 18:2ω6 in the soil by 14.0% and 43.2%, respectively. Total bacterial PLFAs in soil were also reduced in presence of O. tyrtaeum, but did not respond to the addition of the rye leaves. In addition, the bacterial community in treatments with O. tyrtaeum differed from that without earthworms and shifted towards an increased dominance of Gram-negative bacteria.The results indicate that litter-decomposing fungi translocate litter-derived carbon via their mycelial network in to the upper mineral soil. Endogeic earthworms decrease fungal biomass by grazing and disruption of fungal hyphae thereby counteracting the fungal-mediated translocation of carbon in soils.     

Cause and duration of mustard incorporation effects on soil-borne plant pathogenic fungi

Two fungal plant pathogens, Rhizoctonia solani AG 2-2 and Fusarium oxysporum f.sp. lini, were studied in relation to general responses of soil fungi and bacteria following incorporation of Brassica juncea. Our aim was to understand to what extent the changes in the biological and physicochemical characteristics of the soil could explain the effects on the studied pathogens and diseases, and to determine the temporal nature of the responses. Short-term effects of mustard incorporation (up to 4 months) were investigated in a microcosm experiment, and compared with a treatment where composted plant material was incorporated. In a field experiment, the responses were followed up to 11 months after removal or incorporation of a mustard crop. In general, responses in the variables measured changed more after incorporation of fresh mustard material than after addition of similar amounts of composted plant material (microcosms) or after removal of the mustard crop (field). The soil inoculum potential of R. solani AG 2-2 decreased directly after incorporation of mustard, but increased later to disease levels above those in the untreated soil. Neither of these effects could be explained by changes in the population density of R. solani AG 2-2. Fusarium spp. were less influenced, although an increase in the suppressiveness to Fusarium wilt was observed after mustard incorporation as compared with the treatment where mustard was removed. The microbial responses to mustard incorporation were more pronounced for bacteria than for fungi. After an initial substantial increase, the bacterial density decreased but remained above the levels in the control treatment throughout the experimental periods. The bacterial community structure was modified up to 8 months after mustard incorporation. We conclude that incorporation of fresh mustard influences soil microbial communities, especially the bacteria, and has a potential to control the pathogenic activity of R. solani 2-2 on a short-term perspective. The time dependency in microbial responses is important and should be taken into consideration for the evaluation of the potential of Brassicas to control plant disease on a field scale.     

Lyophyllum sp. strain Karsten alleviates pH pressure in acid soil and enhances the survival of Variovorax paradoxus HB44 and other bacteria in the mycosphere

In previous work, Variovorax paradoxus strain HB44, next to Burkholderia terrae BS001 and Dyella japonica BS003, were found to be selected in the mycosphere of the tricholomataceous fungi Laccaria proxima and Lyophyllum sp. strain Karsten in an acid soil denoted G. V. paradoxus HB44 showed poor survival in G bulk soil, irrespective of prior soil sterilization, and this poor survival also occurred for B. terrae BS001 and D. japonica BS003. In contrast, the survival rate of strain HB44 in two other soils, with pH values > 5.5, was significantly raised. Also, significantly enhanced strain HB44 survival in G soil was found if the pH was raised to 5.5 or 6.5, and it was even shown to grow (in the presence of the exogenous carbon source glycerol) at such pH values in the sterile G soil. This behaviour was similar to that of the V. paradoxus type strain. Strikingly, Lyophyllum sp. strain Karsten, when colonizing the sterilized G soil, significantly raised the soil pH from about 4.6 to ≥5.0. The pH raise was dependent on time, hyphal development, as well as on initial soil pH, but was consistent throughout. The modulated soil pH conditions were shown to be permissive for the survival and growth of strain HB44, and this was extended to strains BS001 and BS003. These findings corroborate the hypothesis that L. sp. strain Karsten provides a suitable habitat for acid-sensitive strains like HB44, BS001 and BS003 in its mycosphere in acid soil, which is strongly defined by the establishment of a growth-permissive pH.     

施用不同生物有机肥对连作黄瓜枯萎病防治效果及其机理初探

采用营养钵育苗和盆钵试验的方法,研究了以枯草芽孢杆菌SQR9和哈茨木霉T37这两株拮抗菌制成的三种生物有机肥（BIOⅠ、 BIOⅡ和BIOⅢ）对黄瓜生长、 土传枯萎病防治效果及其生防菌株在黄瓜根系中的定殖情况的影响。结果表明, 1）未施用生物有机肥的对照处理（CK）完全发病,有机肥处理（OF）发病率高达88.2%。施用BIO处理都不同程度地降低了黄瓜土传枯萎病的发病率,施用生物有机肥BIOⅠ、 BIOⅡ和BIOⅢ后发病率分别降至51.0%、 19.6%、 13.7%。2）与对照相比,BIOⅠ、 BIO Ⅱ和BIO Ⅲ处理能够显著提高黄瓜生物量,分别是对照的 2.55、 2.46 和 2.58 倍。3）菌株SQR9和T37通过有效的根际定殖降低了病原真菌对根系的侵染,荧光定量PCR（real-time PCR）测定黄瓜根际尖孢镰刀菌数量的结果表明,施用BIOⅡ和BIOⅢ能够将病原菌数量控制在103 copies/g, 土,而对照土壤高达 107 copies/g, 土。     

The abundance of hollow-bearing trees in urban dry sclerophyll forest and the effect of wind on hollow development

Anthropogenic change, particularly in urban landscapes, has resulted in the fragmentation of indigenous vegetation into often small isolated ‘remnants’. The persistence of arboreal fauna in small urban remnants in part depends on the distribution and abundance of habitat resources within the remnant. We surveyed 44 small (<2.5 ha) eucalypt remnants located within the south-eastern suburbs of metropolitan Melbourne to ascertain the abundance of hollow-bearing trees, an important ecological resource. The probability of a live or dead tree containing a hollow was investigated in relation to site variables that influenced exposure to wind, a factor thought to increase the propensity of hollow formation in eucalypt trees. A total of 2678 live and 224 dead eucalypt trees were surveyed, of which 350 live (12%) and 70 dead (31%) trees were hollow-bearing. The probability of a tree being hollow-bearing was strongly positively associated with the diameter of the tree, however, past management practices have lead to a paucity of large (>80 cm DBH) trees in small urban remnants. We found that variables that measured exposure to wind were correlated with the chance that a live tree will be hollow-bearing while reducing the chance that a dead tree will be hollow-bearing. Although highly variable, the number of hollow-bearing trees contained within small urban remnants (mean of 5.8 ha⁻¹) fell well below that contained in areas of un-logged non-urban forest. Our results indicated that large numbers of hollow-bearing tree are unlikely to be recruited into urban remnants for a significant time-span and as such there is an increased importance placed on maintaining the current inventory of hollow-bearing trees for the maintenance of biodiversity in urban areas.     

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Impacts of earthworms and arbuscular mycorrhizal fungi (Glomus intraradices) on plant performance are not interrelated

Earthworms and arbuscular mycorrhizal fungi (AMF) might interactively impact plant productivity; however, previous studies reported inconsistent results. We set up a three-factorial greenhouse experiment to study the effects of earthworms (Aporrectodea caliginosa Savigny and Lumbricus terrestris L.) and AMF (Glomus intraradices N.C. Schenck & G.S. Sm.) on the performance (productivity and shoot nutrient content) of plant species (Lolium perenne L., Trifolium pratense L. and Plantago lanceolata L.) belonging to the three functional groups grasses, legumes and herbs, respectively. Further, we investigated earthworm performance and plant root mycorrhization as affected by the treatments. Our results accentuate the importance of root derived resources for earthworm performance since earthworm weight (A. caliginosa and L. terrestris) and survival (L. terrestris) were significantly lower in microcosms containing P. lanceolata than in those containing T. pratense. However, earthworm performance was not affected by AMF, and plant root mycorrhization was not modified by earthworms. Although AMF effectively competed with T. pratense for soil N (as indicated by δ¹⁵N analysis), AMF enhanced the productivity of T. pratense considerably by improving P availability. Remarkably, we found no evidence for interactive effects of earthworms and AMF on the performance of the plant species studied. This suggests that interactions between earthworms and AMF likely are of minor importance.     

Selection of Variovorax paradoxus-like bacteria in the mycosphere and the role of fungal-released compounds

At two occasions (2004 and 2006), a similar cluster of culturable bacteria was found to be selected in the mycosphere of the basidiomycetous fungus Laccaria proxima (Agaricales, Tricholomataceae) in the field. The bacteria, identified as related to Variovorax paradoxus, comprised 7.3-9% of the total culturable bacterial community in the L. proxima mycosphere, but were not found in corresponding bulk soil (<0.3%). One strain, denoted HB44, was selected in order to unravel the basis of the V. paradoxus mycosphere competence in in vitro experiments with the former Laccaria laccata, which was recently reclassified as Lyophyllum sp. strain Karsten (Agaricales, Tricholomataceae). In liquid culture experiments, L. strain Karsten was shown to be an avid producer of glycerol, next to acetate and formate, which constituted the most abundant carbonaceous compounds released. Strain HB44 was able to grow avidly at the expense of the glycerol liberated by the fungus, as evidenced by proton NMR analysis of the fungal exudates in the medium before and after bacterial growth. In sterilized field soil, strain HB44 survived significantly better in the presence than in the absence of L. strain Karsten. Addition of a glycerol series to the sterilized soil (without the fungus) resulted in the persistence or growth of strain HB44, but only if the pH of this soil was previously set at 5.5. Thus, we provide evidence for the contention that tricholomataceous fungi can create specific niches in soil for the V. paradoxus-related strain HB44 and that glycerol may be a main carbon source that drives the selection of this organism.     

Carbohydrate use and assimilation by litter and soil fungi assessed by carbon isotopes and BIOLOG assays

Soil fungi are integral to decomposition in forests and other habitats, yet identifying probable functional roles of different taxa is problematic. Here, we compared carbohydrate assimilation patterns derived from stable isotope analyses on cultures with patterns of metabolic activity measured on Biolog^® SF-P plates for 12 taxa of soil- and litter-inhabiting saprotrophic fungi isolated from Douglas-fir (Pseudotsuga menziesii) ecosystems. To determine the relative assimilation of carbon from malt extract versus sucrose by ¹³C stable isotope analyses, we cultured fungi with malt extract (consisting primarily of glucose and maltose) plus either C₃- or C₄-derived sucrose as carbon sources. Rhodotorula graminis and F. oxysporum assimilated the highest proportion of sucrose, a Mortierella isolate and an unidentified sterile isolate (FPC 341) assimilated the lowest proportion of sucrose, and remaining cultures assimilated similar and intermediate proportions of sucrose. On Biolog plates, low metabolic activity of Mortierella and FPC 341 on sucrose and R. graminis and F. oxysporum on maltose were qualitatively consistent with isotopic results. Assimilation of sucrose calculated isotopically was correlated with the ratio of sucrose: maltose activity calculated from Biolog assays (r²=0.45, P=0.0145, n=12). Metabolic activity on Biolog plates for six other common soil carbohydrates were also determined: glucose, fructose, galactose, cellobiose, lactose, and glycogen. Metabolic activity was greatest overall on maltose and glucose and lowest on fructose. Two of the isolates (Aspergillus flavus and F. oxysporum) had higher metabolic activity on the glucose-containing disaccharide cellobiose than on glucose, strongly suggesting preferential uptake of cellobiose compared to glucose and suggesting the potential ability to use cellulose. The high metabolic activity of these cultures on galactose, a primary constituent of hemicellulose, also suggested cellulolytic capabilities. Our results indicated that stable isotope studies and Biolog assays may provide complementary information to characterize metabolic potential of fungi in forest litter and soil.