Rhizosphere Colonization and Control of Meloidogyne spp. by Nematode-trapping Fungi

The ability of nematode-trapping fungi to colonize the rhizosphere of crop plants has been suggested to be an important factor in biological control of root-infecting nematodes. In this study, rhizosphere colonization was evaluated for 38 isolates of nematode-trapping fungi representing 11 species. In an initial screen, Arthrobotrys dactyloides, A. superba, and Monacrosporium ellipsosporum were most frequently detected in the tomato rhizosphere. In subsequent pot experiments these fungi and the non-root colonizing M. geophyropagum were introduced to soil in a sodium alginate matrix, and further tested both for establishment in the tomato rhizosphere and suppression of root-knot nematodes. The knob-forming M. ellipsosporum showed a high capacity to colonize the rhizosphere both in the initial screen and the pot experiments, with more than twice as many fungal propagules in the rhizosphere as in the root-free soil. However, neither this fungus nor the other nematode-trapping fungi tested reduced nematode damage to tomato plants.     

New insights into the evolution of subtilisin-like serine protease genes in Pezizomycotina

Background  

Subtilisin-like serine proteases play an important role in pathogenic fungi during the penetration and colonization of their hosts. In this study, we perform an evolutionary analysis of the subtilisin-like serine protease genes of subphylum Pezizomycotina to find if there are similar pathogenic mechanisms among the pathogenic fungi with different life styles, which utilize subtilisin-like serine proteases as virulence factors. Within Pezizomycotina, nematode-trapping fungi are unique because they capture soil nematodes using specialized trapping devices. Increasing evidence suggests subtilisin-like serine proteases from nematode-trapping fungi are involved in the penetration and digestion of nematode cuticles. Here we also conduct positive selection analysis on the subtilisin-like serine protease genes from nematode-trapping fungi.     

Plant species richness,identity and productivity differentially influence key groups of microbes in grassland soils of contrasting fertility

The abundance of microbes in soil is thought to be strongly influenced by plant productivity rather than by plant species richness per se. However, whether this holds true for different microbial groups and under different soil conditions is unresolved. We tested how plant species richness, identity and biomass influence the abundances of arbuscular mycorrhizal fungi (AMF), saprophytic bacteria and fungi, and actinomycetes, in model plant communities in soil of low and high fertility using phospholipid fatty acid analysis. Abundances of saprophytic fungi and bacteria were driven by larger plant biomass in high diversity treatments. In contrast, increased AMF abundance with larger plant species richness was not explained by plant biomass, but responded to plant species identity and was stimulated by Anthoxantum odoratum. Our results indicate that the abundance of saprophytic soil microbes is influenced more by resource quantity, as driven by plant production, while AMF respond more strongly to resource composition, driven by variation in plant species richness and identity. This suggests that AMF abundance in soil is more sensitive to changes in plant species diversity per se and plant species composition than are abundances of saprophytic microbes.     

Hyphal growth and mycorrhiza formation by the arbuscular mycorrhizal fungus Glomus claroideum BEG 23 is stimulated by humic substances

Effects of humic substances (humic acid or fulvic soil extract) or saprophytic microorganisms (Paecilomyces lilacinus and an unidentified actinomycete) on growth of mycelium and mycorrhiza formation by Glomus claroideum BEG23 were studied in a hydroponic system. Humic substances stimulated root colonization and production of extraradical mycelium by the mycorrhizal fungus. Both humic and fulvic acids tended to decrease populations of culturable bacteria and fungi in the cultivation system, indicating a moderately antibiotic activity. The addition of saprophytic microorganisms able to use humic substances to the cultivation system further stimulated the development of the mycorrhizal fungus. However, stimulation of G. claroideum was also observed when the saprophytic microorganisms were heat-killed, suggesting that their effect was not linked to a specific action on humic substances. The results indicate that humic substances may represent a stimulatory component of the soil environment with respect to arbuscular mycorrhizal fungi.     

Assessment of germination and carnivorous activities of a nematode-trapping fungus Arthrobotrys dactyloides in fungistatic and fungicidal soil environment

Carnivorism is the ability of nematode-trapping fungi to trap and digest the nematodes by sophisticated devices called traps. Delivery of nematode-trapping fungi in soil for bio-control of pest nematodes often fails or gives inconsistent results. Possible reasons for failure could be the effect of soil fungistasis on germination of nematode-trapping fungi in soil environment, use of avirulent species and sensitivity of these fungi to fungicidal residues in soil. Exploitation of nematode-trapping fungi for nematode control demands that it be compatible with fungicides applied in soil or crops and proliferate in soil. This investigation represents is one of the first to evaluate the effect of fungicides on the nematode-trapping fungus Arthrobotrys dactyloides. A. dactyloides showed in vitro carnivorous potential against Meloidogyne incognita, Meloidogyne javanica, Meloidogyne graminicola, Helicotylenchus dihystera and Heterodera cajani. Conidia of A. dactyloides exposed to agricultural soils showed poor germination but formed conidial traps, which captured and killed the soil nematodes. Conidial traps, which trapped the nematodes, grew well in all soils after killing and nutrient absorption from nematode body. Soil amended with 20 mg ai kg⁻¹ of carbendazim and thiram, 30 mg ai kg⁻¹ of mancozeb, 50 mg ai kg⁻¹ of captan, and 100 mg ai kg⁻¹ of carboxin completely checked the conidial trap formation and nematode capturing. 30, 50 and 100 mg ai kg⁻¹ of metalaxyl adversely affected the conidial trap formation and nematode capturing in soil. Propiconazole inhibited 15.2% conidial trap formation up to 50 mg ai kg⁻¹ but caused 93.3% inhibition of conidial traps formation and complete inhibition of nematode capturing at 100 mg ai kg⁻¹. Sulphur, triademefon, and tricyclazole showed least toxic effect on conidial trap formation and nematode capturing activities of A. dactyloides in soil up to 100 mg ai kg⁻¹.     

The living strategy of nematophagous fungi

The infection structures, trophism, and ecological character of nematophagous fungi are reviewed in this article on the basis of data extracted from the literature and the most recent experiments conducted in this area. Traditionally, nematophagous fungi are classified into four groups according to their modes of attacking nematodes: nematode-trapping fungi using adhesive or mechanical hyphal traps, endoparasitic fungi using their spores, eggparasitic fungi invading nematode eggs or females with their hyphal tips, and toxin-producing fungi immobilizing nematodes before invasion. In the present review, we focus on the first two groups. The living strategies of these nematophagous fungi depend on the diversity of their infection structures, such as different traps and spore types, which determine the modes of infecting nematodes. The diversity of trophic modes of nematophagous fungi is an important prerequisite for fungal survival and activity in soil. The abundance and activity of Hirsutella rhossiliensis and H. minnesotensis, representatives of endoparasites and potential biocontrol agents against nematodes, are highly dependent on environmental factors. Comprehensive understanding of the survival and activity of nematophagous fungi in soil is fundamental for the exploitation of these fungi as successful biocontrol agents.     

Fungal Parasitism of Heterodera glycines Eggs as Influenced by Egg Age and Pre-colonization of Cysts by Other Fungi

The objective of this study was to determine the effect of egg age and pre-colonization of cysts by a saprophytic or parasitic fungus on parasitism of Heterodera glycines eggs by other parasitic fungi. In agar and in soil tests, fungi generally parasitized more eggs in early developmental stages than eggs containing a juvenile. The effect of pre-colonization of cysts by a fungus on parasitism of eggs by other fungi depended on the fungi involved. In most cases, pre-colonization of cysts by an unidentified, saprophytic fungal isolate (A-1-24) did not affect parasitism of eggs in the cysts subsequently treated with other fungi. However, pre-colonization of cysts by A-1-24 reduced fungal parasitism of eggs in cysts subsequently treated with Cylindrocarpon destructans isolate 3. In agar tests, pre-colonization of cysts by Chaetomium cochliodes, a saprophytic or weakly parasitic fungus, reduced parasitism of eggs in cysts subsequently treated with Verticillium chlamydosporium Florida isolate, Fusarium oxysporum, Fusarium solani, ARF18, and another sterile fungus. However, in soil tests, pre-colonization of cysts by C. cochliodes had no effect on parasitism of eggs by subsequent fungal parasites. In another test, parasitism of eggs by V. chlamydosporium in cysts was not affected by pre-colonizing fungi C. destructans, F. oxysporum, and F. solani but was reduced by Mortierella sp., Pyrenochaeta terrestris, and C. cochliodes. Parasitism of eggs in cysts by ARF18 was reduced by pre-colonizing fungi C. destructans, F. oxysporum, F. solani, P. terrestris, and C. cochliodes but not Mortierella sp.     

The arbuscular mycorrhizal fungus Rhizophagus intraradices and other microbial groups affect plant species in a copper-contaminated post-mining soil

Background and aimArbuscular mycorrhizal fungi (AMF) have an important role in plant-microbe interactions. But, there are few studies in which the combined effect of AMF with a stress factor, such as the presence of a metal, on plant species were assessed. This study investigated the effect of arbuscular mycorrhizal (AM) fungus Rhizophagus intraradices and other soil microbial groups in the presence of copper on three plant species in a microcosm experiment.MethodsTwo grass species Poa compressa and Festuca rubra and one herb species Centaurea jacea were selected as model plants in a pot-design test in which soils were artificially contaminated with copper. Treatments were bacteria (control), saprophytic fungi, protists, and a combined treatment of saprophytic fungi and protists, all in the presence or absence of the AM fungal species. After sixty days, plants were harvested and the biomass of grass and herb species and microbial respiration were measured.ResultsThe results showed almost equal above- and belowground plant biomass and microbial respiration in the treatments in the presence or absence of R. intraradices. The herb species C. jecea responded significantly to the soil inoculation with AM fungus, while grass species showed inconsistent patterns. Significant effect of AMF and copper and their interactions was observed on plant biomass when comparing contaminated vs. non-contaminated soils.ConclusionStrong effect of AMF on the biomass of herb species and slight changes in plant growth with the presence of this fungal species in copper-spiked test soils indicates the importance of mycorrhizal fungi compared to other soil microorganisms in our experimental microcosms.     

A leaf-rolling weevil benefits from general saprophytic fungi in polysaccharide degradation

Insects, especially those feeding on leaf litter, widely form symbiosis with fungi. As dead plant tissues provide insects with poor-quality diets, which contain relatively high levels of indigestible lignin and cellulose, some saprophytic fungi may increase nutrient availability by polysaccharide degradation. Although the inherited, obligate bacterial symbionts are well documented, the non-inherited, facultative fungal symbionts are relatively overlooked. Females of the leaf-rolling weevil Heterapoderopsis bicallosicollis, a specialist of Triadica sebifera, construct leaf-rolls that serve as retreats from which larvae feed internally. We found that fungi associated with leaf-rolls were not transported by the female, but likely originated from the soil. To determine the effects of fungi on H. bicallosicollis development, fungal growth was reduced by a dry treatment. This treatment decreased adult weight and survival, and prolonged larval duration significantly. We further tested the hypothesis that fungi degrade leaf-roll polysaccharides, by a fungus inoculation experiment. Three dominant fungi (Penicillium sp., Aspergillus sp. and Cladosporium sp.) decreased the levels of soluble carbohydrate, cellulose, and lignin in inoculation experiments. Soluble carbohydrate, cellulose, and lignin of leaf-rolls all were found to decrease gradually during insect development. We conclude that these saprophytic fungi form facultative associations with H. bicallosicollis and benefit weevil nutrition by polysaccharide decomposition. Our study highlights the significance of fungal symbionts in insect nutritional ecology.     

Detection of Helminthosporium solani from soil and plant tissue with species-specific PCR primers

Two PCR primer pairs specific for Helminthosporium solani, which causes silver scurf on potato tubers, were designed from nucleotide sequences of the nuclear ribosomal internal transcribed spacer regions of H. solani. Both primer pairs amplified a single product with DNA from 48 North American and European isolates of H. solani, but not with DNA from 42 other fungi. Primers also amplified a single product with DNA extracted from silver scurf lesions on potato tubers and other plant tissue inoculated with spores of H. solani. Detection of the fungus in infested soil was only possible with nested PCR and after processing soil with a bead beater. Specific amplification of H. solani DNA can be used to study the saprophytic and pathogenic activity of this fungus in soil and plant tissue.     

Biocontrol: Fungi as Nematode Control Agents

The fungal antagonists of nematodes consist of a great variety of organisms belonging to widely divergent orders and families of fungi. They include the nematode-trapping fungi, endoparasitic fungi, parasites of nematode eggs and cysts, and fungi which produce metabolites toxic to nematodes. The diversity, adaptations, and distribution of nematode-destroying fungi and taxonomic problems encountered in their study are reviewed. The importance of nemato-phagous fungi in soil biology, with special emphasis on their relationship to populations of plant-parasitic nematodes, is considered. While predacious fungi have long been investigated as possible biocontrol agents and have often exhibited spectacular results in vitro, their performance in field studies has generated little enthusiasm among nematologists. To date no species has demonstrated control of any plant pest to a degree achieved with nematicides, but recent studies have provided a much clearer concept of possibilities and problems in the applied use of fungal antagonists. The discovery of new species, which appear to control certain pests effectively under specific conditions, holds out some promise that fungi may be utilized as alternatives to chemical control after a more thorough and expanded study of their biology and ecology.     

捕食线虫真菌在北京地区果园中的季节性分布

通过对北京海淀区2个苹果园和2个葡萄园根际捕食线虫真菌2年的季节性变化调查,共分离到199个捕食线虫真菌菌株,隶属3个属16个种,Arthrobotrysdactyloides分离频率最高,其次是A.oligospora,A.conoides和Stylopagesp.。捕食线虫真菌的数量在晚春夏初最丰富,秋季次之,冬夏最少。通过统计分析,捕食线虫真菌数量的季节性变化与温度线性相关,而与线虫总数S形相关,与土壤湿度和植物寄生线虫数量无显著相关。     

Germination of the fully myco-heterotrophic orchid Cyrtosia septentrionalis is characterized by low fungal specificity and does not require direct seed-mycobiont contact

We examined the level of specificity exhibited by seeds of Cyrtosia septentrionalis for germination-inducing fungi. Three wood-decomposing Armillaria species (A. gallica, A. mellea subsp. nipponica and A. tabescens) known to colonize adult plants and an unidentified species of Polyporales isolated from naturally growing protocorms were tested. Xylobolus annosus, a free-living decomposer of Russulales, was included as a control. Seed germination occurred in a sawdust-based medium in sealed and unsealed containers in the presence of all fungi, indicating low mycorrhizal specificity in germination. Moreover, germination occurred even in modified containers in which the fungus was physically isolated from the orchid seeds, indicating that direct seed-fungus contact is not required. Higher germination percentages were observed in sealed containers in which a modified atmosphere, consisting of a lower O₂ and a higher CO₂ concentration, had been established in the air above the inoculated medium as a result of the saprophytic activity of the fungus. In nature, atmospheric conditions more effective for seed germination might be established by the action of mycobionts in decomposing wood. Seeds germinated in the presence of the unknown Polyporales species underwent further growth and development, as compared with the other fungi tested, when covered with soil.     

华北退化荒地建植豆类和禾本植物人工草地对土壤真菌群落结构和功能的影响

阐明植被恢复过程中土壤真菌群落的变化及其生态功能,对于制定科学有效的退化生态系统管理措施有重要参考价值。利用扩增子高通量测序技术和生物信息学分析解析了华北退化荒地自然恢复（对照组,CK）和建植豆科植物和禾本植物人工草地（分别为LG和GG处理）过程中土壤真菌群落结构和功能群特征差异。结果表明：（1）退化荒地土壤表层样品中共获取6315个真菌OTU,隶属于17门60纲145目347科896属,优势菌门为Ascomycetes、Mortierellomycota和Basidiomycetes,LG处理相较于CK的Basidiomycetes相对丰度明显升高,GG处理相较于CK的Mortierellomycota相对丰度明显提高。（2）土壤真菌功能群类型以腐生真菌为主,共生真菌次之,病原真菌占比最少。建植人工草地对腐生真菌和共生真菌相较于病原真菌的功能群组成影响更明显,并导致腐生真菌相对丰度升高,共生真菌相对丰度降低。（3）土壤真菌群落结构受植物物种丰富度、根系生物量等植被参数变化的显著（P<0.05）影响,且与土壤有机碳、总氮、速效氮、总磷等土壤养分水平显著（P<0.05）相关。本研究的结果有助于深入理解建植人工草地对土壤真菌群落结构和功能的影响,并为华北退化荒地植被恢复策略提供理论依据。     

Antifungal activity of extracellular hydrolases produced by autolysing Aspergillus nidulans cultures

Carbon-starving Aspergillus nidulans cultures produce high activities of versatile hydrolytic enzymes and, among these, ChiB endochitinase and EngA ??-1,3-endoglucanase showed significant antifungal activity against various fungal species. Double deletion of engA and chiB diminished the antifungal activity of the fermentation broths and increased conidiogenesis and long-term viability of A. nidulans, but decreased the growth rate on culture media containing weak carbon sources. Production of ChiB and EngA can influence fungal communities either directly due to their antifungal properties or indirectly through their effects on vegetative growth. Our data suggest saprophytic fungi as promising future candidates to develop novel biocontrol technologies.     

腐生型兰科植物研究进展

腐生型兰科植物又称完全菌根异养型兰(Fully Mycoheterotrophic Orchids),无绿叶,不含叶绿素,不能进行光合作用制造有机物,完全依靠与其共生的真菌提供营养。近年来,腐生兰独特的生活方式引起了生物学家对其生理生态及进化问题的广泛关注,但目前仍缺乏系统深入的研究。为此,作者通过整理《中国植物志》和近年来报道的相关文献,发现分布在中国的腐生型兰科植物约有23属81种,主要集中在天麻属(Gastrodia)(24种)、鸟巢兰属(Neottia)(8种)、无叶兰属(Aphyllorchis)(6种)、山珊瑚属(Galeola)(5种)等。由于腐生兰特殊的营养需求,相比于其他营养类型的兰科植物,其菌根真菌有较大区别。腐生兰的菌根真菌主要有两大类,即外生菌根真菌(Ectomycorrhizal fungi,ECM)和非丝核菌类的腐生菌(Non-rhizoctonia SAP fungi)。该文还综述了腐生兰与其菌根真菌的专一性、营养来源以及系统进化等问题,提出了目前研究存在的问题,并对今后的研究工作进行了展望,以期为腐生兰的资源保护及再生研究提供参考。