Streptomyces as symbionts: an emerging and widespread theme?

Streptomyces bacteria are ubiquitous in soil, conferring the characteristic earthy smell, and they have an important ecological role in the turnover of organic material. More recently, a new picture has begun to emerge in which streptomycetes are not in all cases simply free-living soil bacteria but have also evolved to live in symbiosis with plants, fungi and animals. Furthermore, much of the chemical diversity of secondary metabolites produced by Streptomyces species has most likely evolved as a direct result of their interactions with other organisms. Here we review what is currently known about the role of streptomycetes as symbionts with fungi, plants and animals. These interactions can be parasitic, as is the case for scab-causing streptomycetes, which infect plants, and the Streptomyces species Streptomyces somaliensis and Streptomyces sudanensis that infect humans. However, in most cases they are beneficial and growth promoting, as is the case with many insects, plants and marine animals that use streptomycete-produced antibiotics to protect themselves against infection. This is an exciting and newly emerging field of research that will become increasingly important as the search for new antibiotics switches to unusual and under-explored environments.     

特殊环境真菌

真菌是生物多样性最丰富和生存环境最多样的生物类群之一,除了我们熟知的土壤、植物残体、水体等一般环境外,还存在极地、高温、高盐等各种特殊环境.对真菌而言,特殊环境是指绝大多数真菌不能生存的环境,而特殊环境真菌是指特殊环境中特有的真菌或适应特殊环境的真菌.特殊环境人为地划分为以下3种类型:第一类,具有某种特定的理化限制因素的环境;第二类,某些特殊基质;第三类,受多因素限制的特殊复杂环境.特殊环境真菌有着独特的适应性,并进化出各种机制来占据生态位.它们在细胞构造、代谢方式、进化机制等方面的特殊性引起了科学家的广泛关注.本文介绍了各种特殊生态环境下生活的真菌类群以及它们的生存机制,并对今后的研究方向进行了展望.     

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.     

Fight to the death: Arabidopsis thaliana defense response to fungal necrotrophic pathogens

Necrotrophic fungi, being the largest class of fungal plant pathogens, pose a serious economic problem to crop production. They are the cause of heavy losses in agriculture worldwide. Understanding the process of plant infection by necrotrophic fungi, including subtle interaction networks connecting such evolutionarily distinct organisms has recently been given high research priority. Such studies are now possible mainly because of the utility of the model plant Arabidopsis thaliana. A. thaliana has a sequenced genome and thousands of mutants available, allowing investigation of virtually all aspects of plant pathogenesis. This review focuses on morphological and molecular changes in A. thaliana, which occur during response to infection by necrotrophic fungi. These responses in relation to resistance and susceptibility of the plant will be discussed.     

Common processes in pathogenesis by fungal and oomycete plant pathogens,described with Gene Ontology terms

Plant diseases caused by fungi and oomycetes result in significant economic losses every year. Although phylogenetically distant, the infection processes by these organisms share many common features. These include dispersal of an infectious particle, host adhesion, recognition, penetration, invasive growth, and lesion development. Previously, many of these common processes did not have corresponding Gene Ontology (GO) terms. For example, no GO terms existed to describe processes related to the appressorium, an important structure for infection by many fungi and oomycetes. In this mini-review, we identify common features of the pathogenic processes of fungi and oomycetes and create a pathogenesis model using 256 newly developed and 38 extant GO terms, with an emphasis on the appressorium and signal transduction. This set of standardized GO terms provides a solid base to further compare and contrast the molecular underpinnings of fungal and oomycete pathogenesis.     

Phylogenetic trait conservatism and the evolution of functional trade-offs in arbuscular mycorrhizal fungi

The diversity of functional and life-history traits of organisms depends on adaptation as well as the legacy of shared ancestry. Although the evolution of traits in macro-organisms is well studied, relatively little is known about character evolution in micro-organisms. Here, we surveyed an ancient and ecologically important group of microbial plant symbionts, the arbuscular mycorrhizal (AM) fungi, and tested hypotheses about the evolution of functional and life-history traits. Variation in the extent of root and soil colonization by AM fungi is constrained to a few nodes basal to the most diverse groups within the phylum, with relatively little variation associated with recent divergences. We found no evidence for a trade-off in biomass allocated to root versus soil colonization in three published glasshouse experiments; rather these traits were positively correlated. Partial support was observed for correlated evolution between fungal colonization strategies and functional benefits of the symbiosis to host plants. The evolution of increased soil colonization was positively correlated with total plant biomass and shoot phosphorus content. Although the effect of AM fungi on infection by root pathogens was phylogenetically conserved, there was no evidence for correlated evolution between the extent of AM fungal root colonization and pathogen infection. Variability in colonization strategies evolved early in the diversification of AM fungi, and we propose that these strategies were influenced by functional interactions with host plants, resulting in an evolutionary stasis resembling trait conservatism.     

Evolution of fungal pathogens in domestic environments?

Specific indoor environments select for certain stress-tolerant fungi and can drive their evolution towards acquiring medically important traits. Here we review the current knowledge in this area of research, focussing on the so-called black yeasts. Many of these melanised stress-tolerant organisms originate in unusual ecological niches in nature, and they have a number of preadaptations that make them particularly suited for growth on human-made surfaces and substrates. Several pathogenic species have been isolated recently from various domestic habitats. We argue that in addition to enriching for - potentially - pathogenic species, the selection pressure and stress acting on microorganisms in indoor environments are driving their evolution towards acquiring the missing virulence factors and further enhancing their stress tolerance and pathogenic potential. Some of the polyextremotolerant fungi are particularly problematic: they can grow at elevated temperatures, and so they have a higher potential to colonise warm-blooded organisms. As several species of black fungi are already implicated in health problems of various kinds, their selection and possible evolution in human environments are of concern.     

cAMP signalling in pathogenic fungi: control of dimorphic switching and pathogenicity

Morphological changes in pathogenic fungi often underlie the development of virulence and infection by these organisms. Our knowledge of the components of the cell signalling pathways controlling morphological switching has, to a large extent, come from studies of pseudohyphal growth of the model organism Saccharomyces cerevisiae, in which control is exerted via changes in the intracellular cAMP and mitogen-activated protein kinase cascades. There is evidence that pathogenic fungi also utilize these pathways to control dimorphic switching between saprobic and pathogenic forms and, as such, the elements of these pathways have potential as drug targets.     

Biolistic transformation of arbuscular mycorrhizal fungi

Gene transfer systems have proved effective for the transformation of a range of organisms for both fundamental and applied studies. Biolistic transformation is a powerful method for the gene transfer into various organisms and tissues that have proved recalcitrant to more conventional means. For fungi, the biolistic approach is particularly effective where protoplasts are difficult to obtain and/or the organisms are difficult to culture. This is particularly applicable to arbuscular mycorrhizal (AM) fungi, being as they are obligate symbionts that can only be propagated in association with intact plants or root explants. Furthermore, these fungi are aseptate and protoplasts cannot be released. Recent advancements in gene transformation systems have enabled the use of biolistic technology to introduce foreign DNA linked to molecular markers into these fungi. In this review we discuss the development of transformation strategies for AM fungi by biolistics and highlight the areas of this technology which require further development for the stable transformation of these elusive organisms.     

The significance of peroxisomes in secondary metabolite biosynthesis in filamentous fungi

Peroxisomes are ubiquitous organelles characterized by a protein-rich matrix surrounded by a single membrane. In filamentous fungi, peroxisomes are crucial for the primary metabolism of several unusual carbon sources used for growth (e.g. fatty acids), but increasing evidence is presented that emphasize the crucial role of these organelles in the formation of a variety of secondary metabolites. In filamentous fungi, peroxisomes also play a role in development and differentiation whereas specialized peroxisomes, the Woronin bodies, play a structural role in plugging septal pores. The biogenesis of peroxisomes in filamentous fungi involves the function of conserved PEX genes, as well as genes that are unique for these organisms. Peroxisomes are also subject to autophagic degradation, a process that involves ATG genes. The interplay between organelle biogenesis and degradation may serve a quality control function, thereby allowing a continuous rejuvenation of the organelle population in the cells.     

植物病原真菌黑色素与致病性关系的研究进展

黑色素是一类生物聚合分子的总称,不同来源的黑色素种类不同,其中报道较多的是DOPA黑色素和DHN黑色素。DOPA黑色素和DHN黑色素具有相似的理化性质但其合成底物和途径不同。DHN黑色素在植物病原直菌中广泛存在,与病原菌致病能力密切相关。病原菌侵染寄主时黑色素沉积存附着胞细胞壁的内层,防止了形成膨压的溶质渗透到细胞外,产生很大的机械压力,保证病原菌侵入寄主。结合作者的研究结果综述了黑色素的种类、性质及黑色素与病原菌致病性关系等方面的研究进展。     

Protists as opportunistic pathogens: public health impact in the 1990s and beyond

Protist organisms (protozoa and fungi) have become increasingly prominent as opportunistic pathogens among persons infected with human immunodeficiency virus (HIV) and among organ transplant recipients--two immunocompromised populations that have increased dramatically in the past two decades. Pneumocystis carinii pneumonia continues to be the most common serious opportunistic infection (OI) among HIV-infected persons in the United States, occurring frequently among persons not previously receiving medical care. Toxoplasmosis, cryptococcosis, cryptosporidiosis, and isosporiasis occur frequently in HIV-infected persons in the developing world. Candidiasis and aspergillosis are common OIs in organ transplant recipients. As these populations of immunosuppressed patients continue to expand worldwide new OIs caused by protist pathogens are likely to emerge.     

植物病原丝状真菌G蛋白偶联受体的研究进展

通过对丝状真菌G蛋白偶联受体(GPCR)的结构、分类以及功能方面进行综述,以期明确丝状真菌与其他模式生物GPCR之间的关系。基于已报道的模式生物及丝状真菌等不同生物中的GPCR,通过SMART保守结构域分析,以及利用Clustal X、MEGA等软件对上述GPCR进行遗传关系分析。明确丝状真菌典型GPCR具有七跨膜结构域,新型GPCR则含有PIPK、RGS等保守结构域,明确不同学者对于GPCR的分类情况,以及新型GPCR所具有的特殊功能,明确模式生物GPCR、丝状真菌GPCR分别各自聚类。丝状真菌中GPCR的数量较模式生物少,不同分类单元中真菌之间GPCR的数量也不尽相同,同时,丝状真菌GPCR除具有典型的七跨膜结构域外,还含有一些其他保守的结构域,上述研究为进一步开展其功能研究提供重要的理论基础。     

Species richness and food web structure of soil decomposer community as affected by the size of habitat fragment and habitat corridors

While most ecologists agree that the effects of fragmentation on diversity of organisms are predominantly negative and that the scale of fragmentation defines their severity, the role of habitat corridors in mitigating those effects still remains controversial. This ambiguousness rests largely on various difficulties in experimentation, a problem partially solved in the present paper by the use of easily manipulated soil communities. In this 2.5‐year‐long field experiment, we investigated the responses of soil decomposer organisms (from microbes to mesofaunal predators) to habitat fragment size, in the presence or absence of habitat corridors connecting the fragments. The habitat fragments and corridors, composed of forest humus soil, were embedded in mineral soil representing an uninhabitable (or nonpreferred) matrix for the decomposer organisms. The results demonstrate that soil decomposer organisms do respond to changes in their habitat size: the species richness of microarthropods (mites and collembolans) increased as the size of the fragments increased. Especially collembolan species and predatory mites proved to be sensitive to the restricted habitat size, which is suggested to be a consequence of the large proportion of rare species and small and fluctuating population sizes in these groups. Contrary to our expectations, the presence of corridors had no positive effects on species richness or abundance of any of the studied faunas, possibly because of the low quality of the corridors. On the other hand, the biomass of soil fungi increased in the presence of corridors, which apparently provided a preferred pathway for vegetative dispersal of the fungi. Our results indicate that despite their characteristic underground environment, the response of soil decomposer organisms – in particular that of microarthropods – to habitat size is not unlike to that of the larger organisms in aboveground habitats.     

植物病原真菌1,8-间苯二酚黑色素研究进展

黑色素是一种广泛分布于生物体中的酚类聚合物疏水色素,分为1,8-间苯二酚(1,8-dihydroxynaphthalene,DHN)黑色素和3,4-二羟基苯丙氨酸(3,4-dihydroxyphenylalanine,L-DOPA)黑色素两种,其中DHN黑色素多存在于子囊菌门的植物病原真菌中。基因组和转录组技术的发展及功能基因组研究的深入,使DHN黑色素合成途径上关键基因在不同病原真菌中被鉴定,而且黑色素与真菌抗逆、发育和致病的关系受到越来越多的关注。本文阐述了DHN黑色素合成途径及其在真菌抗辐射与抗极端温度中的作用,以及黑色素对真菌侵染和细胞发育的影响,旨在加深人们对黑色素介导真菌与环境和寄主协同进化的认识,这对黑色素的基础研究和开发利用具有重要意义。     

Ecophysiology and breeding of mycoparasitic Trichoderma strains (a review)

Losses due to plant diseases may be as high as 10-20% of the total worldwide food production every year, resulting in economic losses amounting to many billions of dollars and diminished food supplies. Chemical control involves the use of chemical pesticides to eradicate or reduce the populations of pathogens or to protect the plants from infection by pathogens. For some diseases chemical control is very effective, but it is often non-specific in its effects, killing beneficial organisms as well as pathogens, and it may have undesirable health, safety, and environmental risks. Biological control involves the use of one or more biological organisms to control the pathogens or diseases. Biological control is more specialized and uses specific microorganisms that attack or interfere with the pathogens. The members of the genus Trichoderma are very promising against soil-born plant parasitic fungi. These filamentous fungi are very widespread in nature, with high population densities in soils and plant litters [1]. They are saprophytic, quickly growing and easy to culture and they can produce large amounts of conidia with long lifetime.     

Ornithine transcarbamylase from Neurospora crassa: purification and properties

Ornithine transcarbamylase catalyzes the synthesis of citrulline from carbamyl phosphate and ornithine. This enzyme is involved in the biosynthesis of arginine in many organisms and participates in the urea cycle of mammals. The biosynthetic ornithine transcarbamylase has been purified from the filamentous fungus, Neurospora crassa. It was found to be a homotrimer with an apparent subunit molecular weight of 37,000 and a native molecular weight of about 110,000. Its catalytic activity has a pH optimum of 9.5 and Km's of about 5 and 2.5 mM for the substrates, ornithine and carbamyl phosphate, respectively, at pH 9.5. The Km's and pH optimum are much higher than those of previously characterized enzymes from bacteria, other fungi, and mammals. These unusual kinetic properties may be of significance with regard to the regulation of ornithine transcarbamylase in this organism, especially in the avoidance of a futile ornithine cycle. Polyclonal antibodies were raised against the purified enzyme. These antibodies and antibody raised against purified rat liver ornithine transcarbamylase were used to examine the structural similarities of the enzyme from a number of organisms. Cross-reactivity was observed only for mitochondrial ornithine transcarbamylases of related organisms.