VA菌根共生的起源和进化

ＶＡ菌根共生的起源和进化赵之伟（云南大学生物学系,昆明６５００９１）ＯｒｉｇｉｎａｎｄＥｖｏｌｕｔｉｏｎｏｆｔｈｅＶＡＭｙｃｏｒｒｈｉｚａｌＳｙｍｂｉｏｓｉｓ．ＺｈａｏＺｈｉｗｅｉ（ＢｉｏｌｏｇｙＤｅｐａｒｔｍｅｎｔｏｆＹｕｎｎａｎＵｎｉｖｅｒｓｉｔ...     

Translocation and Utilization of Fungal Storage Lipid in the Arbuscular Mycorrhizal Symbiosis

The arbuscular mycorrhizal (AM) symbiosis is responsible for huge fluxes of photosynthetically fixed carbon from plants to the soil. Carbon is transferred from the plant to the fungus as hexose, but the main form of carbon stored by the mycobiont at all stages of its life cycle is triacylglycerol. Previous isotopic labeling experiments showed that the fungus exports this storage lipid from the intraradical mycelium (IRM) to the extraradical mycelium (ERM). Here, in vivo multiphoton microscopy was used to observe the movement of lipid bodies through the fungal colony and to determine their sizes, distribution, and velocities. The distribution of lipid bodies along fungal hyphae suggests that they are progressively consumed as they move toward growing tips. We report the isolation and measurements of expression of an AM fungal expressed sequence tag that encodes a putative acyl-coenzyme A dehydrogenase; its deduced amino acid sequence suggests that it may function in the anabolic flux of carbon from lipid to carbohydrate. Time-lapse image sequences show lipid bodies moving in both directions along hyphae and nuclear magnetic resonance analysis of labeling patterns after supplying 13C-labeled glycerol to either extraradical hyphae or colonized roots shows that there is indeed significant bidirectional translocation between IRM and ERM. We conclude that large amounts of lipid are translocated within the AM fungal colony and that, whereas net movement is from the IRM to the ERM, there is also substantial recirculation throughout the fungus.     

Net Accumulation of Minerals and Water and the Carbon Budget in an Expanding Leaf of Tomato

The changes in f. wt, d. wt, nitrogen, potassium, calcium, phosphorus,magnesium and carbon of the seventh leaf of a tomato plant weremeasured at seven occasions from 10 days to 30 days after leafemergence. Measurements of CO₂ exchange by the leaf during bothlight (70 W m^–2, 7 h) and dark (17 h) periods and thechange in carbon content over these two periods enabled a carbonbalance to be constructed on these seven occasions. Changesin the sugars and starch contents of the leaf over these twoperiods at each occasion were measured. With the exception of calcium the rates of accumulation of allsubstances increased to their maxima when the leaf was 22–24-days-old.Carbon fixation per unit f. wt. increased to a maximum whenthe leaf was 16-days-old. In a 10-day-old leaf the rate of carbonfixation was already four-fifths of maximum and one-quartermore than that at 30 days. The rate of night respiratory lossof carbon per unit fresh weight decreased as the leaf expanded.In a 10-day-old leaf, the amount of carbon lost by night respirationaccounted for one-quarter of that fixed in the same day. Thisfraction fell to one-tenth when the leaf was 22-days-old andremained constant thereafter. The amount of carbon being importedto the leaflets of a 10-day-old leaf was less than one-quarterof that accumulated in 1 day. Thus, the contribution of theimported carbon to the leaf growth up to this stage is relativelysmall. The transition of the seventh leaf from being a net importerto being a net exporter occurred when the leaf was 13-days-old. The sucrose content per unit f. wt was higher in the youngerthan in the older leaves and was not correlated to the transitionfrom net import to net export. The accumulation and breakdownof starch in a leaf were related not only to the growth of theleaf but also to the development of the whole plant. Lycopersicon esculentum, tomato, leaf, accumulation of minerals, water content, carbon budget     

Response of lentil to VA Mycorrhizal inoculation and plant available P levels of unsterile soils

Summary Responses of lentil in unsterile soils at low, medium and high levels of plant available soil P toGlomus fasciculatum inoculation were evaluated. It was observed that growth, dry matter accumulation, nodulation, and nitrogen fixation were considerably improved in VAM inoculated plants over uninoculated control at low and medium levels of plant available soil P.     

Relative Importance of Deterministic and Stochastic Processes in Driving Arbuscular Mycorrhizal Fungal Assemblage during the Spreading of a Toxic Plant

Both deterministic and stochastic processes are expected to drive the assemblages of arbuscular mycorrhizal (AM) fungi, but little is known about the relative importance of these processes during the spreading of toxic plants. Here, the species composition and phylogenetic structure of AM fungal communities colonizing the roots of a toxic plant, Ligularia virgaurea, and its neighborhood plants, were analyzed in patches with different individual densities of L. virgaurea (represents the spreading degree). Community compositions of AM fungi in both root systems were changed significantly by the L. virgaurea spreading, and also these communities fitted the neutral model very well. AM fungal communities in patches with absence and presence of L. virgaurea were phylogenetically random and clustered, respectively, suggesting that the principal ecological process determining AM fungal assemblage shifted from stochastic process to environmental filtering when this toxic plant was present. Our results indicate that deterministic and stochastic processes together determine the assemblage of AM fungi, but the dominant process would be changed by the spreading of toxic plants, and suggest that the spreading of toxic plants in alpine meadow ecosystems might be involving the mycorrhizal symbionts.     

浙江镜湖湿地3种菌根植物根部真菌群落多样性及对金属元素富集能力的研究

城市湿地（urban wetland）植物多样性水平高,并具有积极的净化和美化环境的作用。本研究以浙江绍兴镜湖城市湿地公园为研究样地,通过ICP-AES检测了人工园区和次生林土壤中几种金属元素（Al、Cr、Cu、K和Zn）的含量,比较不同类型的菌根植物——黑麦草（Lolium perenne L.）（AM）、六月霜（Monochasma sauatieri Franch）（AM）和乌饭树（Vaccinium bracteatum Thunb）（ERM）对金属元素的富集作用。通过高通量测序分析各宿主植物根部真菌群落的组成和结构,比较其对宿主植物金属富集能力的影响。结果表明：（1）人工园区土壤中5种金属元素的含量均显著高于次生林下的土壤;（2）乌饭树对Al的富集能力较强,黑麦草和六月霜富集Cr能力都较强;（3）3种植物根部真菌主要来自Ascomycota,而乌饭树根部Basidiomycota也是优势真菌,AM真菌在黑麦草和六月霜根部真菌群落结构中较少,而公认的ERM真菌Helotiales和Sebacinales在乌饭树根部真菌群落中的比例较高,且与所测金属元素无显著相关性。绍兴镜湖城市湿地公园次生林土壤环境保持良好。     

Narrow Fungal Mycorrhizal Diversity in a Population of the Orchid Coppensia doniana

In nature, orchids are fully dependent on mycorrhizal fungi to germinate their seeds. These fungi can penetrate root cells and form pelotons, hyphal coils responsible for providing simple sugars for the orchid embryo. During the achlorophyllous seedling stage, orchids depend on fungi; and some species remain dependent through life, while others establish photosynthesis but, to varying degrees, remain facultatively dependent or responsive to fungal colonization as adults. The aim of this study was to identify how many clades of fungi can establish mycorrhizal associations with Coppensia doniana, a widespread orchid in Campos do Jordão (Brazil) and to demonstrate how morphological features can be useful to group and identify these fungi. Plants were collected during the dry season of 2009 near Campos do Jordão State Park. Fungi were isolated by transferring root segments containing pelotons to media. Three main clades of fungi were formed by either qualitative or quantitative morphological data. We identified these fungi as two morphotypes of Ceratorhiza (anamorphic stage of Ceratobasidium) and one uninucleate Rhizoctonia. The internal transcribed spacer (ITS) sequencing data corroborated the morphological features showing the same three clades; all isolates showed a high similarity with Ceratobasidium ITS sequences in GenBank. Thus, it was possible to demonstrate a high affinity between this orchid species of Oncidiinae and Ceratobasidium. Morphological data associated with multivariate statistics proved to be a useful tool in multilevel taxonomy of these orchid‐associated fungi. Abstract in Portuguese is available in the online version of this article.     

Genome-Wide Analysis of AMT Gene Family and its Response to Mycorrhizal Symbiosis in Maize

Journal of Plant Growth Regulation - Nitrogen can greatly increase the yield of crops, but excessive use of nitrogen fertilizer in agriculture will cause environmental pollution. Therefore, it is...     

Effects of Inoculum Additions in the Presence of a Preestablished Arbuscular Mycorrhizal Fungal Community

Communities of arbuscular mycorrhizal fungi (AMF) are crucial for promoting plant productivity in most terrestrial systems, including anthropogenically managed ecosystems. Application of AMF inocula has therefore become a widespread practice. It is, however, pertinent to understand the mechanisms that govern AMF community composition and their performance in order to design successful manipulations. Here we assess whether the composition and plant growth-promotional effects of a synthetic AMF community can be altered by inoculum additions of the isolates forming the community. This was determined by following the effects of three AMF isolates, each inoculated in two propagule densities into a preestablished AMF community. Fungal abundance in roots and plant growth were evaluated in three sequential harvests. We found a transient positive response in AMF abundance to the intraspecific inoculation only in the competitively weakest isolate. The other two isolates responded negatively to intra- and interspecific inoculations, and in some cases plant growth was also reduced. Our results suggest that increasing the AMF density may lead to increased competition among fungi and a trade-off with their ability to promote plant productivity. This is a key ecological aspect to consider when introducing AMF into soils.     

Seasonal Dynamics of Arbuscular Mycorrhizal Fungal Communities in Roots in a Seminatural Grassland

Symbiotic arbuscular mycorrhizal fungi (AMF) have been shown to influence both the diversity and productivity of grassland plant communities. These effects have been postulated to depend on the differential effects of individual mycorrhizal taxa on different plant species; however, so far there are few detailed studies of the dynamics of AMF colonization of different plant species. In this study, we characterized the communities of AMF colonizing the roots of two plant species, Prunella vulgaris and Antennaria dioica, in a Swedish seminatural grassland at different times of the year. The AMF small subunit rRNA genes were subjected to PCR, cloning, sequencing, and phylogenetic analysis. Nineteen discrete sequence types belonging to Glomus groups A and B and to the genus Acaulospora were distinguished. No significant seasonal changes in the species compositions of the AMF communities as a whole were observed. However, the two plant species hosted significantly different AMF communities. P. vulgaris hosted a rich AMF community throughout the entire growing season. The presence of AMF in A. dioica decreased dramatically in autumn, while an increased presence of Ascomycetes species was detected.     

云南热带,亚热带蕨类植物根际土壤中的VA菌根真菌

对云南热带,亚热带地我的２５６种蕨类植物的ＶＡ菌根状况进行了调查;从具有ＶＡ菌根的蕨类植物根际土壤中分离鉴定了分属于无梗２囊霉巨孢囊霉球囊霉,硬囊霉属和盾孢囊霉属的２５种ＶＡ菌根真菌,其中,８种属于国内新记录种。本文对这些植物的ＶＡ菌根状况及其进化进行了讨论,并对２５种真菌的分类特征进行了描述。     

Osmotic Adjustment in Leaves of VA Mycorrhizal and Nonmycorrhizal Rose Plants in Response to Drought Stress

Osmotic adjustment in Rosa hybrida L. cv Samantha was characterized by the pressure-volume approach in drought-acclimated and unacclimated plants brought to the same level of drought strain, as assayed by stomatal closure. Plants were colonized by either of the vesicular-arbuscular mycorrhizal fungi Glomus deserticola Trappe, Bloss and Menge or G. intraradices Schenck and Smith, or were nonmycorrhizal. Both the acclimation and the mycorrhizal treatments decreased the osmotic potential (Ψ_π) of leaves at full turgor and at the turgor loss point, with a corresponding increase in pressure potential at full turgor. Mycorrhizae enabled plants to maintain leaf turgor and conductance at greater tissue water deficits, and lower leaf and soil water potentials, when compared with nonmycorrhizal plants. As indicated by the Ψ_π at the turgor loss point, the active Ψ_π depression which attended mycorrhizal colonization alone was 0.4 to 0.6 megapascals, and mycorrhizal colonization and acclimation in concert 0.6 to 0.9 megapascals, relative to unacclimated controls without mycorrhizae. Colonization levels and sporulation were higher in plants subjected to acclimation. In unacclimated hosts, leaf water potential, water saturation deficit, and soil water potential at a particular level of drought strain were affected most by G. intraradices. G. deserticola had the greater effect after drought preconditioning.     

The Flavin Content of Clovers Relative to Symbiosis with a Riboflavin-requiring Mutant of Rhizobium trifoli

A riboflavin-requiring auxotroph of Rhizobium trifolii (T1/D-his(r)-15) formed ineffective root nodules on red clover and on two cultivars of subterranean clover, but produced almost fully effective nodules on several other cultivars of subterranean clover. Fluorescence and bioassay measurements of the flavin content of the roots and shoots of these cultivars revealed no differences between cultivars which could be correlated with the differences in symbiotic response. The concentration of flavin in nodules formed by the auxotroph (in the absence of riboflavin), by the effective parent strain (T1), or by a partly effective mutant (penicillin-resistant) of T1 was roughly proportional to the effectiveness of the nodules. Effective nodules contained 20 times as much flavin, and ineffective nodules 3 to 4 times as much flavin as non-nodulated root tissue. Approximately 20 to 30% of the flavins in both root and nodule tissue was flavin adenine dinucleotide and 70 to 80% was riboflavin + flavin mononucleotide. Most of the flavin adenine dinucleotide in macerated nodules was associated with host cell fragments, and none was detected in a cell-free fraction. Bacteroids accounted for approximately 20% of flavins in effective nodules and also contained more riboflavin + flavin mononucleotide than cultured rhizobial cells. The total flavin content of noninoculated roots increased from about 1.2 nmoles to 1.7 nmoles flavin/g of tissue after 3 days' exposure to 80 mum riboflavin. Exposure of only the upper or lower portion of preinoculated roots indicated negligible translocation, as effective nodulation occurred only on parts of the root in direct contact with riboflavin. Plants grown in a medium containing combined nitrogen (100 or 300 mum nitrogen added as (NH(4))(2)SO(4)), but no added riboflavin showed an increased root flavin content (about 2.1 nmoles flavin/g tissue) and a partly effective response when inoculated with the mutant. Nitrogen also promoted some upward translocation of exogenous riboflavin in the roots.     

Fungal and Bacterial Colonization of Submerged Leaf Litter in a Mediterranean Stream

Microbial colonization dynamics of fungi and bacteria were analyzed in an intermittent Mediterranean forested stream using two different leaf substrata (Platanus acerifolia and Populus nigra). Results showed that fungal and bacterial biomass accumulation was stimulated on both leaves due to a flooding episode that increased dissolved inorganic nitrogen (DIN) and dissolved oxygen (DO) availability in the stream water. Leaf mass loss coincided with the parallel increase in microbial biomass and extracellular enzymatic activities after the flood event. Differences in litter quality favoured bacterial biomass accumulation and β‐glucosidase and cellobiohydrolase enzymatic activities in the soft Populus species. Microbial heterotrophs colonization of submerged leaf litter and organic matter use in Mediterranean‐type streams are modulated by environmental conditions, especially the hydrological variability. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Origin of a Fungal Symbiont of Perennial Ryegrass by Interspecific Hybridization of a Mutualist with the Ryegrass Choke Pathogen, Epichloe Typhina

Seed-borne fungal symbionts (endophytes) provide many cool-season grass species with biological protection from biotic and abiotic stresses. The endophytes are asexual, whereas closely related sexual species of genus Epichloe (Clavicipitales) cause grass choke disease. Perennial ryegrass (Lolium perenne) is a host of two endophyte taxa, LpTG-1 (L. perenne endophyte taxonomic grouping one = Acremonium lolii) and LpTG-2, as well as the choke pathogen, Epichloe typhina (represented by isolate E8). Relationships among these fungi and other Epichloe species were investigated by analysis of gene sequences, DNA polymorphisms and allozymes. The results indicate that LpTG-2 is a heteroploid derived from an interspecific hybrid. The LpTG-2 isolates had two copies each of nine out of ten genes analyzed (the exception being the rRNA gene locus), and the profiles for seven of these were composites of those from E. typhina E8 and A. lolii isolate Lp5. Molecular phylogenetic analysis grouped the two β-tubulin genes of LpTG-2 into separate clades. One (tub2-1) was related to that of E. typhina E8, and the other (tub2-2) to that of A. lolii. The mitochondrial DNA profile of LpTG-2 was similar to that of A. lolii, but its rRNA gene sequence grouped it with E. typhina E8. A proposed model for the evolution of LpTG-2 involves infection of a L. perenne-A. lolii symbiotum by E. typhina, followed by hybridization of the two fungi. Such interspecific hybridization may be a common and important mechanism for genetic variation in Epichloe endophytes.     

Effect of Inorganic Nutrients on Relative Contributions of Fungi and Bacteria to Carbon Flow from Submerged Decomposing Leaf Litter

The relative contributions of fungi and bacteria to carbon flow from submerged decaying plant litter at different levels of inorganic nutrients (N and P) were studied. We estimated leaf mass loss, fungal and bacterial biomass and production, and microbial respiration and constructed partial carbon budgets for red maple leaf disks precolonized in a stream and then incubated in laboratory microcosms at two levels of nutrients. Patterns of carbon flow for leaf disks colonized with the full microbial assemblage were compared with those colonized by bacteria but in which fungi were greatly reduced by placing leaf disks in colonization chambers sealed with membrane filters to exclude aquatic hyphomycete conidia but not bacterial cells. On leaves colonized by the full microbial assemblage, elevated nutrient concentrations stimulated fungi and bacteria to a similar degree. Peak fungal and bacterial biomass increased by factors of 3.9 and 4.0; cumulative production was 3.9 and 5.1 times higher in the high nutrient in comparison with the low nutrient treatment, respectively. Fungi dominated the total microbial biomass (98.4 to 99.8%) and cumulative production (97.3 and 96.5%), and the fungal yield coefficient exceeded that of bacteria by a factor of 36 and 27 in low- and high-nutrient treatments, respectively. Consequently, the dominant role of fungi in leaf decomposition did not change as a result of nutrient manipulation. Carbon budgets indicated that 8% of leaf carbon loss in the low-nutrient treatment and 17% in the high-nutrient treatment were channeled to microbial (essentially fungal) production. Nutrient enrichment had a positive effect on rate of leaf decomposition only in microcosms with full microbial assemblages. In treatments where fungal colonization was reduced, cumulative bacterial production did not change significantly at either nutrient level and leaf decomposition rate was negatively affected (high nutrients), suggesting that bacterial participation in carbon flow from decaying leaf litter is low regardless of the presence of fungi and nutrient availability. Moreover, 1.5 and 2.3 times higher yield coefficients of bacteria in the reduced fungal treatments at low and high nutrients, respectively (percentage of leaf carbon loss channeled to bacterial production), suggest that bacteria are subjected to strong competition with fungi for resources available in leaf litter.     

Role of Organic Carbon Sources and Sulfate in Controlling Net Methylmercury Production in Riverbank Sediments of the South River,VA (USA)

Mercury (Hg) transport and methylmercury (MeHg) production in riverbank sediments are complex processes influenced by site-specific physical and biogeochemical conditions. The South River watershed in VA, USA, contains elevated concentrations of Hg in riverbank and floodplain sediments, which has the potential to methylate. The role of specific organic carbon sources in promoting methylation reactions in natural sediments under dynamic flow conditions is not well understood. Four saturated column experiments were conducted, including a control column, which received South River water as an influent solution, and three columns that received South River water amended with: acetate (5.8 mM); lactate (5.7 mM); and lactate (5.7 mM) with SO₄^2? (10.1 mM). The amendments were selected to promote growth of different microorganisms to gain an understanding of the microbial processes, controlling rates of methylation. The column receiving lactate and SO₄^2? had the highest MeHg concentrations in the effluent and in the pore water near the effluent at 1.8 and 4.9 μg L^?1, respectively. At the cessation of the column experiments, the lactate–sulfate column sediments contained the highest populations of enumerable sulfur-reducing bacteria and the highest solid-phase MeHg at 530 ± 100 ng g^?1 dry wt. from the interval closest to the influent. The results suggest that the form and availability of electron donors and acceptors are primary factors controlling rates of methylation in the South River sediment.     

Alteration of Soil Carbon Pools and Communities of Mycorrhizal Fungi in Chaparral Exposed to Elevated Carbon Dioxide

We examined the effects of atmospheric carbon dioxide (CO₂) enrichment on belowground carbon (C) pools and arbuscular mycorrhizal (AM) fungi in a chaparral community in southern California. Chambers enclosing intact mesocosms dominated by Adenostoma fasciculatum were exposed for 3.5 years to CO₂ levels ranging from 250 to 750 ppm. Pools of total C in bulk soil and in water-stable aggregates (WSA) increased 1.5- and threefold, respectively, between the 250- and 650-ppm treatments. In addition, the abundance of live AM hyphae and spores rose markedly over the same range of CO₂, and the community composition shifted toward dominance by the AM genera Scutellospora and Acaulospora. Net ecosystem exchange of C with the atmosphere declined with CO₂ treatment. It appears that under CO₂ enrichment, extra C was added to the soil via AM fungi. Moreover, AM fungi were predominant in WSA and may shunt C into these aggregates versus bulk soil. Alternatively, C may be retained longer within WSA than within bulk soil. We note that differences between the soil fractions may act as a potential feedback on C cycling between the soil and atmosphere.