Carbon export from arbuscular mycorrhizal roots involves the translocation of carbohydrate as well as lipid

Arbuscular mycorrhizal (AM) fungi take up photosynthetically fixed carbon from plant roots and translocate it to their external mycelium. Previous experiments have shown that fungal lipid synthesized from carbohydrate in the root is one form of exported carbon. In this study, an analysis of the labeling in storage and structural carbohydrates after (13)C(1) glucose was provided to AM roots shows that this is not the only pathway for the flow of carbon from the intraradical to the extraradical mycelium (ERM). Labeling patterns in glycogen, chitin, and trehalose during the development of the symbiosis are consistent with a significant flux of exported glycogen. The identification, among expressed genes, of putative sequences for glycogen synthase, glycogen branching enzyme, chitin synthase, and for the first enzyme in chitin synthesis (glutamine fructose-6-phosphate aminotransferase) is reported. The results of quantifying glycogen synthase gene expression within mycorrhizal roots, germinating spores, and ERM are consistent with labeling observations using (13)C-labeled acetate and glycerol, both of which indicate that glycogen is synthesized by the fungus in germinating spores and during symbiosis. Implications of the labeling analyses and gene sequences for the regulation of carbohydrate metabolism are discussed, and a 4-fold role for glycogen in the AM symbiosis is proposed: sequestration of hexose taken from the host, long-term storage in spores, translocation from intraradical mycelium to ERM, and buffering of intracellular hexose levels throughout the life cycle.     

Mycorrhizal sink strength influences whole plant carbon balance of Trifolium repens L.

A comparative analysis of daily carbon (C) budgets and aspects of the C physiology of clover ( Trifolium repens L.) colonized by vesicular-arbuscular (VA) mycorrhizal fungi was carried out over a 70 d growth period under conditions designed to ensure that shoots of mycorrhizal (M) and non-mycorrhizal (NM) plants were of similar nutrient status. C budgets did not differ on day 24 but by day 42 M plants had a significantly higher rate of photosynthesis than their NM counterparts when expressed on a whole shoot basis or unit dry weight basis. As both sets of plants were of the same size it was concluded that this greater C gain was the result of increased sink strength provided by the mycorrhizal fungus. By day 53 M plants had become larger than their uncolonized counterparts and a sink-induced stimulation in the rate of photosynthesis was no longer apparent. M plants had higher root sucrose, glucose and fructose pools from day 24. Analyses suggested that these sugars were utilized for trehalose and lipid synthesis, for the production of the large extramatrical mycelium and for the support of the respiratory demands of the M root system. Increased C allocation to roots of M plants was associated with a stimulation of the activities of cell wall and cytoplasmic invertases and of sucrose synthase in roots colonized by VA fungi. Such increases in enzyme activity may provide the mechanism enabling increased partitioning of carbohydrate both to the M root system and the fungal symbiont.     

13C incorporation into signature fatty acids as an assay for carbon allocation in arbuscular mycorrhiza

The ubiquitous arbuscular mycorrhizal fungi consume significant amounts of plant assimilated C, but this C flow has been difficult to quantify. The neutral lipid fatty acid 16:1omega5 is a quantitative signature for most arbuscular mycorrhizal fungi in roots and soil. We measured carbon transfer from four plant species to the arbuscular mycorrhizal fungus Glomus intraradices by estimating (13)C enrichment of 16:1omega5 and compared it with (13)C enrichment of total root and mycelial C. Carbon allocation to mycelia was detected within 1 day in monoxenic arbuscular mycorrhizal root cultures labeled with [(13)C]glucose. The (13)C enrichment of neutral lipid fatty acid 16:1omega5 extracted from roots increased from 0.14% 1 day after labeling to 2.2% 7 days after labeling. The colonized roots usually were more enriched for (13)C in the arbuscular mycorrhizal fungal neutral lipid fatty acid 16:1omega5 than for the root specific neutral lipid fatty acid 18:2omega6,9. We labeled plant assimilates by using (13)CO(2) in whole-plant experiments. The extraradical mycelium often was more enriched for (13)C than was the intraradical mycelium, suggesting rapid translocation of carbon to and more active growth by the extraradical mycelium. Since there was a good correlation between (13)C enrichment in neutral lipid fatty acid 16:1omega5 and total (13)C in extraradical mycelia in different systems (r(2) = 0.94), we propose that the total amount of labeled C in intraradical and extraradical mycelium can be calculated from the (13)C enrichment of 16:1omega5. The method described enables evaluation of C flow from plants to arbuscular mycorrhizal fungi to be made without extraction, purification and identification of fungal mycelia.     

Aberrant nutritional regulation of carbohydrate synthesis by parasitized Manduca sexta L

The present studies confirm that storage carbohydrate synthesis from [1-(13)C]glucose is elevated in Manduca sexta parasitized by Cotesia congregata, despite a decrease in the rate of metabolism of the labeled substrate. Further, the results demonstrate that a similar pattern of carbohydrate synthesis and glucose metabolism was induced in normal larvae by administration of the glycolytic inhibitor, iodoacetate. (13)C enrichment of C6 of trehalose and glycogen demonstrated randomization of the C1 label at the triose phosphate step of the glycolytic/gluconeogenic pathway and suggested that gluconeogenesis, that is, de novo carbohydrate formation, contributed to the synthesis of carbohydrate in both normal and parasitized insects. Accounting for differences in the (13)C enrichment in C1 of trehalose and glycogen due to direct labeling from [1-(13)C]glucose, the mean C6/C1 labeling ratios in trehalose and glycogen of parasitized larvae and insects treated with iodoacetate were greater than the mean ratio observed in normal larvae, suggesting a greater contribution of gluconeogenesis to trehalose labeling in parasitized insects. This conclusion was confirmed by additional investigations on the metabolism of [3-(13)C]alanine by normal and parasitized insects. The pattern of (13)C enrichment in hemolymph trehalose observed in normal larvae maintained on a low carbohydrate diet indicated a large contribution of gluconeogenesis, while gluconeogenesis contributed very little to trehalose labeling in normal insects maintained on a high carbohydrate diet. Parasitized insects maintained on a high or a low carbohydrate diet displayed a significantly greater contribution of gluconeogenesis to trehalose labeling than was observed in normal larvae maintained on the same diets. In conclusion, these investigations indicate that regulation over the utilization of dietary glucose for trehalose and glycogen synthesis as well as the dietary regulation of de novo carbohydrate synthesis were altered by parasitism.     

The uptake, metabolism, transport and transfer of nitrogen in an arbuscular mycorrhizal symbiosis

Nitrogen (N) is known to be transferred from fungus to plant in the arbuscular mycorrhizal (AM) symbiosis, yet its metabolism, storage and transport are poorly understood. In vitro mycorrhizas of Glomus intra-radices and Ri T-DNA-transformed carrot roots were grown in two-compartment Petri dishes. (15)N- and/or (13)C-labeled substrates were supplied to either the fungal compartment or to separate dishes containing uncolonized roots. The levels and labeling of free amino acids (AAs) in the extra-radical mycelium (ERM) in mycorrhizal roots and in uncolonized roots were measured by gas chromatography/mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC). Arginine (Arg) was the predominant free AA in the ERM, and almost all Arg molecules became labeled within 3 wk of supplying (15)NH(4) (+) to the fungal compartment. Labeling in Arg represented > 90% of the total (15)N in the free AAs of the ERM. [Guanido-2-(15)N]Arg taken up by the ERM and transported to the intra-radical mycelium (IRM) gave rise to (15)N-labeled AAs. [U-(13)C]Arg added to the fungal compartment did not produce any (13)C labeling of other AAs in the mycorrhizal root. Arg is the major form of N synthesized and stored in the ERM and transported to the IRM. However, NH(4) (+) is the most likely form of N transferred to host cells following its generation from Arg breakdown.     

Arginine bi-directional translocation and breakdown into ornithine along the arbuscular mycorrhizal mycelium

Bi-directional translocation and degradation of Arginine (Arg) along the arbuscular mycorrhizal (AM) fungal mycelium were testified through ¹⁵N and/or ¹³C isotopic labeling. In vitro mycorrhizas of Glomus intraradices and Ri T-DNA-transformed carrot roots were grown in dual compartment Petri dishes. [¹⁵N- and/or¹³C]Arg was supplied to either the fungal compartment or the mycorrhizal compartment or separate dishes containing the uncolonized roots. The levels and labeling of free amino acids (AAs) in the mycorrhizal roots and in the extraradical mycelia(ERM) were measured by gas chromatography/mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC). The ERM of AM fungi exposed in either NH₄ ⁺ or urea as sole external nitrogen source had much higher ¹⁵N enrichment of Arg, compared with those in nitrate or exogenous Arg; however, glycerol supplied as an external carbon source to the ERM had no significant effect on the level of Arg in the ERM. Meanwhile, Arg biosynthesized in the ERM could be translocated intact to the mycorrhizal roots and thereby the level of Arg in the mycorrhizal roots increased to about 20% after culture of ERM in 4 mmol/L NH₄ ⁺ for 6 weeks. Also Arg was found to be bi-directionally transported along the AM fungal mycelium through [U-¹³C]Arg labeling either in the mycorrhizal compartment or in the fungal compartment. Once Arg was translocated to the potential N-limited sites, it would be further degraded into ornithine (Orn) and urea since either [U-¹³C] or [U-¹⁵N/U-¹³C]Orn was apparently shown up in the mycorrhizal root tissues when [U-¹³C] or [U-¹⁵N/U-¹³C]Arg was labeled in the fungal compartment, respectively. Evidently Orn formation indicated the ongoing activities of Arg translocation and degradation through the urea cycle in AM fungal mycelium. Supported by Science and Technology Department of Zhejiang Province (Grant No. 2006C22009).     

Influence of three vesicular-arbuscular mycorrhizal fungi (Glomaceae) on the activity of specific enzymes in the root system of Cucumis sativus L.

The influence of three vesicular-arbuscular mycorrhizal (VAM) Glomus species on the activity of enzymes in the roots of Cucumis sativus was tested. Cucumber plants were grown in a split-root system, in which colonized and uncolonized roots of a single plant could be separated. The activity of the host root malate dehydrogenase (MDH), glucose 6-phosphate dehydrogenase (Gd), glutamate oxaloacetate transaminase (GOT) and glutamate dehydrogenase (GDH) was measured on a densitometer after separation of the host and fungal enzymes on polyacrylamide gels.The results showed that only minor changes in the activity of the host root enzymes occurred after VAM inoculation. Gd was stimulated by VAM and phosphorus, and one of the fungi decreased the activity of GDH in the host plant when both parts of the root system were colonized.     

13C Incorporation into Signature Fatty Acids as an Assay for Carbon Allocation in Arbuscular Mycorrhiza

The ubiquitous arbuscular mycorrhizal fungi consume significant amounts of plant assimilated C, but this C flow has been difficult to quantify. The neutral lipid fatty acid 16:1ω5 is a quantitative signature for most arbuscular mycorrhizal fungi in roots and soil. We measured carbon transfer from four plant species to the arbuscular mycorrhizal fungus Glomus intraradices by estimating ¹³C enrichment of 16:1ω5 and compared it with ¹³C enrichment of total root and mycelial C. Carbon allocation to mycelia was detected within 1 day in monoxenic arbuscular mycorrhizal root cultures labeled with [¹³C]glucose. The ¹³C enrichment of neutral lipid fatty acid 16:1ω5 extracted from roots increased from 0.14% 1 day after labeling to 2.2% 7 days after labeling. The colonized roots usually were more enriched for ¹³C in the arbuscular mycorrhizal fungal neutral lipid fatty acid 16:1ω5 than for the root specific neutral lipid fatty acid 18:2ω6,9. We labeled plant assimilates by using ¹³CO₂ in whole-plant experiments. The extraradical mycelium often was more enriched for ¹³C than was the intraradical mycelium, suggesting rapid translocation of carbon to and more active growth by the extraradical mycelium. Since there was a good correlation between ¹³C enrichment in neutral lipid fatty acid 16:1ω5 and total ¹³C in extraradical mycelia in different systems (r² = 0.94), we propose that the total amount of labeled C in intraradical and extraradical mycelium can be calculated from the ¹³C enrichment of 16:1ω5. The method described enables evaluation of C flow from plants to arbuscular mycorrhizal fungi to be made without extraction, purification and identification of fungal mycelia.     

Carbohydrate and Amino Acid Metabolism in the Ectomycorrhizal Ascomycete Sphaerosporella brunnea during Glucose Utilization : A C NMR Study

Nuclear magnetic resonance spectroscopy was utilized to study the metabolism of [1-¹³C]glucose in mycelia of the ectomycorrhizal ascomycete Sphaerosporella brunnea. The main purpose was to assess the biochemical pathways for the assimilation of glucose and to identify the compounds accumulated during glucose assimilation. The majority of the ¹³C label was incorporated into mannitol, while glycogen, trehalose and free amino acids were labeled to a much lesser extent. The high enrichment of the C1/C6 position of mannitol indicated that the polyol was formed via a direct route from absorbed glucose. Randomization of the ¹³C label was observed to occur in glucose and trehalose leading to the accumulation of [1,6-¹³C]trehalose and [1,6-¹³C]glucose. This suggests that the majority of the glucose carbon used to form trehalose was cycled through the metabolically active mannitol pool. The proportion of label entering the free amino acids represented 38% of the soluble ¹³C after 6 hours of continuous glucose labeling. Therefore, amino acid biosynthesis is an important sink of assimilated carbon. Carbon-13 was incorporated into [3-¹³C]alanine and [2-¹³C]-, [3-¹³C]-, and [4-¹³C]glutamate and glutamine. From the analysis of the intramolecular ¹³C enrichment of these amino acids, it is concluded that [3-¹³C]pyruvate, arising from [1-¹³C]glucose catabolism, was used by alanine aminotransferase, pyruvate dehydrogenase, and pyruvate carboxylase (or phosphoenolpyruvate carboxykinase). Intramolecular ¹³C labeling patterns of glutamate and glutamine were similar and are consistent with the operation of the Krebs cycle. There is strong evidence for (a) randomization of the label on C2 and C3 positions of oxaloacetate via malate dehydrogenase and fumarase, and (b) the dual biosynthetic and respiratory role of the citrate synthase, aconitase, and isocitrate dehydrogenase reactions. The high flux of carbon through the carboxylation (presumably pyruvate carboxylase) step indicates that CO₂ fixation is an important component of the carbon metabolism in S. brunnea, and it is likely that this anaplerotic role is particularly prevalent during NH₄⁺ assimilation. The most relevant information resulting from this investigation is (a) the occurrence of the mannitol cycle, (b) a large part of the trehalose pool is synthesized after the cycling of glucose-carbon through the mannitol cycle, and (c) pyruvate (or phosphoenolpyruvate) carboxylation plays an important role in the primary metabolism of glucose-fed mycelia.     

菌根真菌与植物共生营养交换机制研究进展

菌根是陆地生态系统普遍存在的、由土壤中的菌根真菌侵染宿主植物根系形成的联合共生体.菌根的建立是以共生体双方的营养交换为基础的:菌根真菌从土壤中吸收氮、磷等营养物质并转运给宿主植物,供其生长;作为交换,植物则以脂质或糖的形式向菌根真菌提供其生长所必需的碳水化合物.近年来,菌根真菌与宿主植物间的营养交换机制一直是研究的热点,国内外对菌根真菌介导的植物营养物质吸收和转运机制的研究也取得了巨大进展.本文综述了丛枝和外生两种菌根真菌与宿主植物间营养交换的最新研究进展,尤其是碳、氮、磷等几种重要营养物质的吸收与双向转运机制,以及营养交换在菌根形成中的潜在调控作用,并对目前存在的关键问题和未来研究方向进行了分析和展望,这对菌根模型的建立及菌根效益的优化具有重要意义.     

Natural Abundance of <Superscript>15</Superscript>N in Nitrogen-Limited Forests and Tundra Can Estimate Nitrogen Cycling Through Mycorrhizal Fungi: A Review

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural ¹⁵N:¹⁴N ratios (expressed as δ¹⁵N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against ¹⁵N when they create compounds for transfer to plants; host plants are depleted in ¹⁵N, whereas mycorrhizal fungi are enriched in ¹⁵N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ¹⁵N increases. Careful analyses of δ¹⁵N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

More than 400 million years of evolution and some plants still can't make it on their own: plant stress tolerance via fungal symbiosis

All plants in natural ecosystems are thought to be symbioticwith mycorrhizal and/or endophytic fungi. Collectively, thesefungi express different symbiotic lifestyles ranging from parasitismto mutualism. Analysis of Colletotrichum species indicates thatindividual isolates can express either parasitic or mutualisticlifestyles depending on the host genotype colonized. The endophytecolonization pattern and lifestyle expression indicate thatplants can be discerned as either disease, non-disease, or non-hosts.Fitness benefits conferred by fungi expressing mutualistic lifestylesinclude biotic and abiotic stress tolerance, growth enhancement,and increased reproductive success. Analysis of plant–endophyteassociations in high stress habitats revealed that at leastsome fungal endophytes confer habitat-specific stress toleranceto host plants. Without the habitat-adapted fungal endophytes,the plants are unable to survive in their native habitats. Moreover,the endophytes have a broad host range encompassing both monocotsand eudicots, and confer habitat-specific stress tolerance toboth plant groups. Key words: Colletotrichum, fungal endophytes, stress tolerance, symbiosis, symbiotic lifestyle Received 19 June 2007; Revised 25 November 2007 Accepted 30 November 2007     

Carbon Metabolism in Spores of the Arbuscular Mycorrhizal Fungus Glomus intraradices as Revealed by Nuclear Magnetic Resonance Spectroscopy

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that colonize the roots of over 80% of plants in all terrestrial environments. Understanding why AM fungi do not complete their life cycle under free-living conditions has significant implications for the management of one of the world's most important symbioses. We used (13)C-labeled substrates and nuclear magnetic resonance spectroscopy to study carbon fluxes during spore germination and the metabolic pathways by which these fluxes occur in the AM fungus Glomus intraradices. Our results indicate that during asymbiotic growth: (a) sugars are made from stored lipids; (b) trehalose (but not lipid) is synthesized as well as degraded; (c) glucose and fructose, but not mannitol, can be taken up and utilized; (d) dark fixation of CO(2) is substantial; and (e) arginine and other amino acids are synthesized. The labeling patterns are consistent with significant carbon fluxes through gluconeogenesis, the glyoxylate cycle, the tricarboxylic acid cycle, glycolysis, non-photosynthetic one-carbon metabolism, the pentose phosphate pathway, and most or all of the urea cycle. We also report the presence of an unidentified betaine-like compound. Carbon metabolism during asymbiotic growth has features in between those presented by intraradical and extraradical hyphae in the symbiotic state.     

A mutualistic symbiosis between a dark septate endophytic fungus, Heteroconium chaetospira, and a nonmycorrhizal plant, Chinese cabbage

Symbiotic microorganisms, such as mycorrhizal fungi, are known to associate with most plants; however members of the Cruciferae are an exception. We investigated nutrient exchange between a dark septate endophytic fungus, Heteroconium chaetospira, and Chinese cabbage plants (Cruciferae) in vitro. Chinese cabbage could not use some amino acids, while the fungus-treated plants were able to use all of the nitrogen forms provided. To demonstrate that nitrogen transfer occurs between the fungus and the host plant, we used a hydrophobic polytetrafluoroethylene (PTFE) membrane compartment system, which restricts diffusion and mass flow of ions and allows only fungal penetration. Our results strongly suggest that H. chaetospira provided nitrogen to the plant, rather than the plant mineralizing available organic nitrogen. In addition carbon transfer from the host plant to the fungus was demonstrated with HPLC and (l3)CO2-labeling experiments. When H. chaetospira colonized host plant roots under low glucose condition, ergosterol content in culture pot (as an index of fungal biomass) increased significantly compared to the fungal treatment without a host plant. Sucrose concentration in the host root significantly decreased as a result of fungal colonization, and mannitol (a specific carbon source to fungal cells) increased in the roots. Sucrose and mannitol in the host root treated with the fungus were labeled clearly by 13C after 1C-labeled CO2 was provided to the plant. These results suggest that the fungus obtained carbon, mainly as sucrose, from the host plant. We show for the first time the existence of a fungus establishing a mutualistic association with a nonmycorrhizal Cruciferae plant.     

Seasonal and environmental changes of mycorrhizal associations and heterotrophy levels in mixotrophic Pyrola japonica (Ericaceae) growing under different light environments

? Premise of the study: Mixotrophy is a strategy whereby plants acquire carbon both through photosynthesis and heterotrophic exploitation of mycorrhizal fungi. In Euro-American Pyroleae species studied hitherto, heterotrophy levels vary according to species, sites of study, and possibly light conditions. We investigated mycorrhizal association and mixotrophy in the Asiatic forest species Pyrola japonica, and their plasticity under different light conditions. ? Methods: Pyrola japonica was sampled bimonthly in sunny and shaded conditions from a deciduous broadleaf forest. We microscopically assessed the rate of fungal colonization and sequenced the ITS to identify the mycorrhizal fungi. We measured (13)C and (15)N isotopic abundances in P. japonica as compared with neighboring autotrophic and mycoheterotrophic plants, to evaluate P. japonica's heterotrophy level. ? Key results: Pyrola japonica formed arbutoid mycorrhizas devoid of fungal mantles, with intracellular hyphal coils and a Hartig net. It tended to be more colonized by mycorrhizal fungi in spring and summer. Most associated fungi belonged to ectomycorrhizal taxa, and 84% of identified fungi were Russula spp. Rate of mycorrhizal colonization and Russula frequency tended to be higher in shaded conditions. Both δ(13)C and δ(15)N values of P. japonica were significantly higher in autotrophic plants, showing that about half of the carbon on average was received from mycorrhizal fungi. Both isotopic values negatively correlated with light availability, suggesting higher heterotrophy levels in shaded conditions. ? Conclusions: The mixotrophic P. japonica undergoes changes in mycorrhizal symbionts and carbon nutrition according to light availability. Our results suggest that during Pyroleae evolution, a tendency to increased heterotrophy emerged in the Pyrola/Orthilia clade.     

Forms of nitrogen uptake, translocation, and transfer via arbuscular mycorrhizal fungi: A review

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that colonize the roots of more than 80% of land plants. Experiments on the relationship between the host plant and AM in soil or in sterile root-organ culture have provided clear evidence that the extraradical mycelia of AM fungi uptake various forms of nitrogen (N) and transport the assimilated N to the roots of the host plant. However, the uptake mechanisms of various forms of N and its translocation and transfer from the fungus to the host are virtually unknown. Therefore, there is a dearth of integrated models describing the movement of N through the AM fungal hyphae. Recent studies examined Ri T-DNA-transformed carrot roots colonized with AM fungi in ¹⁵N tracer experiments. In these experiments, the activities of key enzymes were determined, and expressions of genes related to N assimilation and translocation pathways were quantified. This review summarizes and discusses the results of recent research on the forms of N uptake, transport, degradation, and transfer to the roots of the host plant and the underlying mechanisms, as well as research on the forms of N and carbon used by germinating spores and their effects on amino acid metabolism. Finally, a pathway model summarizing the entire mechanism of N metabolism in AM fungi is outlined.     

丛枝菌根真菌在内的土壤真菌对入侵植物紫茎泽兰的正反馈：证据来自野外实验

包括紫茎泽兰在内的许多外来植物都能够与新入侵生境的丛枝菌根真菌（ AMF）形成互利共生,因此菌根真菌如何调节外来植物种的入侵是当前亟待研究的问题。测定了紫茎泽兰入侵不同阶段（紫茎泽兰呈零星丛状分布于本地植物群落中[部分入侵生境]及紫茎泽兰单优群落形成期[入侵生境]）的土壤化学性状,而后通过野外试验,采用杀真菌剂处理,研究了包括AMF在内的土壤真菌对紫茎泽兰入侵的反馈作用。紫茎泽兰入侵改变了土壤化学性状。施用杀真菌剂降低了紫茎泽兰叶面积、叶片碳、氮、磷、和δ13 C含量。综合分析发现,在紫茎泽兰与本地植物混生群落中,土壤真菌能够增加紫茎泽兰叶片碳和δ13 C含量,但是不能提高紫茎泽兰的光合作用,表明碳和δ13 C含量的提高,不是光合作用的结果,而是通过其他机制实现的。因此可以得出,在部分入侵生境中,碳从土壤或临近植物经由菌丝网向紫茎泽兰转移。紫茎泽兰入侵不同阶段土壤养分的变化利于紫茎泽兰种群建立,同时利于紫茎泽兰借助真菌（尤其是AMF）从土壤或临近植物转移碳,促进种群扩散,这可能是紫茎泽兰入侵的机制之一。     

Plugging into the network: belowground connections between germlings and extraradical mycelium of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs; nevertheless their spores can germinate in the absence of host plants. Such inconsistent behavior is balanced by diverse survival strategies. The ability of AM fungal hyphae to fuse might represent a fundamental survival strategy because germlings could plug into compatible mycorrhizal networks, thus gaining access to plant-derived carbon before asymbiotic growth arrest. An in vivo experimental system was used to grow extraradical mycelium produced by Glomus mosseae colonizing three different plant species and germlings of the same isolate. After symbiotic and asymbiotic mycelia came into contact we showed that germling hyphae fused with symbiotic network hyphae and established protoplasm connections with nuclei occurring in fusion bridges. The frequency of anastomoses between germling and symbiotic hyphae was 4.9-23.9%. Prefusion and postfusion incompatible responses, with protoplasm withdrawal in interacting hyphae, were evident in some hyphal contacts. Given the multigenomic nature of AMF, the mingling of germling nuclei with those of the mycorrhizal network through perfect fusions might represent a means for the maintenance of genetic diversity in the absence of sexual recombination.     

Influence of glycogen storage on vascular smooth muscle metabolism

The role of glycogen as an oxidative substrate for vascular smooth muscle (VSM) remains controversial. To elucidate the importance of glycogen as an oxidative substrate and the influence of glycogen flux on VSM substrate selection, we systematically altered glycogen levels and measured metabolism of glucose, acetate, and glycogen. Hog carotid arteries with glycogen contents ranging from 1 to 11 micromol/g were isometrically contracted in physiological salt solution containing 5 mM [1-(13)C]glucose and 1 mM [1, 2-(13)C]acetate at 37 degrees C for 6 h. [1-(13)C]glucose, [1, 2-(13)C]acetate, and glycogen oxidation were simultaneously measured with the use of a (13)C-labeled isotopomer analysis of glutamate. Although oxidation of glycogen increased with the glycogen content of the tissue, glycogen oxidation contributed only approximately 10% of the substrate oxidized by VSM. Whereas [1-(13)C]glucose flux, [3-(13)C]lactate production from [1-(13)C]glucose, and [1, 2-(13)C]acetate oxidation were not regulated by glycogen content, [1-(13)C]glucose oxidation was significantly affected by the glycogen content of VSM. However, [1-(13)C]glucose remained the primary ( approximately 40-50%) contributor to substrate oxidation. Therefore, we conclude that glucose is the predominate substrate oxidized by VSM, and glycogen oxidation contributes minimally to substrate oxidation.