Molecular identification and phylogeny of arbuscular mycorrhizal fungi

The fossil record and molecular data show that the evolutionary history of arbuscular mycorrhizal fungi (Glomales) goes back at least to the Ordovician (460 million years ago), coinciding with the colonization of the terrestrial environment by the first land plants. At that time, the land flora only consisted of plants on the bryophytic level. Ribosomal DNA sequences indicate that the diversity within the Glomales on the family and genus level is much higher than previously expected from morphology-based taxonomy. Two deeply divergent lineages were found and described in two new genera, Archaeospora and Paraglomus, each in its own family. Based on a fast-growing number of available DNA sequences, several systems for molecular identification of the Glomales within roots have been designed and tested in the past few years. These detection methods have opened up entirely new perspectives for studying the ecology of arbuscular mycorrhiza.     

中国大陆地区丛枝菌根真菌菌种资源的分离鉴定与形态学特征

【目的】分离收集保藏中国大陆各个地区不同生态环境的丛枝菌根真菌菌种资源,为丛枝菌根的研究提供资源、奠定基础。【方法】以高粱为宿主植物,采用诱导培养、单孢培养和扩繁培养分离土壤样品中的丛枝菌根真菌菌种并鉴定。【结果】从我国大陆的45个地区50余种宿主植物根区土壤中分离到丛枝菌根真菌135株,隶属于23个种;对各个菌株的形态特征进行了描述。【结论】我国蕴藏着丰富的丛枝菌根真菌菌种资源,文中描述的菌种资源是目前从我国大陆地区获得的种类和数量最多、覆盖范围最广的AM真菌菌种资源。     

Improved PCR primers for the detection and identification of arbuscular mycorrhizal fungi

A set of PCR primers that should amplify all subgroups of arbuscular mycorrhizal fungi (AMF, Glomeromycota), but exclude sequences from other organisms, was designed to facilitate rapid detection and identification directly from field-grown plant roots. The small subunit rRNA gene was targeted for the new primers (AML1 and AML2) because phylogenetic relationships among the Glomeromycota are well understood for this gene. Sequence comparisons indicate that the new primers should amplify all published AMF sequences except those from Archaeospora trappei. The specificity of the new primers was tested using 23 different AMF spore morphotypes from trap cultures and Miscanthus sinensis, Glycine max and Panax ginseng roots sampled from the field. Non-AMF DNA of 14 plants, 14 Basidiomycota and 18 Ascomycota was also tested as negative controls. Sequences amplified from roots using the new primers were compared with those obtained using the established NS31 and AM1 primer combination. The new primers have much better specificity and coverage of all known AMF groups.     

高磷土壤中丛枝菌根真菌的分离鉴定

【背景】丛枝菌根(arbuscular mycorrhiza,AM)真菌具有广泛的寄主范围、环境适应性和优良的植物促生能力。然而,土壤的高磷水平严重抑制了AM真菌生长及AM形成。【目的】分离鉴定出耐较高有效磷含量的华南土著AM真菌菌株,为菌根学研究工作提供新颖材料。【方法】采用经典形态学和分子系统学方法鉴定高磷土壤中AM真菌。【结果】从有效磷含量为53-131 (平均值±标准差为88.2±17.6) mg/kg的根区土壤中鉴定出7属25种AM真菌,包括无梗囊霉属(Acaulospora) 12种、球囊霉属(Glomus) 7种、隔球囊霉属(Septoglomus) 2种、近明球囊霉属(Claroideoglomus) 1种、根孢囊霉属(Rhizophagus) 1种、硬囊霉属(Sclerocystis) 1种和类球囊霉属(Paraglomus) 1种,其中幼套近明球囊霉(Claroideoglomus etunicatum)和蜜色无梗囊霉(Acaulospora mellea)是优势种。在(87.7±8.0) mg/kg的高磷水平下,AM真菌仍能形成丛枝和泡囊。但当有效磷含量达到(99.7±1.2) mg/kg时,菌根侵染率和丛枝丰度显著下降,但仍能够形成泡囊。【结论】从广州市南沙区有效磷含量为(88.2±17.6) mg/kg的耕地植物根区土壤中,鉴定出具有耐高磷潜力的7属25种AM真菌,幼套近明球囊霉和蜜色无梗囊霉等分离株可作为后续高磷抑制机制解析及耐高磷AM真菌菌剂研发工作的试验菌株。     

Molecular identification of arbuscular mycorrhizal fungi in roots: Perspectives and problems

Molecular identification methods are about to revolutionize studies on ecology of arbuscular mycorrhiza. These techniques offer the unique opportunity to investigate communities of arbuscular mycorrhizal fungi (AMF) within roots. Recent technical advances are reviewed, discussing their drawbacks and advantages. An experimental approach to analyze AMF communities within roots using a molecular identification method is presented. Sample results from the analysis of trap cultures from a current project are shown.     

Detection of a novel intracellular microbiome hosted in arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are important members of the plant microbiome. They are obligate biotrophs that colonize the roots of most land plants and enhance host nutrient acquisition. Many AMF themselves harbor endobacteria in their hyphae and spores. Two types of endobacteria are known in Glomeromycota: rod-shaped Gram-negative Candidatus Glomeribacter gigasporarum, CaGg, limited in distribution to members of the Gigasporaceae family, and coccoid Mollicutes-related endobacteria, Mre, widely distributed across different lineages of AMF. The goal of the present study is to investigate the patterns of distribution and coexistence of the two endosymbionts, CaGg and Mre, in spore samples of several strains of Gigaspora margarita. Based on previous observations, we hypothesized that some AMF could host populations of both endobacteria. To test this hypothesis, we performed an extensive investigation of both endosymbionts in G. margarita spores sampled from Cameroonian soils as well as in the Japanese G. margarita MAFF520054 isolate using different approaches (molecular phylotyping, electron microscopy, fluorescence in situ hybridization and quantitative real-time PCR). We found that a single AMF host can harbour both types of endobacteria, with Mre population being more abundant, variable and prone to recombination than the CaGg one. Both endosymbionts seem to retain their genetic and lifestyle peculiarities regardless of whether they colonize the host alone or together. These findings show for the first time that fungi support an intracellular bacterial microbiome, in which distinct types of endobacteria coexist in a single cell.     

Differential growth response to arbuscular mycorrhizal fungi and plant density in two wild plants belonging to contrasting functional types

The effect of arbuscular mycorrhizal fungi (AMF) on plant growth was examined in two wild plant species belonging to contrasting functional types: an annual forb (Bidens pilosa, Asteraceae) and a deciduous shrub (Acacia caven, Fabaceae) at three contrasting plant densities (one, two, and three individuals per pot). AMF had a slightly negative effect on B. pilosa when the species grew in isolation while they positively affected A. caven. Positive effects of AMF on shoot mass of A. caven decreased at higher plant densities, while shoot mass of individuals of B. pilosa showed less marked differences between plant densities. When considering total biomass per pot, AMF positively affected A. caven growth while negatively affecting B. pilosa, at all three plant densities. Root/shoot ratio per pot was negatively affected by AMF but not plant density in both species. These findings highlight the importance of including plants belonging to different life forms and/or traits in research regarding the interaction between AMF and intraspecific plant competition.     

Genetic processes in arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi (Glomeromycota) colonize roots of the majority of land plants and facilitate their mineral nutrient uptake. Consequently, AM fungi play an important role in terrestrial ecosystems and are becoming a component of sustainable land management practices. The absence of sexual reproductive structures in modern Glomeromycota combined with their long evolutionary history suggest that these fungi may represent an ancient asexual lineage of great potential interest to evolutionary biology. However, many aspects of basic AM fungal biology, including genome structure, within-individual genetic variation, and reproductive mode are poorly understood. These knowledge gaps hinder research on the mechanisms of AM fungal interactions with individual plants and plant communities, and utilization of AM fungi in agricultural practices. I present here the current state of research on the reproduction in AM fungi and indicate what new findings can be expected in the future.     

丛枝菌根真菌生物地理学研究进展

丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)普遍存在于陆地生态系统中,能与绝大多数高等植物形成菌根共生体系。AMF能够促进植物对矿质养分的吸收,增强植物的抗逆能力,在维持生态系统稳定性和生产力中发挥着重要作用。AMF生物地理学主要研究AMF的生物地理分布格局和群落构建机制,对于理解AMF在不同生态系统中的重要性至关重要。总结了AMF生物地理学最新研究进展及研究方法,提出了AMF生物地理学研究理论框架。认为AMF在自然界中并非简单随机分布,宿主植物、地理因子、气候因子和土壤因子共同决定AMF的群落结构,不同尺度下的AMF群落构建符合生态位-中性连续统假说,但在不同尺度下这些驱动因子的相对重要性不同。在全球尺度和区域尺度下,AMF的地理分布格局主要受地理距离和气候因子的影响,中性理论的作用大于生态位理论。随着空间尺度的缩减,宿主植物和环境因子对AMF群落的影响胜过地理距离和扩散限制的作用,生态位理论取代中性理论在AMF群落构建中的主导地位。此外,很多研究发现,同一生境中AMF的群落构建机制并非一成不变,会随环境的变化而发生改变。在未来的研究中,应在野外调查和公共数据库的基础上加强整合分析和数据挖掘工作,从而进一步丰富和完善AMF生物地理学理论。     

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.     

Conserved and reproducible bacterial communities associate with extraradical hyphae of arbuscular mycorrhizal fungi

Extraradical hyphae (ERH) of arbuscular mycorrhizal fungi (AMF) extend from plant roots into the soil environment and interact with soil microbial communities. Evidence of positive and negative interactions between AMF and soil bacteria point to functionally important ERH-associated communities. To characterize communities associated with ERH and test controls on their establishment and composition, we utilized an in-growth core system containing a live soil–sand mixture that allowed manual extraction of ERH for 16S rRNA gene amplicon profiling. Across experiments and soils, consistent enrichment of members of the Betaproteobacteriales, Myxococcales, Fibrobacterales, Cytophagales, Chloroflexales, and Cellvibrionales was observed on ERH samples, while variation among samples from different soils was observed primarily at lower taxonomic ranks. The ERH-associated community was conserved between two fungal species assayed, Glomus versiforme and Rhizophagus irregularis, though R. irregularis exerted a stronger selection and showed greater enrichment for taxa in the Alphaproteobacteria and Gammaproteobacteria. A distinct community established within 14 days of hyphal access to the soil, while temporal patterns of establishment and turnover varied between taxonomic groups. Identification of a conserved ERH-associated community is consistent with the concept of an AMF microbiome and can aid the characterization of facilitative and antagonistic interactions influencing the plant-fungal symbiosis.Subject terms: Symbiosis, Microbiome     

Detection and identification of cytokinins produced by mycorrhizal fungi

Several fungi including six species of the genus Rhizopogon, 22 species of Hebeloma and one of Agaricus have been screened for production of cytokinins. The screening was done by culturing cytokinin-requiring soybean callus tissue alongside the fungus on a medium lacking a cytokinin supply. Growth of the soybean callus indicated production of cytokinins by the fungus. Of the fungi tested, only R. ochraceorubens A. H. Smith gave off sufficient cytokinin to be detected. Although a number of mycorrhizal species are now known to make and give off cytokinins, an even larger number apparently do not do so under the conditions of screening employed. An unidentified ectendotrophic species definitely gave off trans-zeatin, which has been crystallized, and probably trans-ribosylzeatin. Suillus punctipes (Pk.) Sing. apparently produced the same two cytokinins.     

Wheat responses to aggressive and non-aggressive arbuscular mycorrhizal fungi

In southwestern Australia fields, colonization of wheat roots by arbuscular mycorrhizal fungi (AMF) is reduced due to repeated use of phosphate (P) fertilizers. We predicted AMF that aggressively colonize wheat roots at low P supply would also aggressively colonize at high P supply, but provide no additional P uptake benefit and reduce growth. Wheat (cv. Kulin) seedlings were non-mycorrhizal (NM) or inoculated separately with 10 isolates of AMF from wheat-belt soils in a glasshouse experiment. Kojonup loamy sand was supplied with P to provide suboptimal and supraoptimal P for growth of NM wheat in this soil. At low P supply, wheat growth was limited by P availability. All AMF isolates colonized wheat roots at 14 days after emergence of seedlings. At 42 days, percentage root length colonized (%RLC) was highest for two isolates of Scutellospora calospora, WUM 12(2) and WUM 12(3), followed by Glomus sp. WUM 51, G. invermaium WUM 10(1), Acaulospora laevis WUM 11(4) and Gigaspora decipiens WUM 6(1). These isolates, designated as `aggressive colonizers', ranged from 50 to 89%RLC. A second group of AMF ranged from 1 to 19%RLC at 42 days. This group, termed `non-aggressive colonizers', included Acaulospora spp. WUM 11(1), WUM 46, and WUM 49 and Glomus sp. WUM 44. High soil P supply increased seedling growth 2–3 fold, but reduced%RLC. Grouping of aggressive and non-aggressive AMF based on colonization rate at high P supply was similar to that at low P. At low P supply, only the two isolates of S. calospora increased wheat growth compared to the NM plant. The remaining aggressive and non-aggressive AMF reduced growth of wheat at low P, while aggressive colonizers reduced growth at high P. At low P supply, the aggressive colonizers increased shoot P concentration, while at high P, shoot P was not affected by AMF. Growth depression by aggressive colonizers was associated with reduced sucrose concentration in roots. Based on the negative growth response under low and high P fertility in the glasshouse, AMF could be expected to produce non-beneficial effects on wheat in the field depending on the P status of the soil and the aggressiveness of AMF in the community. This revised version was published online in June 2006 with corrections to the Cover Date.     

Isolation and identification of soil bacteria growing at the expense of arbuscular mycorrhizal fungi

Soil-microorganism symbioses are of fundamental importance for plant adaptation to the environment. Research in microbial ecology has revealed that some soil bacteria are associated with arbuscular mycorrhizal fungi (AMF). However, these interactions may be much more complex than originally thought. To assess the type of bacteria associated with AMF, we initially isolated spores of Glomus irregulare from an Agrostis stolonifera rhizosphere. The spores were washed with sterile water and plated onto G. irregulare mycelium growing in vitro in a root-free compartment of bicompartmented Petri dishes. We hypothesized that this system should select for bacteria closely associated with the fungus because the only nutrients available to the bacteria were those derived from the hyphae. Twenty-nine bacterial colonies growing on the AMF hyphae were subcultured and identified using 16S rRNA gene sequences. All bacterial isolates showed high sequence identity to Bacillus cereus, Bacillus megaterium, Bacillus simplex, Kocuria rhizophila, Microbacterium ginsengisoli, Sphingomonas sp. and Variovorax paradoxus. We also assessed bacterial diversity on the surface of spores by PCR-denaturating gradient gel electrophoresis. Finally, we used live cellular imaging to show that the bacteria isolated can grow on the surface of hyphae with different growing patterns in contrast to Escherichia coli as a control.     

龙脑香科植物对丛枝菌根真菌的影响

在天然林地和温室盆栽条件下,比较研究了龙脑香科植物对丛枝菌根(Arbuscular mycorrhizas,AM)真菌孢子密度、相对多度、频度、属的组成、丰度和侵染状况等方面的影响.结果表明,用坡垒作盆栽寄主加富培养后,菌根侵染率、泡囊、丛枝和侵入点都低于原采样植物,以原坡垒土壤中栽植苗木的侵染率为最高,可达20.3%;而以望天树根围土壤栽植的苗木为最低,仅为10.6%;坡垒还不同程度地改变了原采样植物根围土壤中AM真菌孢子的密度、相对多度、频度、属的组成、丰度等.在4种土壤中,栽植坡垒苗木后,AM真菌的孢子密度都有不同程度的增长.采用与原采样相同种类的植物作为AM真菌加富培养的寄主更有利于促进AM真菌的生长发育、保持AM的多样性.     

Role of mitochondria in the response of arbuscular mycorrhizal fungi to strigolactones

The plant signals strigolactones activate seed germination of the parasitic weeds (Striga and Orobanche), growth of arbuscular mycorrhizal (AM) fungi and have recently been described as a new class of plant hormones that inhibit shoot branching. In AM fungi, the synthetic strigolactone analogue GR24 rapidly stimulates mitochondrial metabolism (within minutes) and biogenesis (within one hour). New gene expression, more active nuclear division and cell proliferation occur later (within days). By using pharmacological approaches to inhibit the mitochondrial ATP synthesis, various steps of the respiratory chain and the mitochondrial protein translation, we further describe the mechanisms underlying the mitochondrial response to GR24. We show with SHAM and KCN inhibition treatments that the respiratory chain of Gigaspora rosea is branched and includes an alternative oxydase. The two electron transports can be used for GR24 activation of hyphal branching but only the alternative one is used for spore germination. By using the inhibitors Oligomycin, Rotenone, Antimycine A and KCN, we show that indirect (proton pumping) and direct inhibition of ATP synthase does not completely abolish the activation of hyphal branching by GR24. However, hyphal branching was totally inhibited with the suppression of mitochondrial biogenesis, confirming the essential role played by mitochondria to amplify the strigolactone response of AM fungi.Key words: strigolactones, mitochondria, catabolism, lipids, AOXStrigolactones are plant signal molecules initially characterized as seed germination stimulants of the parasitic weeds Orobanche and Striga.¹ Recent findings have revealed that natural strigolactones or the synthetic strigolactone analogue GR24 can also elicit hyphal branching of arbuscular mycorrhizal (AM) fungi, a cellular response typically observed when these fungi grow close to a living root.²Moreover when plants are affected in strigolactone production their capacity to be mycorrhized is strongly reduced.^3,4 An additional role of strigolactones, as possible novel plant hormones inhibiting shoot branching, has recently been discovered.^4,5 The cellular modes of action of strigolactones on seeds of parasitic weeds and on plant axillary buds are not known. In previous studies we showed, on the AM fungus Gigaspora rosea, that GR24 elicits a fast (within minutes) synthesis and utilisation of NADH by mitochondria. This boost of mitochondrial activity was correlated, after one hour of GR24 treatment, with an enhancement of cellular ATP production. This enhancement of oxydative metabolism precedes active mitochondrial proliferation.^6,7 Taken together, these results suggested that mitochondrial activation is a key event required for switching the fungus to a pre-symbiotic stage, characterized by a more active nuclear proliferation, an upregulation of several genes and an activated cell proliferation (the so called branching response).⁶In order to clarify the role of mitochondria in the GR24 response, we carried out a series of pharmacological experiments where different steps of the oxidative phosphorylation were specifically inhibited. We first inhibited mitochondrial ATP synthesis with oligomycin, an inhibitor of ATP synthase subunit Fo, or with a cocktail of Rotenone, Antimycine A and KCN (inhibitors that block the proton pumping from the matrix to the inter membrane space) (Fig. 1). Surprisingly only 36.7% to 50% of the hyphal branching response was suppressed. Hyphal elongation (not shown) was not affected by the treatment with the cocktail of the 3 last inhibitors suggesting that catabolic pathways other than the mitochondrial oxidative phosphorylation produced enough ATP to sustain normal hyphal activity. The same hypothesis can be proposed to explain that the GR24 branching response was only partially reduced. As triglycerides are the main carbon storage form in AM fungi,⁸ lipid catabolism, in the absence of oxidative phosphorylation, can only provide ATP (actually GTP) during the oxidation of acetyl CoA in the citric acid cycle. Under normal condition we can postulate that most ATP production associated with lipid degradation is sustained in mitochondria by the respiratory chain. Thus, if the citric acid cycle still works in mitochondria despite the inhibition of the respiratory chain, it implies that mitochondria are able (i) to achieve the reoxydation of NADH (produced during the β-oxidation of fatty acids and the citric acid cycle) in the absence of a fonctional complex I and (ii) to transfer electrons from NADH to O₂ in the absence of a functional cytochrome c oxidase. This would be possible if the respiratory chain of G. rosea contained an alternative NADH dehydrogenase and alternative oxidase (AOX) as already described in other fungi.⁹ To investigate the hypothesis that G. rosea possesses a mitochondrial alternative oxydase, single and double inhibitions of the AOX and the cytochrome c oxidase were carried out. Only a simultaneous inhibition of AOX and cytochrome c oxidase by SHAM and KCN, respectively, led to a reduction of 57% of the branching response intensity (Fig. 2). This result strongly suggests that G. rosea mitochondria have a ramified respiratory chain and that the role of O₂ as a final electron acceptor is important to get the GR24 branching response. In contrast with the GR24 branching response, germination of G. rosea spores could be completely inhibited with a SHAM treatment, suggesting that spore germination is exclusively dependent on the activity of AOX (data not shown), as it is for seed germination of Orobanche sp.¹⁰Open in a separate windowFigure 1Inhibition of the ATP synthase or loss of proton gradient is not sufficient to totally suppress the hyphal branching activation by GR24. Spores of Gigaspora rosea were grown for six days on solid M medium in the dark at 28°C under 2% CO₂ and then treated with 10⁻⁷ M GR24 alone or simultaneously with the various inhibitors. A 28 µM oligomycin treatment was applied to specifically inhibit ATP synthesis. H⁺ pumping activities by the respiratory complexes I, III and IV were inhibited by treating the germinated spores with a mix of 20 µM Rotenone, 19 µM antimycin A and 1.5 mM KCN. After 24 h, the number of new apices formed was counted. Values are the mean of three biological replicates. Thirty spores were used in each experimental condition. Means with the same letter are not significantly different (ANOVA test and Tuckey post-hoc Analysis). Data were computed with the R software.Open in a separate windowFigure 2Partial inhibition of the hyphal branching activation by GR24 requires conjugated action of KCN and SHAM. Spores of Gigaspora rosea were grown for six days on solid M medium in the dark at 28°C under 2% CO₂ and then treated with 10⁻⁷ M GR24 and with 1.5 mM KCN, 5 mM SHAM or both. After 24 h, the number of new hyphal apices formed was counted. Values are the mean of three biological replicates. Twenty to 30 spores were used in each treatment. Means with the same letter are not significantly different (ANOVA test and Tuckey post-hoc Analysis). Data were computed with the R software.The other catabolic pathway that could produce ATP other than in mitochondria is the glycolytic pathway. AM fungi, like plants, can also degrade fatty acids through the glyoxylate cycle and produce succinate.¹¹ Succinate can thereafter serve as a biosynthetic intermediate and be later oxidized through the glycolytic pathway. Another, more likely, possibility is that the catabolism of glycogen, an additional form of carbon storage in AM fungi,¹² is also stimulated by strigolactones.Whereas we can speculate that strigolactones do not directly stimulate oxidative phosphorylation (otherwise inhibition of oxidative phosphorylation would have had more impact on GR24-induced hyphal branching), mitochondrial complete oxidation process seems necessary to get a maximal hyphal branching response. This was further demonstrated by an inhibition experiment of mitochondrial biogenesis. This biogenesis requires mitochondrial protein biosynthesis,^13,14 a process that can be specifically inhibited with chloramphenicol. Preliminary assays showed that after a chloramphenicol treatment, mitochondria of GR24 treated hyphae did not change their shape or movements like normally observed in GR24-treated hyphae⁷ (data not shown). This indicates that, at this concentration, chloramphenicol was able to inhibit mitochondrial biogenesis in the fungus. When germinated spores of G. rosea were treated with both chloramphenicol and GR24 for 24 h, GR24-induced branching was decreased by 46.8%, whereas no significant effect was visible on control hyphae not treated with GR24 (Fig. 3).Open in a separate windowFigure 3Inhibition of the synthesis of mitochondrial proteins in Gigaspora rosea affects hyphal branching activation by GR24. Spores of Gigaspora rosea were grown for six days on solid M medium in the dark at 28°C under 2% CO₂ and then treated with 10⁻⁷ M GR24, 1 mg/ml chloramphenicol or both. After 24 h, the number of new hyphal apices formed was counted. Values are the mean of three biological replicates. Ten to 20 spores were used in each treatment. Means with the same letter are not significantly different (ANOVA test and Tuckey post-hoc Analysis). Data were computed with the R software.In conclusion, our data suggest that Gigaspora rosea possesses a ramified respiratory chain with an alternative NADH dehydrogenase and an AOX, the activity of the latter being essential for spore germination. They also suggest that, in the activation of hyphal branching by strigolactones in AM fungi, upstream catabolic processes, such as the β-oxidation of fatty acids (followed by the citric acid cycle) or glycogen degradation (followed by glycolysis), could be activated before oxidative phosphorylation. This is in agreement with a recent report¹⁵ which showed that lipid catabolism of Glomus intraradices is enhanced in response to root exudates. The stimulation of mitochondrial biogenesis that occurs later would be a consequence of these first metabolic activations and would then largely amplify them.Similar mechanisms of catabolic activation could be involved in the strigolactone induction of seed germination of parasitic weeds by strigolactones.¹⁶ In contrast, it is more difficult to imagine that strigolactone inhibition of plant lateral bud outgrowth also requires an activation of ATP synthesis. In order to evaluate to which extent they are conserved, the mode of action of strigolactones on the three biological systems need to be further investigated.     

Scale-dependent niche axes of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are mutualistic with most species of plants and are known to influence plant community diversity and composition. To better understand natural plant communities and the ecological processes they control it is important to understand what determines the distribution and diversity of AMF. We tested three putative niche axes: plant species composition, disturbance history, and soil chemistry against AMF species composition to determine which axis correlated most strongly with a changing AMF community. Due to a scale dependency we were not able to absolutely rank their importance, but we did find that each correlated significantly with AMF community change at our site. Among soil properties, pH and NO₃ were found to be especially good predictors of AMF community change. In a similar analysis of the plant community we found that time since disturbance had by far the largest impact on community composition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.