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natalJuly 11, 2006
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subset: commoncrawl


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text:
Editor: Philippe Lemanceau
Plant species and soil type cooperatively shape the structure and function of microbial communities in the rhizosphere
Article first published online: 25 FEB 2009
DOI: 10.1111/j.1574-6941.2009.00654.x
© 2009 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved
FEMS Microbiology Ecology
Volume 68, Issue 1, pages 1–13, April 2009
Additional Information
How to Cite
Berg, G. and Smalla, K. (2009), Plant species and soil type cooperatively shape the structure and function of microbial communities in the rhizosphere. FEMS Microbiology Ecology, 68: 1–13. doi: 10.1111/j.1574-6941.2009.00654.x
Publication History
- Issue published online: 5 MAR 2009
- Article first published online: 25 FEB 2009
- Received 16 September 2008; revised 4 December 2008; accepted 14 December 2008.First published online 25 February 2009.
- Abstract
- Article
- References
- Cited By
- microbial communities;
- environmental factors;
- plant–microorganism interaction
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
The rhizosphere is of central importance not only for plant nutrition, health and quality but also for microorganism-driven carbon sequestration, ecosystem functioning and nutrient cycling in terrestrial ecosystems. A multitude of biotic and abiotic factors are assumed to influence the structural and functional diversity of microbial communities in the rhizosphere. In this review, recent studies on the influence of the two factors, plant species and soil type, on rhizosphere-associated microbial communities are discussed. Root exudates and the response of microorganisms to the latter as well as to root morphology were shown to shape rhizosphere microbial communities. All studies revealed that soil is the main reservoir for rhizosphere microorganisms. Many secrets of microbial life in the rhizosphere were recently uncovered due to the enormous progress in molecular and microscopic tools. Physiological and molecular data on the factors that drive selection processes in the rhizosphere are presented here. Furthermore, implications for agriculture, nature conservation and biotechnology will also be discussed.
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
The term 'rhizosphere' was coined by Hiltner in 1904 to describe the portion of soil where microorganism-mediated processes are under the influence of the root system. Functions of the rhizosphere are of central importance for plant nutrition, health and quality. The well-studied rhizosphere effect describes the phenomenon that, in comparison with bulk soil, the biomass and activity of microorganisms is enhanced as a result of exudation of compounds by the root (Sørensen, 1997; Raaijmakers et al., 2009). Because of the enormous importance of plant–microorganism interactions in the rhizosphere for carbon sequestration, ecosystem functioning and nutrient cycling in natural ecosystems as well as in agricultural and forest systems (Singh et al., 2004), it is crucial to understand the factors influencing the microbial communities in this habitat.
The use of polyphasic approaches combining novel cultivation-independent and more traditional techniques to study microbial communities led to a significantly better understanding of community structure and function in the rhizosphere in the last decade. Several biotic and abiotic factors influencing the structural and functional diversity of bacterial communities (Fig. 1), for example, climate and season, grazers and animals, pesticide treatments, soil type and structure and plant health and developmental stage, were investigated (Lemanceau et al., 1995; Siciliano et al., 2001; Graner et al., 2003; reviewed in Garbeva et al., 2004; Jousset et al., 2006; Rasche et al., 2006b). Different soil types are assumed to harbour specific microbial communities, as recently shown in a continental-scale study of soil bacterial communities (Fierer & Jackson, 2006). In contrast to what we know about the biodiversity of macroorganisms, the microbial biogeography is controlled primarily by edaphic variables, especially by pH (Fierer & Jackson, 2006). Furthermore, the bacterial community composition changed with age of soil that developed over c. 77 000 years of intermittent aeolian deposition, and the overall diversity, richness and evenness of the communities increased (Tarlera et al., 2008). Plants affect these indigenous microbial populations in soil; each plant species is thought to select specific microbial populations. Root exudates are a driving force in this process, but researchers are only beginning to understand the role of single compounds in mediating belowground interactions (reviewed in Bais et al., 2006; Haichar et al., 2008). The composition of root exudates varies from plant to plant and affects the relative abundance of microorganisms in the vicinity of the root (Somers et al., 2004). Plants not only provide nutrients for microorganisms, but some plant species also contain unique antimicrobial metabolites in their exudates. Many of them are used as medical plants, for example camomile, thyme and eucalyptus. The existing huge diversity of plant species with an estimated range from 310 000 to 422 000 species (Pitman & Jörgensen, 2002) and corresponding secondary metabolites of plants (Buchanan et al., 2000) affects below-ground diversity. Interestingly, invasive plants can have major effects on microbial communities in soil (Van der Putten et al., 2007). There is no doubt that both factors, soil properties as well as plant species, influence the structure and function of microbial communities. However, the extent to which both factors contribute to microbial communities is not fully understood.
There are several contrasting reports in the literature indicating plant or soil type as dominant factor (Grayston et al., 1998; Girvan et al., 2003; Nunan et al., 2005). This review will present historical and more recent findings about plant specificity of rhizosphere communities and will analyse the background for this phenomenon on the basis of examples and physiological data. Furthermore, conclusions for agriculture, nature conservation and biotechnology will be discussed.
Each plant species is colonized by specific microbial populations
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
History and overview of plant specificity
Many phytopathogenic organisms, bacteria as well as fungi, have coevolved with plants and show a high degree of host specificity (Raaijmakers et al., 2009). However, up till now, the molecular basis of this host specificity is only partially understood. Another well-studied example is rhizobia–legume interactions, which are highly specific (Long, 2001). Studies on the basis of cultivation-dependent methods gave first hints of different composition of the bacterial community on plants (Kremer et al., 1990; Lemanceau et al., 1995; Maloney et al., 1997; Germida et al., 1998). In a study by Germida & Siciliano (2001), the evolutionary relationship in plant–microorganism interactions was revealed: old wheat cultivars were colonized by phylogenetically diverse rhizobacteria, whereas the rhizosphere of modern cultivars was dominated by fast-growing Proteobacteria. Cultivar specificity was also reported for oilseed rape (Graner et al., 2003). A plant species-dependent composition of bacterial and fungal rhizosphere isolates with in vitro antagonistic activity towards the plant-pathogenic fungus Verticillium dahliae Kleb. was shown by Berg et al. (2002, 2005b, c, 2006). There is currently an ongoing debate about the pros and cons of cultivation-dependent and -independent techniques (Nichols, 2007; Ritz, 2007), but it is clear that both methods have their limitations and not all microorganisms can be investigated (Bent & Forney, 2008). Cultivation-based methods address only the culturable bacteria, which are thought to represent only a small proportion (0.1–10%) of the total bacteria present in soil and in the rhizosphere (Amann et al., 1995).
The analysis of nucleic acids directly extracted from rhizosphere soils provided an opportunity to study a much broader spectrum of microorganisms residing in the rhizosphere. Most frequently rRNA gene fragments are amplified from total community DNA and subsequently analysed by fingerprinting techniques. Cultivation-independent fingerprinting methods clearly showed the influence of the plant species on the structure of the microbial community. Marschner et al. (2001) concluded that the bacterial community composition in the rhizosphere is affected by a complex interaction between soil type, plant species and root zone location. They analysed three plant species (chickpea, rape and Sudan grass) grown in intact cores of three Californian soils (sandy soil, sandy loam and clay) by PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments. Other studies indicated plant species (Wieland et al., 2001; Kowalchuk et al., 2002) or soil type (Da Silva et al., 2003; Salles et al., 2004) as the dominant factor influencing the composition of the rhizosphere microbial community. However, in the latter studies plant specificity was also found, but to a lesser degree. Soil type instead of maize cultivar type was the overriding determinative factor that influenced the community structures of the Paenibacillus communities in the rhizosphere investigated by Da Silva et al. (2003). Salles et al. (2004) found using genus-specific DGGE that plant species affected Burkholderia community structure to a lesser extent than did land use history. However, canonical correspondence analysis demonstrated plant specificity of Burkholderia in maize, grass, barley and oat. The majority of studies, especially those performed in the field or in native vegetation, clearly demonstrate a high degree of plant specificity. In Canyonlands National Park (Utah), bacterial communities associated with the rhizospheres of the native bunchgrasses Stipa hymenoides and Hilaria jamesii, the invading annual grass Bromus tectorum and the interspaces colonized by cyanobacterial soil crusts were compared using terminal restriction fragment length polymorphism (T-RFLP; Kuske et al., 2002). Major differences were observed in the rhizospheres of the three plant species and they were most apparent with analysis of the Acidobacterium division. In another study analysing eight herbaceous plants, native to Germany, in the greenhouse experiment, clear plant-specific single strand conformation polymorphism profiles for bacteria, Alphaproteobacteria, Actinobacteria and Pseudomonas were found (Dohrmann & Tebbe, 2005). In an experiment to test the hypothesis that plant species are a major driver of bacterial rhizoplane community composition, further evidence for the complexity of bacterial communities in grassland soils and resultant difficulties in distinguishing plant-mediated effects was provided. Nunan et al. (2005) analysed field-grown root-associated communities of Agrostis capillaris, Agrostis vinealis, Deschampsia cespitosa, Festuca rubra and Poa pratensis by analysis of the plastid tRNA leucine (trnL) UAA gene intron, and plant-related bacterial communities using T-RFLP and DGGE. Although microbial fingerprints show high similarity between the grass species, statistical analysis of data generated by different fingerprinting techniques demonstrated an influence of plant community composition on bacterial communities and also indicated the significant influence of other factors (topography and uncharacterized, environmental factors). Additionally, studies on sugar beet-associated microorganisms in different European countries clearly showed an influence of soil type and geographical region on specific communities (Zachow et al., 2008b).
Using cultivation-independent methods, evidence for the impact of plant species was not only found for the structure of rhizosphere communities but also for the function. Briones et al. (2002) found cultivar-specific differences for ammonia-oxidizing bacteria (AOB) in rice rhizospheres by a multiphasic approach, including DGGE of the amoA gene, analysis of libraries of cloned amoA, fluorescently tagged oligonucleotide probes targeting the 16S rRNA gene of AOBs as well as metabolism rates obtained by the 15N dilution technique. Impact of plant species on the composition of nirK-type denitrifier communities in eight nonleguminous grassland plant species was shown by Bremer et al. (2007). In this 2-year microcosm study, denitrifier community composition was analysed by T-RFLP of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. Especially less-abundant T-RFs occurred in relation to several plant species. Interestingly, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, for example, through root exudates.
The majority of studies analysed plant specificity on the bacterial communities; the structure and function on rhizosphere fungi is only poorly understood. In a study using 13C labelling and phospholipid fatty acid (PLFA) analysis to examine the microbial dynamics associated with rhizosphere C cycling, the fungi were the most highly labelled and active group (Butler et al., 2003). Broeckling et al. (2008) showed that two model plant species (Arabidopsis thaliana and Medicago truncatula) are able to maintain resident soil fungal populations, but unable to maintain nonresident soil fungal populations. In a hierarchical set of greenhouse experiments, the key role of root exudates for this process was identified. On Tenerife Island, a natural laboratory to study terrestrial biodiversity, a high similarity at a correspondence analysis biplot between fungal and plant populations was found (Zachow et al., 2008a).
Extensive studies related to the question of plant specificity of bacterial as well as fungal community were carried out for host plants of the phytopathogenic fungus V. dahliae Kleb. These studies will be explained in more detail in the following section.
Example: Plant specificity of microbial communities in the rhizosphere of Verticillium host plants
In order to test the hypothesis that each plant species selects their own specific microbial community, three phylogenetically different and economically important crops – strawberry, potato and oilseed rape – were grown under field conditions in a randomized plot design. All three plant species are hosts of the soil-borne fungal pathogen V. dahliae Kleb. that causes high yield losses world wide (Tjamos et al., 2000). In a first 2-year study, rhizosphere samples taken at different plant growth stages from one field site (Braunschweig, Germany) were analysed by cultivation-dependent and -independent methods. DGGE analysis of 16S rRNA gene fragments amplified from total community DNA revealed plant species-dependent pattern (Smalla et al., 2001). DGGE patterns of oilseed rape and potato rhizosphere communities were more similar to each other than to the strawberry patterns. Interestingly, all rhizospheres showed some bands in common but also specific bands, for example Nocardia populations were identified as strawberry-specific bands. The proportion and composition of bacteria with in vitro antagonistic activity towards V. dahliae was also shown to be influenced by the plant species (Berg et al., 2002). In addition, plant-specific genotypes of 34 Pseudomonas putida A isolates were observed, suggesting that these bacteria were specifically enriched in each rhizosphere.
The extent to which this plant specificity can be observed at different sites was still unclear. Therefore, in a second trial, Verticillium host plants oilseed rape and strawberry were analysed at different sites in Germany (Rostock, Berlin, Braunschweig). Again, plant specificity was detected in the rhizosphere by bacterial and group-specific (Alphaproteobacteria, Betaproteobacteria and Actinobacteria) DGGE profiles (Costa et al., 2006a). The plant species was a determinant factor in shaping similar actinobacterial communities in the strawberry rhizosphere from different bulk soil communities in both years. Cloning and sequencing of 16S rRNA gene fragments obtained from dominant DGGE bands detected in the bacterial profiles of the Rostock site revealed that Streptomyces sp. and Rhizobium sp. were among the dominant ribotypes in the strawberry rhizosphere, while sequences from Arthrobacter sp. corresponded to dominant bands from oilseed rape bacterial fingerprints. The rhizosphere effect on the bacterial antagonists was shown by their enhanced proportion, by enrichment of specific species and genotypes as evidenced by BOX-PCR, and by a reduced diversity of antagonistic bacteria in the rhizosphere in comparison with bulk soil (Berg et al., 2006). This effect was influenced by both the plant species and the site of its cultivation. Bacteria of the genus Pseudomonas, which is an important and often dominant plant-associated genus known for its beneficial interaction with the host plant (Haas & Défago, 2005) were studied in more detail. Also in the rhizosphere of Verticillium host plants, the majority of bacterial isolates with antagonistic activity towards V. dahliae belonged to Pseudomonas (Berg et al., 2002, 2006). The factors sampling site, plant species and year-to-year variation were shown to significantly influence the community structure of Pseudomonas in rhizosphere soils (Costa et al., 2006b, 2007). A dominant band in the gacA fingerprints, which was detected in the strawberry rhizosphere profiles of all three sites had the same sequences as Pseudomonas isolates antagonistic towards V. dahliae that carried the phlD biosynthetic locus. The sequences determined for the isolates and the reamplified band showed closest relatedness to Pseudomonas fluorescens strain F113 (97%gacA gene sequence identity in 492 bp sequences), a biocontrol agent and 2,4-DAPG producer.
The results for bacterial communities raised the hypothesis that a plant-dependent rhizosphere effect might also be seen for fungal communities. To address this idea, the fungal communities of Verticillium host plants were also analysed for their structures and antagonistic functions. Using cultivation-independent techniques, plant-specific composition of fungal communities in the rhizosphere could also be detected, but not in all cases (Costa et al., 2006a). Higher heterogeneity of DGGE profiles within soil and rhizosphere replicates was observed for the fungi than for bacteria. In contrast to bacteria, the proportion of fungal isolates with antagonistic activity toward V. dahliae was similar for both bulk and rhizosphere soil (Berg et al., 2005c). However, a plant- and site-dependent specificity of the composition of antagonistic morphotypes and their genotypic diversity was found. Trichoderma strains displayed high diversity in all soils, but a high degree of plant specificity was shown by BOX-PCR fingerprints.
Summarizing all results, an influence of plant species on the structure (microbial fingerprints) and function (microorganisms with antagonistic activity) of rhizosphere communities was shown in each case, but the extent of the plant-dependent rhizosphere effect differed for the taxa studied and method used. The effect of plant species for fungal communities was less pronounced than for bacteria.
Cultivar specificity of microbial communities in the rhizosphere
In several studies, an influence not only of plant species but also of the cultivars on the rhizosphere microbial communities was shown (Germida & Siciliano, 2001; Briones et al., 2002; Graner et al., 2003; Milling et al., 2004). Genetically modified plants with altered root exudates are interesting model systems to study cultivar-specific effects, which were found in several studies for root-associated bacterial as well as fungal communities (Mansouri et al., 2002; Castaldini et al., 2005; Oliver et al., 2008). T4-lysozyme potato plants (Düring et al., 1993) are one of the best-investigated models. In a 6-year field release of T4 lysozyme-producing potatoes cv. Désirée, the changes in structure and function of plant-associated bacterial populations were monitored by a polyphasic approach. Bacteria and group-specific DGGE fingerprints revealed an influence of the transgene on potato-associated bacteria, but the influence of environmental factors was much stronger (Heuer et al., 2002). No statistically significant differences between transgenic and nontransgenic plants were found for cultivated bacteria (Lottmann et al., 1999). In contrast, in an additional experiment analysing colonization of bacteria, the lysozyme-tolerant strain P. putida QC14-3-8 was recovered from the transgenic T4 lysozyme plant in higher abundances than from the control lines (Lottmann et al., 2000). Furthermore, using a root hair Bacillus subtilis model, roots from potato lines expressing the T4 lysozyme gene always showed significantly (1.5–3.5-fold) higher killing (Ahrenholtz et al., 2000). Differences in the relative abundance of endophytic fungi colonizing the roots of T4-lysozyme-producing potatoes and the parental line could also be detected (Götz et al., 2006). While an influence of transgenic T4 lysozyme on microbial strains was shown in in vitro experiments, the influence was negligible in field trials, compared with other environmental factors. These results obtained for T4-lysozyme potatoes were confirmed in greenhouse experiments (Rasche et al., 2006a, b) and in other studies analysing transgenic plants (Bruinsma et al., 2002). Interestingly, in another approach comparing zeaxanthine-producing potatoes with different potato cultivars, the effect of cultivar was apparent and much higher than that of the transgene (Schloter M, pers. commun.).
Impact of methods and techniques on rhizosphere research
The multitude of studies on the effect of the soil type, the plant species and the cultivar on rhizosphere microbial communities and the findings reported are often difficult to compare, and thus it is even more difficult to come up with some general conclusion. The reason for this dilemma is manifold, and thus we felt that a discussion of the general challenges to sample the rhizosphere and to analyse the microorganisms residing in the rhizosphere might be useful for the reader of this minireview. In most studies, the rhizosphere is defined as the soil that is closely adhering to the root (Smalla et al., 2001; Sanguin et al., 2006). Thus, plants carefully dug out of the soil were vigorously shaken and the remaining root-attached soil is treated as the rhizosphere. Clearly, the type of soil and its moisture influences the amount of root-adhering soil. Therefore, in other studies root washes or other cleaning procedures were performed before obtaining the microbial fraction from the surface of the root (Briones et al., 2002; Bremer et al., 2007). Sometimes this special microenvironment is differentiated from the rhizosphere and called rhizoplane (Nunan et al., 2005). Another critical point is which part of the root system should be sampled. Microscopic studies of gfp-tagged inoculants revealed highly heterogeneous colonization patterns (Götz et al., 2006). Exemplarily, different colonization patterns are shown for Pseudomonas trivialis: on lettuce roots a dense colonization in lines was found while on sugar beet cell clusters were observed (Fig. 2). A great variability of the colonization patterns among roots, even of the same age, is problematic for the subsequent analysis. The use of subsamples of the whole root system and, depending on the size of the plant, composite samples might contribute to overcome this variability. However, the representative picture of the kind and abundance of microorganisms in the rhizosphere obtained when analysing composite samples is certainly an underestimate of the existing diversity. As root exudation changes during plant growth development, the sampling time might be another critical factor in the experimental design. Studies which aimed to explore the extent to which plant species and the soil determine the microbial diversity have been performed at different levels of complexity, which again might have influenced the findings reported. The systems used range from very simple, often axenic or hydroponic systems, to more complex microcosm or mesocosm studies performed under growth chamber or greenhouse conditions to field tests. Although each system has its benefit, it is difficult to extrapolate in vitro results to natural conditions if the experimental design does not allow for the natural rhizosphere development. This factor is often neglected in plant ecology, or even in agricultural experiments. Last but not least, the resolution of the methods used to analyse the microbial community might be decisive for the findings reported. While the cultivation-dependent methods usually focus on the small proportion of culturable aerobic bacteria and in some cases on particular taxa such as Pseudomonas, the analysis of total nucleic acids or PLFA directly extracted from the rhizosphere soil or the use of microscopic methods in combination with FISH should provide a much broader spectrum of the rhizosphere microbial community. Altogether, multiphasic approaches yielded in comprehensive pictures of rhizosphere communities. New techniques have a great impact on our understanding; this was shown for example for transcriptome profiling (Mark et al., 2005; Yuan et al., 2008), microarrays (Sanguin et al., 2006), in vivo expression technology and differential fluorescence induction promoter traps as tools for exploring niche-specific gene expression (Rediers et al., 2005), new methods for the in situ analysis of antifungal gene expression using flow cytometry combined with green fluorescent protein-based reporter fusions (De Werra et al., 2008), and ultradeep sequencing (Velicer et al., 2006). Stable isotope probing used to determine bacterial communities assimilating each carbon source in the rhizosphere of four plant species resulted in plant species-specific pattern (Haichar et al., 2008). Also metagenomic approaches are established to analyse the plant–soil interface (Erkel et al., 2006; reviewed in Leveau, 2007).
In most studies, the effect of the plant and the soil type on the relative abundance of the numerically dominant bacteria and fungi was analysed. Thus, very little is known about how these factors affect less common colonizers of the rhizosphere – a general problem in microbial ecology recently termed 'the tragedy of the uncommon', which was formulated by Bent & Forney (2008). With this multitude of factors in mind, the often contrasting findings among studies on the factors shaping rhizosphere might not be too surprising.
Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
The rhizosphere represents a highly dynamic and complex interface for chemical, physical and biological interactions. Plant roots continuously produce and excrete compounds into the rhizosphere (Uren, 2000; Rovira, 2005). Root exudates are a key factor for the enrichment of specific microbial populations in the rhizosphere; they consist of ions, free oxygen and water, enzymes, mucilage and a diverse array of carbon-containing primary and secondary metabolites (Uren, 2000). From 10% up to 44% of the photosynthetically fixed carbon is excreted by the root (Bais et al., 2006). Organic acids, sugars, amino acids, lipids, coumarins, flavonoids, proteins, enzymes, aliphatics and aromatics are examples of the primary substances found at the soil–root interface. Among them, the organic acids have received considerable attention owing to their role in providing substrates for microbial metabolism and for serving as intermediates for biogeochemical reactions in soil. The amount and composition is specific for each plant family or species. Especially, plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microorganisms involved in nutrient acquisition (reviewed by Dakora & Phillips, 2002): extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation, and organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. A transgenic tobacco overexpressing ferritin (P6) was shown to accumulate more iron than the wild type, leading to a reduced availability of iron in the rhizosphere and shifts in the pseudomonad community (Robin et al., 2006, 2007). Theoretically, the composition of microorganisms that colonize the rhizosphere can be a result of a positive or negative selection procedure, or both. However, little is known about this important and complex issue. Stenotrophomonas maltophilia is a member of the rhizobacterial populations of cruciferous plants, which produce particularly high levels of sulphur-containing compounds, for example amino acids such as methionine (Debette & Blondeau, 1980). Stenotrophomonas maltophilia requires methionine (Ikemoto et al., 1980). Because of the fact that root exudates are highly plant species–specific, the use of specific compounds can explain plant specificity of microbial communities. Interestingly, flavonoids, a diverse class of polyphenolic compounds secreted by plants, often serve as signals in plant–microorganism interactions (Shaw et al., 2006). For example, the secretion of isoflavones in soybean roots attracts the beneficial bacterium Bradyrhizobium japonicum as well as the pathogen Phytophthora sojae (Morris et al., 1998). Flavonoids isolated from white lupin roots additionally mobilized inorganic phosphorus and decreased soil microbial respiration, citrate mineralization and soil phosphohydrolase (Tomasi et al., 2008), which indicates their central importance for plant–microorganism interaction as well as soil quality. On the other hand, plants produce and secrete a variety of secondary metabolites, which can be toxic to microorganisms. Those plants, which are known for their high production of toxins, for example walnut (Juglans regia L.) for the production of juglone are colonized by specific microbial populations that can degrade or detoxify metabolites via specific hydrolases (Rettenmaier & Lingens, 1985). Another strategy to survive despite the occurrence of toxins is with the help of efflux pumps, which pump toxic components outside the bacterial cell. In addition, production of toxins can force diversification of microbial communities (Czárán et al., 2002). A whole genome analysis of gene expression of P. putida during interaction with maize roots showed the importance of two selective forces of Pseudomonas cells to colonize the rhizosphere: stress adaptation and availability of particular nutrients (Matilla et al., 2008). Specifically, genes involved in nutrition (amino acid uptake and metabolism of aromatic compounds) and adaptation (induction of efflux pumps and enzymes for glutathione metabolism) were preferentially expressed in the rhizosphere.
There are microbial species that are typically associated with only a few or a single plant species. This fact is well known for pathogens as well as beneficial interactions, for example Rhizobium–legumes. Sinorhizobium meliloti effectively colonize plant genera of Medicago, Melilotus and Trigonella, whereas Rhizobium leguminosarum induce nodules in Pisum vicea, Lens and Lathyrus plants (Bais et al., 2006). Other microbial species such as Pseudomonas and the fungus Trichoderma are ubiquitous. However, it was shown even for ubiquitous species such as P. fluorescens/putida that plant-specific (Lemanceau et al., 1995; Berg et al., 2002, 2006; Weller et al., 2002; Picard & Bosco, 2008) and soil-specific (Latour et al., 1996, 1999) genotypes exist. Not only the plant but also the microbial metabolism has an essential influence on colonization of the rhizosphere. However, it is still possible to detect new taxonomic groups in the rhizosphere such as nonthermophilic members belonging to the archaeal division Crenarchaeota (Sliwinski & Goodman, 2004; Simon et al., 2005). Also for crenarchaeal consortia, a high degree of plant specificity was shown in a study analysing native terrestrial plant groups, including bryophytes (mosses), lycopods (club mosses), pteridophytes (ferns), gymnosperms (conifers) and angiosperms (seed plants) (Sliwinski & Goodman, 2004).
Competitive colonization of the rhizosphere and establishment in the root zone is important for successful functioning of rhizosphere organisms (Weller, 1988; Lugtenberg et al., 2002; reviewed in Compant et al., 2005). Steps of colonization include recognition, adherence, invasion (only endophytes and pathogens), colonization and growth, and several strategies to establish interaction. Plant roots initiate cross talk with soil microorganisms by producing signals that are recognized by the microorganisms, which in turn produce signals that initiate colonization (Bais et al., 2006). To participate and react in this cross talk, motile organisms are preferred (Lugtenberg et al., 2002). For example, swarming of Serratia liquefaciens appeared to be specifically induced by exudates of Pisum sativum (Eberl et al., 1999). Phenotypes of P. fluorescens F113 with enhanced motility were selected during rhizosphere colonization of alfalfa (Martinez-Granero et al., 2006). Under gnotobiotic conditions, several of these highly motile variants were more competitive than the wild-type strain, displacing it from the root tip within 2 weeks. The importance to recognize and adhere to plant roots for all plant-associated microorganisms is underlined in many studies. Factors that contribute to recognition include the ability to sense and use root exudates composed of small organic molecules such as carbonic acids, amino acids or sugars, etc. Chemotaxis especially to plant root exudates is an important trait for colonization of the rhizosphere: this was shown for pathogenic and symbiotic plant-associated bacteria, for example Ralstonia solanacearum (Yao & Allen, 2007) as well as R. leguminosarum (Miller et al., 2007). Interestingly, chemotaxis experiments of cyanobacteria with host plants such as Gunnera and Blasia and nonhost plants such as Arabidopsis showed that the capability to attract cyanobacteria might be widespread in plants (Nilsson et al., 2006). Using comparative transcriptome analysis of Pseudomonas aeruginosa, root exudates of sugar beet altered gene expression of genes involved in chemotaxis (Mark et al., 2005). An early step in the establishment of a plant–bacterium interaction is attachment of cells to plant roots, in which, for example, fimbriae and cell-surface proteins are involved. For the colonization of Pseudomonas on plant roots, flagella, pili, O-antigen of lipopolysaccharides, the growth rate and the ability to grow on root exudates are important (Dekkers et al., 1998; Lugtenberg & Dekkers, 1999). Attachment also is an initial step for the formation of microbial biofilms on plant roots as reviewed in Rodríguez-Navarro et al. (2007). The same authors explain mechanisms of rhizobial attachment to legumes: the first phase of attachment, which is a weak, reversible and unspecific binding of plant lectins, a Ca2+-binding bacterial protein, and bacterial surface polysaccharide and a second attachment step, which requires the synthesis of bacterial cellulose fibrils that cause a tight and irreversible binding of the bacteria to the roots. In Agrobacterium, cyclic glucans, capsular polysaccharide, and cellulose fibrils also appear to be involved in the attachment to plant cells (Rodríguez-Navarro et al., 2007). Not only bacteria but also fungi attach to the root surface. Fungal adhesion to plants is a key step for establishment of interaction (Tucker & Talbot, 2001). It was also shown for fungi that root exudates play a key role in this process (Broeckling et al., 2008). Another study established that the Nectria haematococca PDA1-CD chromosome carries a gene(s) for the utilization of homoserine, a compound found in large amounts in pea root exudates (Rodriguez-Carres et al., 2008). Competition studies demonstrated that an isolate that lacks the cluster containing genes for pea pathogenicity, but carries a portion of the CD chromosome that includes the homoserine utilization gene(s) is more competitive in the pea rhizosphere than an isolate without the CD chromosome. Interestingly, besides chemical signals the electric potential produced by electrogenic ion transport on the root also is able to attract microorganisms, for example oomycete zoospores (Van West et al., 2002). Some plant–microorganism interactions have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root, for example Sinorhizobium (Jones et al., 2007). For the grass endophyte Azoarcus, the putative type IV pilus retraction protein PilT is not essential for the bacterial colonization of the plant surface, but twitching motility is necessary for invasion of and establishment inside the plant (Böhm et al., 2007). Plant-associated bacteria used quorum-sensing signals and two-component regulatory systems to coordinate, in a cell density-dependent manner or in response to changing environmental conditions, the expression of important factors for host colonization and invasion (Soto et al., 2006). Using a highly sensitive biosensor increased AHL availability in intact rhizosphere microbial communities compared with that in bulk soil was shown by De Angelis et al. (2007). The success of invasion and survival within the host also requires that bacteria overcome plant defence responses triggered after microbial recognition, a process in which surface polysaccharides, antioxidant systems, ethylene biosynthesis inhibitors and virulence genes are involved (Soto et al., 2006). However, not only is the plant–microorganism interaction important for the composition of microbial communities in the rhizosphere, but the microorganisms can influence or select each other. This was shown for the influence of pathogens (Rasche et al., 2006b), introduction of beneficial microorganisms in the rhizosphere (Scherwinski et al., 2008) as well as specific interactions such as those between Bacillus cereus and bacteria of the Cytophaga–Flavobacterium group (Peterson et al., 2006).
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
Plant species, as well as the soil type, have a substantial influence on the structure and function of rhizosphere-associated microbial populations. As shown in many studies, there is no general decision about the key player: both factors can dominate depending on biotic and abiotic conditions. The rhizosphere is a unique microenvironment in terrestrial ecosystems and integrated in a complex microbial and macrobial network and food chains. However, the influence of the plant species themselves was clearly shown in almost all the studies discussed. Also the experimental design and applied methods influenced the findings. For example, community fingerprints of PCR products obtained with group-specific primers have a higher resolution than those generated with universal primers. Furthermore, the plant species and their phylogenetic positions and life style can have an influence on the results; for example, monocotyledonous grass species showed highly similar rhizosphere communities in diverse studies.
The finding of plant specificity might have implications for different areas. For agricultural applications, it is important to consider the crop species in the context with their associated microorganisms, because they fulfil important functions for plant growth, health and quality. Each crop selects its specific antagonistic microorganisms. This fact should be exploited in breeding strategies as it is already carried out with the resistance against phytopathogenic microorganisms (reviewed in Smith et al., 1999). Furthermore, the plant-dependent composition of antagonists has to be considered in crop rotations, mixed cropping strategies and soil treatments. Plant specificity is important for applications in biocontrol. However, the question of whether allochthonous or autochthonous bicontrol agents are more successful has to be solved in scientific studies. Also for issues of nature conservation, it is important to recognize that plant-specific microorganisms exist. Although the total number of plants worldwide remains unknown, Pitman & Jörgensen (2002) found that between 22% and 47% of the world's plants are endangered. If plants become extinct an unknown microbial diversity might be affected as well. An improved knowledge of specific plant–microorganism interactions in the rhizosphere could also be important for reforestation projects in which forests and woodlands that have been depleted should be replanted with native tree stock. Also, for invasive plants, the microorganism–plant interactions are an important issue influencing their competition within the native flora. Furthermore, the impact of climate change on plant–microorganism interaction, for example on plant pathogens, is important to predict.
Because of the fact that a high proportion of root-associated bacteria and fungi possess an antagonistic potential against other microorganisms, rhizosphere microorganisms are an important bioresource for bioactive substances, i.e. antibiotics, biosurfactants, enzymes and osmoprotective substances. In this respect, an enormous untapped pool of biological resources might be harboured in hot spots of plant diversity, such as Mediterranean heath lands or tropical rainforests. Interestingly, the rhizosphere is also a reservoir for opportunistic pathogens that have an increasing importance for human health (Berg et al., 2005a). To understand the ecological behaviour and factors of pathogenicity of these emerging pathogens is an important issue in the fight against diseases. One way could be to find new plant-derived metabolites to inhibit their colonization or regulation of pathogenicity. There are many open questions in rhizosphere microbial ecology, for example, to understand the enrichment process in the rhizosphere and the mechanisms involved in specificity (recognition, adherence). All of our knowledge is mainly based on agricultural or model systems and crops, but it is a major task for future research to expand these studies to native plants in natural ecosystems.
Plant–microorganism interaction might have contributed to plant evolution: to what extent did rhizosphere communities contribute to the exorbitant evolutionary diversity of >420 000 plants? More comparative, biogeographic studies are needed to assess a possible role of microbial communities as selective constraints for the diversification of plant populations and vice versa.
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
We thank Martin Grube (Graz) for stimulating discussions and Ilse-Marie Jungkurth (Braunschweig) for her support. We acknowledge Christin Zachow, Henry Müller and Massimiliano Cardinale (Graz) for providing Fig. 2. We also acknowledge the bilateral project BRA05/21 for providing an excellent environment for finalizing this review.
- Top of page
- Abstract
- Introduction
- Each plant species is colonized by specific microbial populations
- Mechanisms contributing to plant specificity and specific beneficial plant–microorganism interactions in the rhizosphere
- Conclusions
- Acknowledgements
- References
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Draghi Ally Asmussen to Leave ECB for German Government
Dec. 16 (Bloomberg) -- Joerg Asmussen is to leave the European Central Bank's Executive Board for a government position, depriving President Mario Draghi of a key German ally.
Asmussen, 47, will become deputy labor minister in German Chancellor Angela Merkel's government and will shortly resign from his post at the ECB, he said in a statement distributed by the Frankfurt-based institution yesterday. Asmussen has been on the six-person Executive Board, which implements monetary policy for the euro area, since January 2012. His term was scheduled to run until 2019.
Asmussen has been a crucial link to Germany for the ECB, criss-crossing the nation to explain what the central bank can and can't do to ease the euro area's debt crisis. Bundesbank President Jens Weidmann, the second German representative on the decision-making Governing Council, has been an outspoken critic of some policies including the bond-buying plan dubbed Outright Monetary Transactions.
"Asmussen's network into the German government will surely be missed," said Carsten Brzeski, senior economist at ING Groep NV in Brussels. "He wasn't afraid to represent the ECB's view toward the German media. He acted as a counterweight and convincingly defended the ECB's line of argument."
Personal Reasons
Weidmann and Asmussen gave opposing evidence to the German Constitutional Court this year in a hearing to determine the legality of the OMT program. While Weidmann argued that bond markets exert discipline on countries' finances, Asmussen said the euro area faced break-up last year as investors began to price in an end to the currency.
Asmussen's early departure from the ECB follows that of Bundesbank President Axel Weber in February 2011 and board member Juergen Stark in September that year. It opens Germany up to criticism that its representatives don't honor their appointments, which won't make it easier for the country to push through its own candidate for Asmussen's successor or to win the ECB presidency at some point.
Other nations may push for a place on the board. Germany, France and Italy, the region's biggest economies, have always had a representative. Spain, the fourth-largest, gave up its seat when Jose Manuel Gonzalez-Paramo left in May 2012.
"I did not take this decision lightly," Asmussen said in the statement. "The reasons behind this decision are purely of a private nature. The seat of the ECB in Frankfurt with my family with my two very young children in Berlin were not compatible over the long term."
Female Candidates
Merkel told reporters in Berlin that she is "pleased that Mr. Asmussen is again putting his experience at the disposal of the coalition." Asmussen, a University of Bonn-trained economist, worked in the finance ministry from 1996 to 2011.
Draghi said in an ECB statement that Asmussen "has been a tremendous help in shaping the monetary policy in the past two years while successfully addressing many other challenges" and that he will "personally miss him."
Jeroen Dijsselbloem, who chairs the group of euro-area finance ministers, will now have to ask for nominations for Asmussen's successor, just over a year after the last change to the board.
The need for a new member may renew calls for a female candidate. When Yves Mersch joined in November 2012, he overcame the longest battle over an appointment in the euro's history as the European Parliament pushed for the position to be given to a woman.
Possible Nomination
German Finance Minister Wolfgang Schaeuble indicated that Bundesbank Vice President Sabine Lautenschlaeger may be nominated to succeed Asmussen. "One of the duties at the ECB is now to build up banking supervision, and that's where she has a lot of experience," he said today in a radio interview with Deutschlandfunk.
A Bundesbank spokesman declined to comment earlier on whether Lautenschlaeger would be a candidate.
The ECB's executive board, which currently consists entirely of men, has previously included two women: Sirkka Hamalainen of Finland and Gertrude Tumpel-Gugerell of Austria.
To contact the reporters on this story: Paul Gordon in Frankfurt at firstname.lastname@example.com; Rainer Buergin in Berlin at firstname.lastname@example.com
To contact the editor responsible for this story: Craig Stirling at firstname.lastname@example.com


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Correction: Fantasia is Not Married
You men who fantasize daily about being singer Fantasia Barrino's boo can breathe a little easier.
Despite the many rumors that she and boyfriend Kendall Taylor tied the knot, they are not yet married. So you still have a chance to be with her — if you can pry her away from her current source of Dopamine (Kendall).
Fantasia's Husband Tattoos Her Name Across His Heart
Fantasia's current husband Kendall Taylor is tired of all the talk that he's just using the cerebrally challenged singer for her money. In response to all the hateful comments on social media, Taylor tattooed Fantasia's name across his heart. Media mogul Nick Cannon made the same mistake — and now he regrets it.
Has Fantasia Known This Man Long Enough to Marry Him?
Apparently singer Fantasia Barrino just up and married a man she barely knows.
Sorry Fellas, Fantasia is Off the Market
R&B Diva Fantasia Barrino has apparently found a new man. It's too early to tell if it's love but Fantasia certainly thinks Kendall Taylor is the man of her dreams. She can't stop gushing about him in her social media posts.Fantasia's poor choices in men is legendary. Hopefully history won't repeat itself.
Fantasia Upset That Her Son Spends Too Much Time With His Stepmother
Singer Fantasia Barrino is upset with her ex-boyfriend Antwaun Cook for allowing his son to spend too much time with his wife, Paula Cook, while she's away at work. Barrino is currently working on Broadway in After Midnight, which leaves her little time to see the toddler.
Radar Online reports Fantasia is upset because Cook has joint custody of the 2-year-old boy, who celebrated his birthday with Cook's wife, Paula.
Morning Glory: Fantasia Barrino
R&B singer Fantasia vacationed in Barbados with family and friends on Tuesday, Sept. 17. The "American Idol" winner canceled a concert scheduled for Thursday, Sept. 19, in Phoenix, so she could take a break in Barbados. Fantasia is nominated for two Soul Train Awards, which will be taped Nov. 8 at the Orleans Arena, and is scheduled to air on Dec. 1, 2013 on BET Centric.
Photos: Islandpaps / Splash News
Fantasia Blames Media for Breaking Up Her Relationship
R&B singer Fantasia blames the media (specifically the blogs) for breaking up her ill-fated relationship with Antwaun Cook.
Like most people with personality disorders, Fantasia pities herself and blames everyone around her for her problems. Fantasia, who is promoting her 4th studio album, Side Effects of You, said the album's ballads about heartbreak are not based on her relationship with the former T-Mobile salesman.
"All of my music is not just based on Antwaun," said Fanty in an interview with Ifelicious
"You know, he plays a big part. I think with the songs like, you know, where I speak about 'I'm a lady, I'm a woman,' and I came in at the beginning of the song and I say 'Look, the world says this love is no good to me, but my heart says that it is.' So I think it's my way of speaking to everybody… You only heard what you heard, you only got what you got. But you never got to see that real love and that relationship, that friendship that we had."


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Microsoft kills Office anti-piracy program
Microsoft last week killed an anti-piracy service that checked whether customers were running legal copies of Office, saying that the program had "served its purpose."
Microsoft last week killed an anti-piracy service that checked whether customers were running legal copies of Office, saying that the program had "served its purpose."
When a photo of Facebook CEO Mark Zuckerberg meeting with the head of China's largest search engine hit the Internet, speculation began to run wild.
Intel will make a big splash at the Consumer Electronics Show on Jan. 6 with its Atom chips featured inside a variety of tablet computers and netbooks, but the chips are not expected to appear in smartphones until the last half of 2011.
More than half of iPhone and Android apps routinely share the personal data of their users with outside companies, a Wall Street Journal investigation has found.
Nimbula's founders developed the Amazon EC2 public cloud system and are now working on Nimbula Director, which aims to partition internal cloud resources by authentication, along the lines of how EC2 works.
Facebook CEO Mark Zuckerberg visited the campus of China's largest search engine Baidu on Monday, meeting with the company's CEO Robin Li and other senior staff.
Nattering nabobs of negativism aside, the marketplace for Windows Phone 7 apps is doing quite well, thank you. A recent analysis from IDC shows that the WP7 marketplace is growing at a faster clip out of the gate than its counterpart for Google's And...
WikiLeaks' posting of classified U.S. Department of State cables, and the Web site's revelation that it will soon post sensitive internal documents from a major U.S. bank, has stoked data security concerns among governments and corporations around th...
Australia's peak technology industry representative body has told Australian businesses to start preparing for the onset of the National Broadband Network or risk being left behind, in the wake of the release yesterday of NBN Co's business plan outli...
Nvidia's much-hyped Tegra 2 CPU has finally found a home in a smartphone courtesy of LG's new Optimus 2X, which will be launched in Korea next month. As such, the Optimus 2X takes the honor of being the first dual-core phone to get to market.
Microsoft's Xbox 360 is the most used console among users aged 13 or older, according to a new survey from media tracker Nielsen. "Most used", meaning everything from playing games to watching movies or catching some tunes to surfing the internet.
We were wrong -- so far -- that Carol Bartz would be ousted as Yahoo CEO by the end of this year, but we were right that Apple's tablet, whose name wasn't known at the end of last year, would be huge. OK, so that second one was probably a given, but ...
Addressing the growing market for tools that handle very large data sets, Microsoft has released a beta set of technologies, called Dryad, to manage and analyze large amounts of information across a cluster of Windows Servers.
Amazon has brought a fleet of major PeopleSoft and JD Edward applications to its Elastic Cloud Compute (EC2) service, following a promise in September to do so. The new services are certified by Oracle.
Hitachi Global Storage Technologies has announced the industry's highest density, single-platter hard disk drive that has an areal density of 636Gbit/inch, almost 100Gbit per square inch more than its closest competitor.
There's never been a better time for a business to refresh its devices. Intel offers a range of business solutions, from Servers to SSD, but also 2 in 1 products and also the NUC unit which is set to revolutionise your desktop. Click here to read more information on these products.
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First impression on unpacking the Q702 test unit was the solid feel and clean, minimalist styling.
For work use, Microsoft Word and Excel programs pre-installed on the device are adequate for preparing short documents.
The Fujitsu LifeBook UH574 allowed for great mobility without being obnoxiously heavy or clunky. Its twelve hours of battery life did not disappoint.
The screen was particularly good. It is bright and visible from most angles, however heat is an issue, particularly around the Windows button on the front, and on the back where the battery housing is located.
My first impression after unboxing the Q702 is that it is a nice looking unit. Styling is somewhat minimalist but very effective. The tablet part, once detached, has a nice weight, and no buttons or switches are located in awkward or intrusive positions.


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House Speaker Zellers visits Worthington
WORTHINGTON -- Curtailing the size of state government was the central theme during a Tuesday visit by Minnesota House of Representatives Speaker Kurt Zellers to the Daily Globe.
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Arsenal FC captain Per Mertesacker said that his side had a "mountain to climb" as they lost to AS Monaco FC on away goals but that their performance in the second leg will give them "confidence" for the rest of the season.


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Tomato leaves yellow, some spotted
I've got three tomato varieties growing in containers -- Juliet hybrid, roma, and brandywine. I transplanted them a couple months ago and for a while they were growing really well -- I've been using Fox Farm Grow Big and then Big Bloom.
Now on all three of them, but especially the brandywine, the leaves are getting yellow and falling off. Some of them are spotted with dark patches and holes in them. I've noticed some red aphids (I think) on the brandywine, but just a handful that I've sprayed off -- not an infestation.
Any other ideas? Is this a sign of overwatering, underwatering, overfertilizing, or something else? I'm using big bloom about twice a week.
I've added a link to photos online -- in one of them you'll see one of the central stems of the brandywine, which has gone slightly brown and mottled, although the stem above it continues to grow.
Any help would be appreciated! Thanks.
Here is a link that might be useful: Pictures of tomatoes
Yeah I have suffered early blight on my last couple of grows and looks very similar to what you have, remove all infected leaves and spray with a Chlorothalonil based fungicide was what has been suggested to me. It will eventually kill the plant if left untreated but, for me at least, a fair amount of fruit had been harvested by that time.


text: Find this wine Find this wine Black Stallion Napa Valley 2008 Good cab. Needs to open first to improve the flavors. 4 of 5 stars. Black. Stallion Estate winery. Pin It Tweet 2 years ago by Kethomas More posts Black Stallion Napa Valley 2013 Clean fresh taste 5 months ago by Dial S. Black Stallion Estate Winery Napa Valley Zinfandel 2012 Liked this as well. Can't buy in stores. Online or napa 6 months ago by Misty B. Black Stallion Winery Napa Valley Cabernet Sauvignon 2010 Really liked. Can't get in stores. Order online. 6 months ago by Misty B. Black Stallion Napa Valley Cabernet Sauvignon 2011 Very nice cab 7 months ago by Sookie49 Black Stallion Napa Valley 2011 Phillip brought. Similar to Jordan. Lower price. 8 months ago by Marcialou123


text:
Because freedom can't protect itself
ACLU client Pascal Abidor was traveling from Montreal to New York on Amtrak when his laptop was searched and confiscated at the Canadian border. Abidor, an Islamic Studies Ph.D. student, was questioned, handcuffed, taken off the train and kept in a holding cell for several hours before being released without charge. When his laptop was returned 11 days later, there was evidence that many of his personal files, including research, photos, and chats with his girlfriend, had been searched.
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Dear Abby: Teen's friend on path to self-destruction
This San Francisco home 'in a deteriorative state' just sold...
Kleiner Perkins prevails in Ellen Pao sex-bias case
Warriors dominate in Memphis to equal franchise win record
Italian court throws out Knox conviction once and for all
Posts filtered on Category
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It's a trip that puts the "mountain" in mountain biking. If you're an avid biker — maybe bat-crap-crazy biker is more accurate — and you love travel and challenges, the nice folks with bike-tour …
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It's not exactly as if National Park fees required a second mortgage in the first place, but next week any lame excuse you might have had for not visiting the parks that involved money is pretty much …


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Bono reveals he may never play guitar again
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text:
Oh man, years on this forum... and, still no Merlin Fat!
Where am I going to find one? Anyone have any advice on finding one? Any shops near you have a "large"?
Any info is greatly appreciated.
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