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Bacterial and fungal populations of healthy and rotted black spruce seedlings are compared. The methodological approach described in this analysis is very helpful and the results are discussed.

Abstract: The inclusion of legumes in semi‐arid native grasslands may promote the productivity and nutritive value of forage. This study was designed to assess the effect of legumes (the introduced legume Medicago sativa or the native legume Dalea purpurea) and soil P fertility (addition of 0, 50, or 200 P2O5 kg/ha at seeding) on the dry matter and nutrient content of native grasses mixtures, compared with the commonly used introduced forage grass Bromus biebersteinii grown with M. sativa. Plant harvests were performed in September 2008, July 2009 and September 2009. Plants nutrient content, δ15N value and dry matter were analysed. Results show that the M. sativa enhanced the N and P concentrations of native grass mixtures early in the summer, as well as the N concentration in Bouteloua gracilis in late summer of the driest year, 2009. The higher AM fungal diversity promoted by M. sativa was positively correlated with the dry matter and nitrogen uptake of M. sativa and with the P concentration of native grasses, in early summer. Overall, this study shows that M. sativa promoted beneficial AM fungal taxa and improved forage production in the semi‐arid prairies. [ABSTRACT FROM AUTHOR]

Anthropogenic interventions play a key role in promoting positive feedback of soil–plant–environment interactions, but systematic reports on how anthropogenic activities influence soil physiochemical, microorganism-induced properties and soil health are still limited. Here, we assessed the impact of anthropogenic interventions, including crop diversification in rotations, soil physical disturbance, synthetic chemical inputs, and biofertilizer use on soil microbial community structure and function, and the consequential effects on agroecosystem productivity and environmental sustainability. Summarizing the results of over 160 medium- to long‐term experiments from various soil-climatic zones across the globe in this review illustrated that (1) increasing crop diversification in rotations could bring positive impacts on soil microorganisms and soil health, especially including legume crops in rotations. (2) However, monocultures such as continuous wheat cropping could negatively impact soil health by enhancing activities of host specific pathogens. (3) Physical agronomic practices such as tillage can alter soil microbial communities by shifting microclimate conditions. (4) Mineral nitrogen fertilizer use, a leading nutrient input, may have exceeded the planetary boundary of N cycling, and is causing soil acidification and decreasing microbial biomass. (5) Synthetic chemicals, essential for disease management (pesticides) and yield sustainability (fertilization) in conventional agroecosystems are often toxic to non-target soil microorganisms, while bio-fungicides and biofertilizers—a more sustainable approach—carry significant risks to trigger succession of the native soil microbial community, thus impacting soil health. The key is to establish a rational balance between anthropogenic activities for agroecosystem productivity and potential negative influences on the soil microbial community and long-term soil health.

The role of arbuscular mycorrhizas in response of plants to soilborne root pathogens is unclear. A time course study was conducted to monitor disease development and expression of mRNA for the defence-related genes phenylalanine ammonia lyase, chalcone synthase, chalcone isomerase, and hydroxyproline-rich glycoprotein in bean (Phasoelus vulgaris L.) plants colonized by the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith and postinfected with the soilborne pathogen Rhizoctonia solani Kühn. Precolonization of bean plants by G. intraradices did not significantly reduce the severity of rot symptoms. RNA blot analysis of the defence-related genes revealed a systemic increase in the four defence genes in response to R. solani infections. On the other hand, precolonization of bean plants with G. intraradices elicited no change in phenylalanine ammonia lyase, chalcone synthase, and chalcone isomerase transcripts. A differential and systemic alteration in the expression of all four defence genes was observed in all tissues only during the pathogenic interaction of arbuscular mycorrhizal beans. Depending on the time after infection with R. solani and the tissue examined, varying responses from stimulation to suppression to no change in transcript levels were detected.Key words: induced resistance, defence-related genes, RNA analysis, Rhizoctonia solani, Glomus intraradices.Le rôle des mycorhizes arbusculaires dans la réaction des plantes aux maladies du sol n'est pas clair. Une étude a été réalisée afin de suivre le développement de la maladie ainsi que l'expression des gènes codant pour les enzymes de défense phénylalanine ammonia-lyase, chalcone synthase, chalcone isomérase, et les glycoprotéines riches en hydroxyproline chez des plants de haricot (Phaseolus vulgaris L.) colonisés par le champignon endomycorhizien arbusculaire Glomus intraradices Schenck & Smith et subséquemment infectés par le parasite racinaire Rhizoctonia solani Kühn. La pré-colonisation des plants de haricot par le champignon endomycorhizien arbusculaire n'a pas significativement réduit la sévérité des symptômes de la maladie. L'analyse de transferts d'ARN sur membrane avec hybridisation à des marqueurs moléculaires désignés (RNA blot analysis) a été réalisé pour les différents gènes de défense étudiés et a révélé une augmentation systémique de l'expression des quatre gènes de défense en réponse à l'infection par le R. solani. La pré-colonisation des plants d'haricot par le G. intraradices n'a pas modifié l'expression des gènes codant pour les phénylalanine ammonia-lyase, chalcone synthase et chalcone isomérase. Une altération systémique de l'expression des quatre gènes de défense a été observée chez les plants mycorhizés et subséquemment infectés par le R. solani. Les résultats obtenus ont varié (stimulation, suppression ou aucun changement dans l'expression des gènes étudiés) en fonction du temps de récolte après l'infection par le R. solani ainsi que de la partie de la plante examinée.Mots clés : résistance induite, gènes reliés à la défense, analyse de l'ARN, Rhizoctonia solani, Glomus intraradices. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) fungi can reduce the incidence and importance of plant root diseases caused by pathogens. The mechanisms involved are not well characterized. We used an in vitro experimental system to test the hypothesis that the extraradical mycelium of AM fungi can interfere directly with microorganisms in the mycosphere and directly or indirectly reduce the population of plant pathogens. This system permitted the isolation of soluble substance released by the extraradical mycelium of Glomus intraradice. The AM fungus was grown on Daucus carota transformed roots in one compartment, while only the extraradical mycelium was allowed to grow in a second compartment. A freezing and centrifugation technique was developed for the extraction and concentration of substance present in the compartment containing only the AM fungal mycelium. Four soil-inhabiting microorganisms were selected, and conidial germination (fungi) or growth (bacteria0 of these was studied in the presence and absence (control) of the extract. In comparison with the control, the results indicated that both the growth Pseudomonas chlororaphis and the conidial germination of Trichoderma harzianum were stimulatedin the presence of the AM fungal extract. In contrast, conidial germination of Fusarium oxysporum f. sp. chrysanthemi was reduced while the growth of Clavibacter michiganensis subsp. michiganensis was not affected. The measured effects in general were directly correlated with extract concentration. Differences in pH were noted between the extract containing substances released by the AM fungus and the non-AM control, but no significant influence of the pH on growth or conidial germination was noted, confirming that substances released by the AM fungus in the growth medium is the main factor explaining differential growth of the microorganisms tested. The results suggest that direct interactions exist between AM fungi and soil microorganisms, which might lead to changes in microbial equilibrium detrimental to pathogens. [ABSTRACT FROM AUTHOR]

Abstract Background A conventional tenet of classical genetics is that progeny inherit half their genome from each parent in sexual reproduction instead of the complete genome transferred to each daughter during asexual reproduction. The transmission of hereditary characteristics from parents to their offspring is therefore predictable, although several exceptions are known. Heredity in microorganisms, however, can be very complex, and even unknown as is the case for coenocytic organisms such as Arbuscular Mycorrhizal Fungi (AMF). This group of fungi are plant-root symbionts, ubiquitous in most ecosystems, which reproduce asexually via multinucleate spores for which sexuality has not yet been observed. Results We examined the number of nuclei per spore of four AMF taxa using high Z-resolution live confocal microscopy and found that the number of nuclei was correlated with spore diameter. We show that AMF have the ability, through the establishment of new symbioses, to pass hundreds of nuclei to subsequent generations of multinucleated spores. More importantly, we observed surprising heterogeneity in the number of nuclei among sister spores and show that massive nuclear migration and mitosis are the mechanisms by which AMF spores are formed. We followed spore development of Glomus irregulare from hyphal swelling to spore maturity and found that the spores reached mature size within 30 to 60 days, and that the number of nuclei per spores increased over time. Conclusions We conclude that the spores used for dispersal of AMF contain nuclei with two origins, those that migrate into the spore and those that arise by mitosis in the spore. Therefore, these spores do not represent a stage in the life cycle with a single nucleus, raising the possibility that AMF, unlike all other known eukaryotic organisms, lack the genetic bottleneck of a single-nucleus stage.

Bacterial and fungal populations associated with the rhizosphere of healthy black spruce (Picea mariana) seedlings and seedlings with symptoms of root rot were characterized by cloned rRNA gene sequence analysis. Triplicate bacterial and fungal rRNA gene libraries were constructed, and 600 clones were analyzed by amplified ribosomal DNA restriction analysis and grouped into operational taxonomical units (OTUs). A total of 84 different bacterial and 31 different fungal OTUs were obtained and sequenced. Phylogenetic analyses indicated that the different OTUs belonged to a wide range of bacterial and fungal taxa. For both groups, pairwise comparisons revealed that there was greater similarity between replicate libraries from each treatment than between libraries from different treatments. Significant differences between pooled triplicate samples from libraries of genes from healthy seedlings and pooled triplicate samples from libraries of genes from diseased seedlings were also obtained for both bacteria and fungi, clearly indicating that the rhizosphereassociated bacterial and fungal communities of healthy and diseased P. mariana seedlings were different. The communities associated with healthy and diseased seedlings also showed distinct ecological parameters as indicated by the calculated diversity, dominance, and evenness indices. Among the main differences observed at the community level, there was a higher proportion of Acidobacteria, Gammaproteobacteria, and Homobasidiomycetes clones associated with healthy seedlings, while the diseased-seedling rhizosphere harbored a higher proportion of Actinobacteria, Sordariomycetes, and environmental clones. The methodological approach described in this study appears promising for targeting potential rhizosphere-competent biological control agents against root rot diseases occurring in conifer nurseries. [ABSTRACT FROM AUTHOR]