Sabine Ravnskov

Arbuscular mycorrhizal phosphate transport under monoxenic conditions using radio-labelled inorganic and organic phosphate.

Compartmented monoxenic cultures of Ri T-DNA transformed carrot roots and a symbiotic arbuscular mycorrhizal fungus demonstrated for the first time that phosphate in an organic form (32P-labelled AMP) may be hydrolysed by extra-radical mycorrhizal hyphae in the absence of other organisms, and subsequently utilized as a mineral nutrient source by the host plant after fungal transport.

Linking fungal communities in roots, rhizosphere, and soil to the health status of Pisum sativum.

Changes in fungal communities associated with healthy and diseased pea roots were investigated using deep amplicon pyrosequencing in three spatial compartments: roots, rhizosphere, and surrounding soil. Thirty root systems were collected from three fields, half of which showing clear symptoms of root rot. In total, 500,461 internal transcribed spacer-1 sequences were obtained that were clustered into 123 (roots), 271 (rhizosphere), and 440 (bulk soil) nonsingleton operational taxonomic units (OTUs). Species richness was highest in bulk soils and lowest in roots; however, no notable differences in richness were observed between samples associated with diseased and healthy roots. Health status and field both had significant effects on fungal community structures in roots, whereas only field had significant effects on communities in rhizosphere and bulk soils. Indicator species analysis across the three fields identified a number of OTUs that were more abundant in healthy roots. Pathogens such as Fusarium oxysporum were abundant in diseased roots in some fields. Patterns of disease and causal agents of root rot were different among the three fields, which were also reflected in fungal communities. In conclusion, health status of roots was only vaguely reflected in rhizosphere and bulk soil fungal communities, whereas health status was more important for shaping root communities.

Effects of Pseudomonas fluorescens DF57 on growth and P uptake of two arbuscular mycorrhizal fungi in symbiosis with cucumber

Abstract The effect of Pseudomonas fluorescens DF57 on growth and P uptake of two arbuscular mycorrhizal (AM) fungi in symbiosis with cucumber plants was studied in compartmentalised growth systems. Hyphae of Glomus intraradices Schenck & Smith (BEG87) or G. caledonium (Nicol. & Gerd.) Trappe & Gerdeman (BEG15) grew into lateral root-free compartments. Non-mycorrhizal plants served as control. The soil in half of the growth units of each mycorrhizal treatment was inoculated with P. fluorescens DF57. P. fluorescens DF57 enhanced hyphal length density of one of the AM fungi, G. caledonium, but this was not reflected in a higher hyphal transport of P from the root-free soil to the plant. The total P content was higher in plants grown in symbiosis with G. intraradices than in plants in the other treatments. G. caledonium and P. fluorescens DF57 had a synergistic effect in that total P content in plants inoculated with G. caledonium was higher in the presence than in the absence of P. fluorescens DF57.

Phosphorus uptake of an arbuscular mycorrhizal fungus is not effected by the biocontrol bacterium Burkholderia cepacia

The biocontrol bacterium Burkholderia cepacia is known to suppress a broad range of root pathogenic fungi, while its impact on other beneficial non-target organisms such as arbuscular mycorrhizal (AM) fungi is unknown. Direct interactions between five B. cepacia strains and the AM fungus, Glomus intraradices (BEG87) were studied in root-free soil compartments separated from a rooting compartment by a fine nylon-mesh. B. cepacia had no effect on AM fungal biomass and energy reserves measured using the signature fatty acid 16:1ω5 from phospholipid fatty acids (PLFAs) and neutral lipid fatty acids (NLFAs), respectively. Hyphal P transport was also unaffected by the biocontrol bacterium, which either stimulated, reduced or had no effect on length of the external mycelium of G. intraradices. The cyclic PLFAs cy17:0 and cy19:0 were suggested to be useful markers for estimation of biomass of B. cepacia. The presence of mycelium of G. intraradices reduced the biomass of three out of five B. cepacia strains as indicated by a reduction in PLFAs cy17:0 and cy19:0, while other bacterial PLFAs were unaffected by mycelium of G. intraradices. On the other hand, two out of five B. cepacia strains reduced the amount of branched PLFAs suggesting a reduction in the population of Gram-positive bacteria in these cases. In conclusion, the B. cepacia seems to have no impact on neither mycorrhiza formation nor on the functioning of the AM fungus G. intraradices in terms of P transport, whereas our results suggest that mycorrhiza might have adverse effects on B. cepacia.

Succession of root-associated fungi in Pisum sativum during a plant growth cycle as examined by 454 pyrosequencing

PurposeRoots are inhabited by a broad range of fungi, including pathogens and mycorrhizal fungi, with functional traits related to plant health and nutrition. Management of these fungi in agroecosystems requires profound knowledge about their ecology. The main objective of this study was to examine succession patterns of root-associated fungi in pea during a full plant growth cycle.MethodsPlants were grown in pots with field soil in a growth chamber under controlled conditions. Fungal communities in pea roots were analyzed at different plant growth stages including the vegetative growth, flowering and senescence, using 454 pyrosequencing.ResultsOne hundred and twenty one non-singleton operational taxonomic units (OTUs) representing fungal species were detected. Pathogenic and arbuscular mycorrhizal fungi dominated during the vegetative growth stage, whereas saprotrophic fungi dominated during plant senescence.ConclusionsIn conclusion, the results from the present study demonstrated highly diverse fungal communities in pea roots with clear succession patterns related to fungal traits.

Molecular characterization of root-associated fungal communities in relation to health status of Pisum sativum using barcoded pyrosequencing

Background and AimsRoot-associated fungi are known to be important for plant health and nutrition, but only few studies have addressed their diversity in relation to plant health status.MethodsFungal diversity in roots of healthy and diseased Pisum sativum plants was examined in terms of barcoded pyrosequencing of the nuclear ribosomal internal transcribed spacer 1. The CLOTU program was used for filtering and clustering of sequences, and Chao 1 estimator was used to calculate fungal richness.ResultsPrincipal component analyses (PCA) showed that the structure of root-associated fungal communities differed between sites and a clear relationship between root-associated fungal communities and plant health status was found. For example, the arbuscular mycorrhizal (AM) fungus Glomus caledonium was prevalent in roots of healthy plants, whereas the pathogenic fungus Phoma sojicola was prevalent in roots of diseased plants.ConclusionsThe present study revealed clear differences in composition of root-associated fungi from four field sites, and correlations between abundance of several root-associated fungi and plant health status were found.

Combined effect of an arbuscular mycorrhizal fungus and a biocontrol bacterium againstPythium ultimum in soil

The combined effect of the arbuscular mycorrhizal (AM) fungusGlomus intraradices and the bacteriumBurkholderia cepacia onPythium ultimum was studied in a growth system with root-free soil compartments allowing examination of microbial interactions without direct interference from roots. Signature fatty acids were used to quantify individual microbial populations. Biomass and amount of energy reserves were estimated using specific phospholipid fatty acids (PLFA) and neutral lipid fatty acids (NLFA), respectively. Population density ofP. ultimum was estimated by colony forming units on selective media. Both biocontrol agents (BCAs) reduced population density ofP. ultimum. Biomass ofB. cepacia measured using signature fatty acids was inhibited by the presence of mycelium ofG. intraradices, which might explain why no additive effect onP. ultimum was found when combining the two BCAs. In conclusion, combiningG. intraradices andB. cepacia did not enhance the biocontrol efficacy againstP. ultimum.

Differential effects of Paenibacillus spp. on cucumber mycorrhizas

The effects of 17 Paenibacillus strains on root colonization by Glomus intraradices or Glomus mosseae and plant growth parameters (shoot and root weight) of mycorrhizal cucumber plants were examined. The Paenibacillus strains were originally isolated from mycorrhizal (G. intraradices) and non-mycorrhizal cucumber rhizosphere and/or hyphosphere, except for strain EJP73, which originated from a Pinus sylvestris-Lactarius rufus ectomycorrhiza. Root colonization of cucumber plants by G. intraradices or G. mosseae was unaffected by all seven strains of Paenibacillus polymyxa, but was decreased or increased by four strains of Paenibacillus macerans and strain EJP73 of Paenibacillus sp. Overall, shoot dry weight of cucumber grown in symbioses with either G intraradices or G. mosseae was unaffected by inoculation with all of the Paenibacillus strains, except for strain MB02-429 of P. macerans, which increased the shoot dry weight in the cucumber-G. mosseae symbiosis. On the other hand, several Paenibacillus strains caused altered root growth. Three strains of P. polymyxa and four strains of P. macerans increased the root fresh weight of the cucumber–G. intraradices symbiosis, whereas three strains of P. polymyxa and one strain of P. macerans as well as Paenibacillus sp. EJP73, decreased the root fresh weight of the cucumber–G. mosseae symbiosis. In conclusion, our results show that bacteria from several species of Paenibacillus differentially affect cucumber mycorrhizas.

Changes in carbon and nitrogen allocation, growth and grain yield induced by arbuscular mycorrhizal fungi in wheat (Triticum aestivum L.) subjected to a period of water deficit

Drought is a major abiotic factor limiting agricultural crop production. One of the effective ways to increase drought resistance in plants could be to optimize the exploitation of symbiosis with arbuscular mycorrhizal fungi (AMF). Hypothesizing that alleviation of water deficits by AMF in wheat will help maintain photosynthetic carbon-use, we studied the role of AMF on gas-exchange, light-use efficiencies, carbon/nitrogen ratios and growth and yield parameters in the contrasting wheat (Triticum aestivum L.) cultivars ‘Vinjett’ and ‘1110’ grown with/without AMF symbiosis. Water deficits applied at the floret initiation stage significantly decreased rates of photosynthetic carbon gain, transpiration and stomatal conductance in the two wheat cultivars. AMF increased the rates of photosynthesis, transpiration and stomatal conductance under drought conditions. Water deficits decreased electron transport rate and increased non-photochemical quenching (NPQ) in ‘1110’ but not in ‘Vinjett’. With AMF, nitrogen concentrations increased in roots of both cultivars, but decreased in grains of ‘Vinjett’ and in side-tiller grains of ‘1110’ regardless of water status. With water deficits, AMF colonization increased plant height in both cultivars. AMF also increased biomass and grain yield in ‘1110’ but not in ‘Vinjett’. The results showed that the improvements in growth and yield were the results of AMF-mediated increases in photosynthesis during drought stress and that the alleviating effect of AMF depended on the wheat cultivar.

Influence of DNA extraction and PCR amplification on studies of soil fungal communities based on amplicon sequencing

Most studies involving next-generation amplicon sequencing of microbial communities from environmental studies lack replicates. DNA extraction and PCR effects on the variation of read abundances of operational taxonomic units generated from deep amplicon 454 pyrosequencing was investigated using soil samples from an agricultural field with diseased pea. One sample was extracted four times, and one of these samples was PCR amplified four times to obtain eight replicates in total. Results showed that species richness was consistent among replicates. Variation among dominant taxa was low across replicates, whereas rare operational taxonomic units showed higher variation among replicates. The results indicate that pooling of several extractions and PCR amplicons will decrease variation among samples.