Fungal Root Colonization Research Articles

Responses of fungal root colonization, plant cover and leaf nutrients to long‐term exposure to elevated atmospheric CO2 and warming in a subarctic birch forest understory

AbstractResponses of the mycorrhizal fungal community in terrestrial ecosystems to global change factors are not well understood. However, virtually all land plants form symbiotic associations with mycorrhizal fungi, with approximately 20% of the plants' net primary production transported down to the fungal symbionts. In this study, we investigated how ericoid mycorrhiza (ErM), fine endophytes (FE) and dark septate endophytes (DSE) in roots responded to elevated atmospheric CO2 concentrations and warming in the dwarf shrub understory of a birch forest in the subarctic region of northern Sweden. To place the belowground results into an ecosystem context we also investigated how plant cover and nutrient concentrations in leaves responded to elevated atmospheric CO2 concentrations and warming. The ErM colonization in ericaceous dwarf shrubs increased under elevated atmospheric CO2 concentrations, but did not respond to warming following 6 years of treatment. This suggests that the higher ErM colonization under elevated CO2 might be due to increased transport of carbon belowground to acquire limiting resources such as N, which was diluted in leaves of ericaceous plants under enhanced CO2. The elevated CO2 did not affect total plant cover but the plant cover was increased under warming, which might be due to increased N availability in soil. FE colonization in grass roots decreased under enhanced CO2 and under warming, which might be due to increased root growth, to which the FE fungi could not keep up, resulting in proportionally lower colonization. However, no responses in aboveground cover of Deschampsia flexuosa were seen. DSE hyphal colonization in grass roots significantly increased under warmer conditions, but did not respond to elevated CO2. This complex set of responses by mycorrhizal and other root‐associated fungi to global change factors of all the fungal types studied could have broad implications for plant community structure and biogeochemistry of subarctic ecosystems.

Essential Oil Content and Composition, Nutrient and Mycorrhizal Status of Some Aromatic and Medicinal Plants of Northern Greece

A field survey was conducted in three northern Greek mountain areas (Chortiatis, Ossa, and Pieria) to investigate the mycorrhizal and nutritional status, and the essential oil content and composition of common medicinal and aromatic plants. A range of values for nutrient status and essential oil contents and composition was established. All plants were found to be mycorrhizal, including Achillea coarctata Poir., Micromeria juliana (L.) Bentham ex Reichenb., and Salvia sclarea L.;. these three are reported as being mycorrhizal for the first time. Arbuscular mycorrhizal fungal root colonization was highest in Pieria, exceeding 80% for all 15 plants sampled, and lower in Chortiatis and Ossa.

Mycorrhizal colonization and growth of Phragmites australis in an intermittent wetland

Arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungal colonization of Phragmites australis roots and its relation to plant productivity parameters was investigated in an intermittent karst wetland Lake Cerknica (Slovenia) with seasonal water level fluctuations. Mycorrhizal colonization was determined five times in 2008–from the beginning to the end of the growing season–in three reed stands with large differences in soil parameters and water level changes. AM and DSE colonization varied significantly between sampling plots and during the growing season (7% < frequency < 87%). The flooding of two of the three reed stands decreased mycorrhiza early in the growing season, but further flooding had no additional effect. Levels of DSE colonization were high in the beginning and the end of the growing season, while a peak in AM colonization was observed towards the end. The observed positive in situ correlation between mycorrhizal colonization and P. australis growth parameters does not allow final mechanistic conclusion: both P. australis and mycorrhiza could be beneficial.

Effectors, effectors et toujours des effectors

Effectors, effectors <i>et toujours des</i> effectors

How Abundant are Root-Colonizing Fungi in Southeastern Louisiana’s Degraded Marshes?

Despite earlier notions that fungi are not important in wetlands, it is becoming clear that root endophytes are abundant in wetlands and potentially can influence plant community dynamics. Little is known about the effects of wetland degradation on these fungi. We assessed two groups of root endophytes in a degrading marsh in southeast Louisiana that historically was a swamp forest dominated by Taxodium distichum (baldcypress) and Nyssa aquatica (water tupelo). We determined percent root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) in each of 18 vascular plant species. Fungi were present in all species that were assessed. In general, monocots were primarily colonized by DSE, whereas dicots were primarily colonized by AMF. Taxodium distichum was heavily colonized by AMF, as was the non-native, invasive Chinese tallow (Triadica sebifera). This study is the first to show that wetland plants in a degraded marsh harbor abundant and diverse root endophytes. These fungi and their interactions with stressed plants may be important in effective management of degrading wetlands.

Cytological investigations on colonization of sorghum roots by the mycoherbicide Fusarium oxysporum f. sp. strigae and its implications for Striga control using a seed treatment delivery system

The application of the potential Striga-mycoherbicide Fusarium oxysporum f. sp. strigae (Foxy 2) by seed coating is an appropriate option for delivering and establishing the biocontrol agent in the rhizosphere, the infection zone of the root parasitic weed Striga hermonthica. Cytological investigations using light and transmission electron microscopy were performed to assess the pattern and extent of colonization of sorghum roots and shoots by Foxy 2, applied as film-coat on seeds. Germination of Foxy 2-treated plants was similar to un-treated plants. During sorghum root development, mycelia of Foxy 2 started from seed coat colonizing root surfaces. The intensity of root colonization increased with time. However, hyphae were not found in the apical zone of the roots. Hyphae growing on the surface of the roots were observed penetrating rhizodermal cells including root hairs, and colonizing the intercellular space and the cells of the cortical parenchyma. Even after four weeks, hyphae were not penetrating the endodermal layer to invade the tissues of the central cylinder. In addition, hyphae of Foxy 2 were also completely absent in the xylem vessels in sorghum shoots even after 11 weeks of sowing, which further proves the non-pathogenicity of the fungus to sorghum. Foxy 2 showed high efficacy in controlling Striga in combination with both tolerant and susceptible sorghum varieties in a root chamber experiment, as reflected by the high percentage of diseased Striga seedlings (95% and 86%, respectively), recorded after 26 days of sowing. The intensity of fungal root colonization, which coincides with the peak of Striga attachments to its host root, will ensure high fungal efficacy and will facilitate the practical use of Foxy 2 against Striga using seed treatment delivery. Indeed, this proof of sorghum non-pathogenicity of the fungus will encourage the acceptance of Foxy 2 by authorities and farmers in Africa.

Fungal root endophytes of the carnivorous plant Drosera rotundifolia

As carnivorous plants acquire substantial amounts of nutrients from the digestion of their prey, mycorrhizal associations are considered to be redundant; however, fungal root endophytes have rarely been examined. As endophytic fungi can have profound impacts on plant communities, we aim to determine the extent of fungal root colonisation of the carnivorous plant Drosera rotundifolia at two points in the growing season (spring and summer). We have used a culture-dependent method to isolate fungal endophytes and diagnostic polymerase chain reaction methods to determine arbuscular mycorrhizal fungi colonisation. All of the roots sampled contained culturable fungal root endophytes; additionally, we have provided molecular evidence that they also host arbuscular mycorrhizal fungi. Colonisation showed seasonal differences: Roots in the spring were colonised by Articulospora tetracladia, two isolates of uncultured ectomycorrhizal fungi, an unidentified species of fungal endophyte and Trichoderma viride, which was present in every plant sampled. In contrast, roots in the summer were colonised by Alatospora acuminata, an uncultured ectomycorrhizal fungus, Penicillium pinophilum and an uncultured fungal clone. Although the functional roles of fungal endophytes of D. rotundifolia are unknown, colonisation may (a) confer abiotic stress tolerance, (b) facilitate the acquisition of scarce nutrients particularly at the beginning of the growing season or (c) play a role in nutrient signalling between root and shoot.

Arbuscular Mycorrhizal Fungi Colonization in Upland Rice as Influenced by Agrochemical Application

Mycorrhizal status of rice under upland conditions was studied using potted seedlings. Percentage of arbuscular mycorrhizal fungi (AMF) root colonization varied between 17.35% and 37.18% over an age series of 7 to 70 days old rice plants. AMF root colonization was increased up to 35–42 days, beyond which the root colonization steadily declined. The vesicles appeared after two weeks and reached their maximum intensity on the 35 th day. The arbuscules were formed late on the 42 nd day (2.93%) and slightly varied up to the 70 th day (3.03%). Higher dosage of urea application suppressed plant growth whereas the superphosphate treatment had no marked impact on plant growth. Generally, application of these agrochemicals registered less influence on the hyphal colonization of AMF in rice plants, whereas arbuscular colonization was adversely affected by higher dosages of fertilizers. There were pronounced decreases in both the plant growth and their AMF colonization due to the application of systemic fungicides, carbendazim and thiophanate methyl. The application of single sprays of fungicides was less deleterious over multiple sprays.

Effects of soil application of olive mill wastewaters on the structure and function of the community of arbuscular mycorrhizal fungi

Recycling of olive mill wastewaters (OMW) into agricultural soils is a controversial issue since benefits to soil fertility should counterbalance potential short-term toxicity effects. We investigated the short-term effects of OMW on the soil–plant system, regarding the diversity, structure and root colonization capacity of arbuscular mycorrhizal (AM) fungi and the respective growth response of Vicia faba L, commonly used as green manure in olive-tree plantations. A compartmentalized pot system was used that allowed the establishment of an AM fungal community in one compartment (feeder) and the application of three OMW dose levels in an adjacent second compartment (receiver). At 0, 10, and 30 days after OMW treatment (DAT), V. faba pre-germinated seeds were seeded in the receiver compartment. At harvest, shoot and root dry weights, AM fungal root colonization, soil hyphal length and P availability were recorded in the receiver compartment. In addition, OMW effects on AM fungal diversity in plant roots were studied by DGGE. A transient effect of OMW application was observed; plant growth and AM fungal colonization were initially inhibited, whereas soil hyphal length was stimulated, but in most cases differences were absent when seeding was performed 30 DAT. Similarly, changes induced in the structure of the root AM fungal community were of transient nature. Cloning and sequencing of all the major DGGE bands showed that roots were colonized by Glomus spp. The transient effects of OMW on the structure and function of AM fungi could be attributed to OMW-derived phytoxicity to V. faba plants or to an indirect effect via alteration of soil nutritional status. The high OMW dose significantly increased soil P availability in the presence of AM fungi, suggesting efficient involvement of AM fungi in organic-P minerilization. Overall our results indicate that soil application of OMW would cause transient changes in the AM fungal colonization of V. faba plants, which, would not impair their long-term plant growth promoting ability.

Phylogenetic trait conservatism and the evolution of functional trade-offs in arbuscular mycorrhizal fungi.

The diversity of functional and life-history traits of organisms depends on adaptation as well as the legacy of shared ancestry. Although the evolution of traits in macro-organisms is well studied, relatively little is known about character evolution in micro-organisms. Here, we surveyed an ancient and ecologically important group of microbial plant symbionts, the arbuscular mycorrhizal (AM) fungi, and tested hypotheses about the evolution of functional and life-history traits. Variation in the extent of root and soil colonization by AM fungi is constrained to a few nodes basal to the most diverse groups within the phylum, with relatively little variation associated with recent divergences. We found no evidence for a trade-off in biomass allocated to root versus soil colonization in three published glasshouse experiments; rather these traits were positively correlated. Partial support was observed for correlated evolution between fungal colonization strategies and functional benefits of the symbiosis to host plants. The evolution of increased soil colonization was positively correlated with total plant biomass and shoot phosphorus content. Although the effect of AM fungi on infection by root pathogens was phylogenetically conserved, there was no evidence for correlated evolution between the extent of AM fungal root colonization and pathogen infection. Variability in colonization strategies evolved early in the diversification of AM fungi, and we propose that these strategies were influenced by functional interactions with host plants, resulting in an evolutionary stasis resembling trait conservatism.

Coexistence of arbuscular mycorrhizae and dark septate endophytic fungi in an undisturbed and a disturbed site of an arid ecosystem

Effect of disturbance on root colonization and vertical distribution of arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) was investigated at two adjacent sites of Lal Suhanra Biosphere Reserve, Pakistan. Disturbance clearly affected AMF and DSE colonization, vertical distribution of AMF and plant community structure. Mean colonization of AMF and DSE was slightly less at the disturbed site. Average spore densities, diversity and richness of AMF and DSE were higher at the undisturbed site. A study of the vertical distribution of AMF associated with the five plant species most common to each study site indicated that beside AMF and DSE colonization disturbance may affect AMF species composition. Correlation of AMF with DSE is also discussed.

Effects of shading on photosynthesis, plant organic nitrogen uptake, and root fungal colonization in a subarctic mire ecosystem

Arctic dwarf shrub ecosystems are predicted to be exposed to lower light intensity in a changing climate where mountain birch forests are expanding. We investigated how shading at 0%, 65%, and 97% affects photosynthesis, organic N uptake, C and N allocation patterns in plants, and root fungal colonization in an ericoid dwarf shrub ecosystem. The ecosystem was labeled by injection of [2-13C,15N]glycine into the soil, and the uptake of 15N and 13C in roots and leaves 24 h later was analysed. Fungal colonization in hair roots was determined visually. Hair root 13C:15N ratios showed that dwarf shrub ecosystems are capable of taking up organic N as intact glycine both under high irradiance levels and under shaded conditions when photosynthesis is strongly reduced. The allocation of 15N to green leaves of Rubus chamaemorus L. increased with shading, whereas the allocation of 13C to leaves of both deciduous and evergreen plant species decreased. Species dominance was correlated with uptake of 13C, i.e., the most productive species also took up the highest amount of glycine. The ecosystem exhibited a tendency towards lower colonization by ericoid mycorrhizal fungi and dark septate endophytes in hair roots when shaded. Thus, shading has implications for processes central to both C and N cycling in subarctic ecosystems. This should be considered in projections of ecosystem responses to climate change and expanding mountain birch forests.

Assessing fungal root colonization for plant improvement

Fungal endophytes display a broad range of symbiotic interactions with their host plants. Current studies on their biology, diversity and benefits are unravelling their high relevance on plant adaptation to environmental stresses. Implementation of such properties may open new perspectives in agriculture and forestry. We aim to exploit the endophytic capacities of the fungal species Fusarium equiseti, a naturally occurring root endophyte which has shown antagonism to plant pathogens, and Pochonia chlamydosporia, a nematophagous fungus with putative endophytic behavior, for plant protection and adaptation to biotic and abiotic stress. A real-time PCR protocol for quantification of the fungal population, together with Agrobacterium-mediated genetic transformation with the GFP gene for confocal microscopy analyses, were designed and applied to assess endophytic development of both these fungal species. Although quantification of both F. equiseti and P. chlamydosporia yielded similar degrees of root colonization, microscopical observations demonstrated differences in infection and development patterns. Furthermore, we found evidences of plant response against endophyte colonization, supporting a balanced antagonism between the endophyte virulence and the plant defenses. Optimization and application of the methodologies presented herein will allow elucidation of beneficial interactions among these endophytes and their host plants.

Arbuscular mycorrhizal fungal association in sandal (Santalum album L.) natural stands in Kerala, India

Arbuscular mycorrhizal (AM) fungal association in Sandal (Santalum album) natural stands in different parts of the Kerala State was studied. AM fungal root colonization in Sandal trees was low and ranged from 5 to 18%. AM fungal spore density was also low and ranged from 56 to 85/10 g soil. Glomalean fungal community in Sandal rhizosphere soils comprised of five genera (Glomus, Acaulospora, Gigaspora, Scutellospora, Entrophospora) with a total of 35 species. Mycorrhization of Sandal seedlings with Glomus fasciculatum, G. mosseae and Acaulospora appendicula showed marked difference in seedling biomass (dry wt.) than untreated control. Highest percentage of mycorrhizal inoculation effect (MIE) of 51.4% was recorded in G. fasciculatum + A. appendicula treatment.

The effect of arbuscular mycorrhizal root colonization on growth and nutrient uptake of two different cowpea (Vigna unguiculata [L.] Walp.) genotypes exposed to drought stress

In the present investigation, the effect of arbuscular mycorrhiza (AM) fungal root colonization on nutrient uptake, growth and soil water depletion of cowpea plants exposed to a drought stress period was studied in a greenhouse experiment. Arbuscular mycorrhizal and nonAM plants of an old, local variety from Brazil ‘Carioca’, and of the breeding improved cultivar ‘Epace-10’ were exposed to 24 days of drought, with or without a subsequent period of water resupply. Control plants remained well watered throughout the growth period. The AM symbiosis greatly enhanced nutrient uptake of drought stressed plants. However, water uptake, growth and net photosynthesis were unaffected by AM root colonization during drought stress. Recovery after water resupply was much faster when plants were inoculated with AM, probably due to an improved plant nutritional status. The cultivar Epace-10 was much more dependent on the AM symbiosis for growth and nutrient uptake compared with Carioca in all irrigation treatments. Only when roots were AM colonized, Epace-10 plants were able to recover from drought stress after water resupply. Our study suggests that the AM symbiosis constitutes an important strategy by which leguminous plants overcome periods of drought. Under conditions of low soil water and P availability, the presence of AM fungal propagules in the field soil appears to be a prerequisite for successful cultivation of cowpea, particularly with respect to the breeding improved cultivar Epace-10.

Influence of Soil Water Status on Arbuscular Mycorrhizas in Three Perennial Grasses in Central Argentina

In this field study, we determined the effects of water stress, rainfed or irrigated conditions on the arbuscular mycorrhizal fungal (AMF) root colonization in the late-seral Stipa clarazii, the earlier-seral S. tenuis and the early-seral S. gynerioides at different developmental stages and sampling dates. We had two goals: (1) to document the levels of AMF root colonization, and (2) to examine the plant stress responses during imposed water stress in the three Stipa species. We tested the hypothesis that the effects of water stress on Stipa grasslands depend upon the interactions between plant AMF status and plant species. No consistent patterns in AMF root colonization frequency were observed relative to soil water levels. This suggests that AMF root colonization is highly plastic and responds to environmental factors that we do not yet understand. Mycorrhiza colonization was greater in the late-seral than in the earlier-seral species. Higher AMF root colonization levels in S. clarazii may partly explain its higher competitive ability in comparison with the earlier-seral species.

Interactions between the arbuscular mycorrhizal fungus Glomus intraradices and the plant growth promoting rhizobacteria Paenibacillus polymyxa and P. macerans in the mycorrhizosphere of Cucumis sativus

Interactions between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and bacteria from the genus Paenibacillus ( P. macerans and P. polymyxa) were examined in a greenhouse pot experiment with Cucumis sativus with and without organic matter amendment (wheat bran). P. polymyxa markedly suppressed AM fungus root colonization irrespective of wheat bran amendment, whereas P. macerans only suppressed AM fungus root colonization in combination with wheat bran amendment. Dual inoculation with P. macerans and G. intraradices in combination with wheat bran amendment also caused severe plant growth suppression. Inoculation with G. intraradices was associated with increased levels of dehydrogenase activity and available P in the growth substrate suggesting that mycorrhiza formation accelerated the decomposition of organic matter resulting in mobilization of phosphorus. Inoculation with both Paenibacillus species increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the AM fungus biomarker 16:1ω5, which was reduced, though not significantly. Similarly, inoculation with G. intraradices increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the Gram-positive bacteria biomarker 15:0 anteiso, which was overall decreased by G. intraradices inoculation. In combination with wheat bran amendment G. intraradices inoculation caused a 39% reduction in the amount of 15:0 anteiso in the treatment with P. polymyxa, suggesting that G. intraradices suppressed P. polymyxa in this treatment. In conclusion, plant growth promoting species of Paenibacillus may have suppressive effects of AM fungi and plant growth, especially in combination with organic matter amendment. The use of an inert plant growth media in the present study allowed us to study rhizosphere microbial interactions in a relative simple substrate with limited interference from other soil biota. However, the results obtained in the present work mainly show potential interactions and should not be directly extrapolated to a soil situation.

Influence of commercial inoculation with Glomus intraradices on the structure and functioning of an AM fungal community from an agricultural site

The use of commercial arbuscular mycorrhizal (AM) inoculants is growing. However, we know little about how resident AM communities respond to inoculations under different soil management conditions. The objective of this study was to simulate the application of a commercial AM fungal inoculant of Glomus intraradices to soil to determine whether the structure and functioning of that soil’s resident AM community would be affected. The effects of inoculation were investigated over time under disturbed or undisturbed soil conditions. We predicted that the introduction of an infective AM fungus, such as G. intraradices, would have greater consequences in disturbed soil. Using a combination of molecular (terminal restriction length polymorphism analysis based on the large subunit of the rRNA gene) and classical methods (AM fungal root colonization and P nutrition) we found that, contrary to our prediction, adding inoculant to soil containing a resident AM fungal community does not necessarily have an impact on the structure of that community either under disturbed or undisturbed conditions. However, we found evidence of positive effects of inoculation on plant nutrition under disturbed conditions, suggesting that the inoculant interacted, directly or indirectly, with the resident AM fungi. The inoculant significantly improved the P content of the host but only in presence of the resident AM fungal community. In contrast to inoculation, soil disturbance had a significant negative impact on species richness of AM fungi and influenced the AM fungal community composition as well as its functioning. Thus, we conclude that soil disturbance may under certain conditions have greater consequences for the structure of resident AM fungal communities in agricultural soils than commercial AM fungal inoculations with G. intraradices.

Spatial Distribution of Arbuscular Mycorrhizal Fungi in the Rhizosphere of the Salt Marsh Plant Inula crithmoides L. Along a Salinity Gradient

The influence of flooding and salinity gradient on the arbuscular mycorrhiza (AM) fungal spore and root colonization level in Inula crithmoides was studied. One transect divided into five plots was established from shoreline (border) to inland in La Mata lagoon, Alicante, Spain. The plots with high salinity level and flooding showed a decreased spore number in soil, and a decrease in the percentage of mycorrhization. The number of mycorrhizal fungal propagules resulting from the most probable number (MPN) experiment was very low. Glomus geosporum was the most frequent fungal species in all plots.

Arbuscular mycorrhizal fungal spore-associated bacteria affect mycorrhizal colonization, plant growth and potato pathogens

Arbuscular mycorrhizal (AM) fungi and their bacterial associates are essential living components of the soil microbiota. From a total of 385 bacteria previously isolated from spores of AM fungi (AMB), 10 were selected based on ability to inhibit growth of plant pathogens. Effects of these isolates on AM fungal colonization, plant growth in potato ( Solanum tuberosum L.) and inhibition of pathogens was investigated. AM fungal root colonization of potato was 7-fold higher in the presence of the Pseudomonas FWC70 isolate in a greenhouse and was 6–9-fold higher in the presence of the three isolates Pseudomonas FWC70, Stenotrophomonas FWC94 and Arthrobacter FWC110 in an outdoor pot experiment. Several growth traits of potato were stimulated by the Pseudomonas isolates FWC16, FWC30 and FWC70 and by the Stenotrophomonas isolate FWC14. All three Pseudomonas isolates showed inhibition against Erwinia carotovora, Phytophthora infestans and Verticillium dahliae but Stenotrophomonas isolates were variable. Protease(s), siderophores and indole acetic acid were produced by all isolates. Chitinase(s) were produced by all Stenotrophomonas and phosphate-solubilizing activity by all Pseudomonas isolates, the Stenotrophomonas FWC14 isolate and the Arthrobacter FWC110 isolate. We conclude that some AMB are multifunctional and production of extracellular enzymes and bioactive compounds are likely mechanisms for their multifunctional activities. Our results show that some AMB are likely to contribute to the often described ability of AM fungi to inhibit pathogens, acquire mineral nutrients and modify plant root growth.