Hyphal Growth Of Arbuscular Mycorrhizal Fungi Research Articles

Carbon allocation to the rhizosphere is affected by drought and nitrogen addition

Abstract Photosynthetic carbon (C) allocated below‐ground can be shared with mycorrhizal fungi in exchange for nutrients, but also added into soil as rhizodeposits that potentially increases plant nutrient supply by supporting microbial nutrient mineralization from organic matter. How water and nitrogen (N) availability affects plant C allocation to the rhizosphere, including both arbuscular mycorrhizal fungi (AMF) symbionts and rhizodeposits, remains largely unknown. We used a 13CO2 pulse labelling experiment to assess the effects of drought and N addition on below‐ground allocation of C to soils and roots (quantified as excess 13C) and tested their relationships with AMF colonization in an Australian grassland. We also examined relationships between AMF and previously reported root respiration and decomposition of rhizodeposits in this study. We found that drought decreased the absolute amount of excess 13C allocated to both soils and roots, likely due to less photosynthetic C fixation. In contrast, proportionally more excess 13C was allocated to soils but less to root biomass with drought, suggesting that relatively more C was allocated to rhizodeposits and to AMF hyphal growth and extension. However, N addition reversed drought effects on below‐ground C allocation by retaining proportionally more excess 13C in roots and less in soils, congruent with higher soil N and phosphorus availability, root biomass and number of root tips compared to drought without N addition. This suggests that the alleviation of nutrient limitation promoted plants to expend relatively more C on root growth and root trait adjustment, but less C on rhizodeposition and mycorrhizal symbiosis. Synthesis. Mycorrhizal colonization related negatively to rhizodeposit decomposition rate but positively to both excess 13C in root biomass and root respiration, suggesting a possible trade‐off in C allocation between mycorrhizal symbiosis and rhizodeposition. We conclude that below‐ground C allocation in this grassland can be mediated by mycorrhizal colonization and is strongly affected by water and nutrient availability.

Contribution of glomalin-related soil proteins to soil organic carbon in trifoliate orange

Arbuscular mycorrhizal fungi (AMF) produce glomalin-related soil protein (GRSP) that influences organic carbon (C) storage in soil; however, how much purified GRSP fractions contribute to soil organic carbon (SOC) is yet not known. The present study evaluated the contribution of GRSP towards changes in SOC in trifoliate orange grown in a rootbox divided into a roots + hyphae chamber (roots colonized by AMF hyphae and AMF extraradical hyphae) and a hyphae chamber (only the presence of AMF extraradical hyphae, without roots). Three AMF species (Diversispora epigaea, Paraglomus occultum and Rhizoglomus intraradices) were inoculated into the roots + hyphae chamber. Following four months of plant growth, P. occultum showed higher AMF hyphal growth in roots, soils and nylon mesh than D. epigaea or R. intraradices. Mycorrhizal inoculation improved the plant growth performance and increased easily extractable GRSP (EE-GRSP) and difficultly extractable GRSP (DE-GRSP) concentrations in both chambers, regardless of AMF species. The C content observed in total GRSP of the soil after purification was 2.71 ± 0.49 mg g−1, while purified EE-GRSP and DE-GRSP showed the C content of 1.01 ± 0.19 mg g−1 and 1.70 ± 010.34 mg g−1, respectively. The C contribution by purified EE-GRSP and DE-GRSP accounted for 8.67 ± 0.95% and 14.59 ± 2.21%, respectively, of total SOC, with a total C contribution of purified GRSPs accounting for 23.26 ± 2.67% of total SOC. A significantly higher C content of GRSP and the C contribution of GRSP to SOC were observed in DE-GRSP than in EE-GRSP, as well as the soil of the roots + hyphae chamber than the soil of the hyphae chamber. The proportionate distribution of water-stable aggregate in 2–4 and 1–2 mm sizes and their stability were higher under AMF hyphae than under non-AMF hyphae. This study thus provided a database evidence of increased contribution of GRSP towards build-up of SOC in response to mycorrhizal symbiosis.

Carbon and phosphorus exchange may enable cooperation between an arbuscular mycorrhizal fungus and a phosphate-solubilizing bacterium.

Arbuscular mycorrhizal fungi (AMF) transfer plant photosynthate underground which can stimulate soil microbial growth. In this study, we examined whether there was a potential link between carbon (C) release from an AMF and phosphorus (P) availability via a phosphate-solubilizing bacterium (PSB). We investigated the outcome of the interaction between the AMF and the PSB by conducting a microcosm and two Petri plate experiments. An in vitro culture experiment was also conducted to determine the direct impact of AMF hyphal exudates on growth of the PSB. The AMF released substantial C to the environment, triggering PSB growth and activity. In return, the PSB enhanced mineralization of organic P, increasing P availability for the AMF. When soil available P was low, the PSB competed with the AMF for P, and its activity was not stimulated by the fungus. When additional P was added to increase soil available P, the PSB enhanced AMF hyphal growth, and PSB activity was also stimulated by the fungus. Our results suggest that an AMF and a free-living PSB interacted to the benefit of each other by providing the C or P that the other microorganism required, but these interactions depended upon background P availability.

The direct response of the external mycelium of arbuscular mycorrhizal fungi to temperature and the implications for nutrient transfer

In this study we investigated the direct effects of temperature on the extra-radical mycelium (ERM) of arbuscular mycorrhizal fungi (AMF) and the resulting impact on the host plant nutrition and biomass production. Plantago lanceolata L. plants colonized by Glomus hoi (experiment 1) and either G. hoi or Glomus intraradices (experiment 2) were grown in compartmented microcosm units. AMF hyphae, but not roots, were permitted access to a second compartment containing a 15N:13C dual-labelled organic patch maintained at different temperature treatments. All plants were maintained at ambient temperature. AMF hyphal growth in the patch compartments was relatively insensitive to temperate but results were variable. G. hoi hyphal length density was 5 times higher at ambient (c. 24 °C) than cooled (c. 11 °C) temperatures but only at the end of the first experiment (105 d after patch addition). In contrast, in the second experiment (86 d after patch addition) AMF hyphal growth was unaffected by temperature in the patch compartment. These differences between experiments are likely due to large variation among replicates in the ERM produced and differences in how the organic patch was applied. In experiment 2, plant biomass and phosphate content differed according to the temperature at which the hyphae of both AMF species grew. Plant biomass was greater when the AMF were grown at c. 18 °C than c. 11 °C but was no different at c. 21 °C. These data show that direct temperature responses by the external hyphae of AMF can independently influence associated host plant growth. However, there were also important differences between the two AMF studied both in the amount of nutrients transferred and the distribution of the nutrients.

Interactions between arbuscular mycorrhizal fungi, rhizobacteria, soil phosphorus and plant cytokinin deficiency change the root morphology, yield and quality of tobacco

Arbuscular mycorrhizal fungi (AMF) and rhizobacteria (Pseudomonas fluorescens) have been targeted for plant growth promotion in order to reduce agrochemical inputs. However, the effects of their interaction on root morphology under different nutrient levels are unknown. Moreover, both soil microorganisms can alter the levels of plant hormonal regulators but no in vivo evidence is available for their interplay with cytokinin (CK) on root morphology. In a full-factorial greenhouse experiment we investigated the effects of interaction between Rhizophagus intraradices, P. fluorescens 8569r, phosphorus (P) amendment and plant CK deficiency on tobacco root morphology, shoot yield and quality. Our results suggest that reduced CK levels may be involved in plant signaling to stimulate AMF hyphal growth in the roots. In addition, we document that a bacteria isolated from the rhizosphere of a non-mycorrhizal plant can function as mycorrhizal helper bacteria, most likely via interplay with phytohormones. The two soil microorganisms, depending on the inoculum combination, P amendment and plant CK levels, modified tobacco root morphology. Our results suggest that the positive interactions between P. fluorescens and AMF depend on soil nutrient status and root hormonal balance. Both microorganism modified shoot yield and these effects seem to result from altered root morphology. Overall, our study support early conclusions that the classification of a soil microorganism as detrimental or beneficial should be based upon their net effects on the plant growth according to circumstances. We suggest this consideration to be extended to the effects of interaction between soil microorganisms on root morphology.

Arbuscular mycorrhizal fungal spores host bacteria that affect nutrient biodynamics and biocontrol of soil-borne plant pathogens.

SummaryThe aim of this research was to isolate and characterize bacteria from spores of arbuscular mycorrhizal fungi (AMF). We designated these bacteria ‘probable endobacteria’ (PE). Three bacterial strains were isolated from approximately 500 spores of Gigaspora margarita (Becker and Hall) using a hypodermic needle (diameter, 200 μm). The bacteria were identified by morphological methods and on the basis of ribosomal gene sequences as Bacillus sp. (KTCIGM01), Bacillus thuringiensis (KTCIGM02), and Paenibacillus rhizospherae (KTCIGM03). We evaluated the effect of these probable endobacteria on antagonistic activity to the soil-borne plant pathogens (SBPPs) Fusarium oxysporum f. sp. lactucae MAFF 744088, Rosellinia necatrix, Rhizoctonia solani MAFF 237426, and Pythium ultimum NBRC 100123. We also tested whether these probable endobacteria affected phosphorus solubilization, ethylene production, nitrogenase activity (NA), and stimulation of AMF hyphal growth. In addition, fresh samples of spores and hyphae were photographed using an in situ scanning electron microscope (SEM) (Quanta 250FEG; FEI Co., Japan). Bacterial aggregates (BAs), structures similar to biofilms, could be detected on the surface of hyphae and spores. We demonstrate that using extraction with an ultrathin needle, it is possible to isolate AMF-associated bacterial species that are likely derived from inside the fungal spores.

Arbuscular mycorrhizal fungus spores host bacteria and their biofilm efficient in nutrient biodynamics and soil-borne plant pathogen suppression

AbstractThe combination of bacteria and arbuscular mycorrhizal fungi (AMF) would be ideal for nutrient biodvnamics in soils and protection from soil-borne plant pathogens (SBPP). During the process of plant colonization AMF interact with bacteria where spores cytoplasm and hvphae provide specific niches for certain populations of bacteria. The clarification of the relationship AMF-bacteria has great potential for sustainable agriculture. The currently research aimed to isolate and characterize bacteria. Which we have designated most probable endobacteria (MPE) from an AMF spore. Three bacterial strains were taken from about 500 spores by using a 200 &x00B5;m diameter hvpodermic needle. These bacteria were identified by DNA and morphological methods. Antagonism to SBPP (Pythium ultimum NBRC 100123, Fusarium oxyxporum f. sp. lactucae MAFF 744088, Rhizoctonia solani MAFF 237426 and Roselinia necatrix - isolated from a Japanese apricot (Prunus mume) by Dr. Norihiko Kobayashi), P solubilization, ethylene production, nitrogenase activity and stimulation of AMF hyphal growth were evaluated. Here we show that the bacteria strongly functioned on nutrient biodynamics (P solubilization and Nitrogenase activity), ethylene production, protection from SBPP and stimulation of AMF hyphal growth. In addition, fresh samples of spores and hyphae were observed and photographed with a new type of scanning electronic microscope (SEM) (FEI Co., Quanta 250FEG), where the hyphae and spores with bacterial slime could be detected. Our results demonstrated the importance of bacteria associated to AMF for the soil ecology having a substantial mpact on sustainability. More specifically, the methods of extraction by ultra-thin needle reveled species associated to AMF, probably from inside spores. Moreover, the observation of bacterial biofilm showed a physical structure that the bacteria could form surround spores and hyphae to help AMF.

Effect of polyamine on growth and development of arbuscular mycorrhizal fungi and host plant in symbiotic culture condition

The effects of exogenous polyamine on the growth and development of arbuscular mycorrhizal fungi (AMF) and host plant in the symbiotic culture condition were investigated by micropropagating grape plantlets infected with AMF (Gigaspora margarita) and then treated with exogenous polyamine (PA) and polyamine biosynthesis inhibitor [methylglyoxal bis (guanylhydrazone), MGBG]. The effects of exogenous PA and MGBG on spore germination, hyphal growth of AMF and development of the host plant in symbiotic culture condition were analyzed. The results indicate that certain doses of exogenous PA have significant effect on symbiotic development, grape plantlets and AMF. AMF arbuscular abundance, hyphal length, spore number and infection rate increase after treatment with exogenous PA. Meanwhile, PA biosynthesis inhibitor (MGBG) has opposite effect which is suppressed by application of exogenous PA. This implies that PA also has significant stimulating effect on the growth and development of AMF in ex vitro conditions. Because exogenous PA activates AMF in the rhizosphere, potential AMF infection is enhanced. Thus, AMF symbiotic association with grape plantlets can establish at early weaning stage and develop with time.

The effect of flavones and flavonols on colonization of tomato plants by arbuscular mycorrhizal fungi of the genera Gigaspora and Glomus

No clear data are available on how flavonoids from different chemical groups affect root colonization by arbuscular mycorrhizal fungi (AMF) and whether flavonoids affecting the presymbiotic growth of AMF also affect root colonization by AMF. In the present work, we compared the effect of flavones (chrysin and luteolin) and flavonols (kaempferol, morin, isorhamnetin, and rutin) on root colonization (number of entry points and degree of root colonization) of tomato plants (Lycopersicum esculentum L.) with the effect of these flavonoids on the presymbiotic growth of these AMF, which has been reported in a recent study. With all tested AMF (Gigaspora rosea, Gigaspora margarita, Glomus mosseae, and Glomus intraradices) a correlation between the number of entry points and the percentage of root colonization was found. When the number of entry points was high, root colonization was also enhanced. Application of the flavones chrysin and luteolin and of the flavonol morin increased the number of entry points and the degree of colonization,whereas the flavonols kaempferol, isorhamnetin, and rutin showed no effect. These results show that in contrast to their effect on the presymbiotic growth of the AMF on the level of root colonization, the tested flavonoids do not exhibit a genus- and species-specificity. Moreover, comparison of our data with the data obtained by J.M. Scervino, M.A. Ponce, R. Erra-Bassells, H. Vierheilig, J.A. Ocampo, and A. Godeas. (2005a. J. Plant Interact. 15: 22-30) indicates that a positive effect on the hyphal growth of AMF does not necessarily result in an enhanced AM root colonization, further indicating that the mode of action of flavonoids at the level of root colonization is more complex.

Effect of Red and Green Algal Extracts on Hyphal Growth of Arbuscular Mycorrhizal Fungi, and on Mycorrhizal Development and Growth of Papaya and Passionfruit

Arbuscular mycorrhizal (AM) fungi enhance terrestrial plant growth by forming a symbiotic relationship with the roots of its host plant. Effects of red and green algal extracts on in vitro hyphal growth of AM fungi, Gigaspora margarita and Glomus caledonium, and on the mycorrhizal development and growth of papaya seedlings and passionfruit cuttings were investigated. Three kinds of red algae, Gracilaria verrucosa, Gelidium amansii and Eucheuma cottonii, and a green alga, Chlorella pyrenoidosa, were extracted with 75% methyl alcohol (MeOH), and these extracts were fractionated by a flash chromatograph equipped with an octa decyl silane column and eluted with 0, 10, 25, 50, and 100% MeOH. Twenty‐five percent MeOH fractions (25% MeOH eluates) of each type of alga most significantly increased in vitro hyphal growth of AM fungi. Inhibition of AM hyphal growth was not observed in any of the fractioned MeOH eluates. Root colonization of papaya and of passionfruit were markedly stimulated by the application of the 25% MeOH eluates of red and green algal extracts. The growth of these plants were promoted in comparison to the control. These results suggest that red and green algae not only contain AM stimulatory substances, but also contribute to mycorrhizal development in higher plants.

EFFECTS OF ARBUSCULAR MYCORRHIZAL FUNGI ON PLANT SECONDARY METABOLISM

Mycorrhizal fungi form the most important mutualistic symbioses on earth with plants. The most prevalent type of mycorrhizal fungi are the arbuscular mycorrhizal (AM) fungi. Much research has shown that the development of AM fungi is correlated with plant secondary metabolism. AM fungi can directly or indirectly affect plant secondary metabolic processes. Secondary metabolites are classified into 3 groups, terpenoids, phenolics and alkaloids. In this paper, we summarize the effects of AM fungi on the 3 groups of secondary metabolites. The relationship between terpenoids and AM fungi have been well studied, and some research has explored interactive mechanisms at the molecular level. Blumenin was first isolated and identified from mycorrhizal cereals, and its biosynthesis has been proven via the Glyceraldehyde 3_phosphate/ pyruvate pathway (MEP) by an isotopic labeling method. Since then, the accumulation of blumenin induced by AM fungi and differences in blumenin levels among different kinds of AM fungi have been observed. Studies on 1_deoxy_D_xylulose_5_phosphate synthase (DXS) and 1_deoxy_D_xylulose_5_phosphate reductoisomerase (DXR), two key enzymes in the biosynthesis of carotenoid metabolism via the MEP pathway, have found to increase the transcription of DXS and DXR in plants with AM fungi. Moreover it was temporarily and spatially correlated with the accumulation of apocarotenoids. Subsequently, two genes were identified: TC78589 encoding DXS2 which is highly expressed in roots inoculated with AM fungi, and TC77051 encoding mevalonate disphosphate decarboxylase, which is catalysed in the synthesis of terpenoids in the mevalonate pathway. Although both genes separately encode enzymes in different pathways, an enhancement of carotenoid biosynthesis has been observed. The interaction between phenolic compounds (such as phytoalexin, wall_bound phenol, flavonoids, isoflavonoids and their derivatives) and AM fungi also has been investigated intensively. It has been shown that some flavonoids stimulated the spore germination and hyphal growth of AM fungi, and the contents of flavonoids increased before the infection of AM fungi. Therefore some investigators hypothesized that flavonoids were a signal compound during the formation of AM fungi. Afterward, increased levels of flavonoids were found after the formation of AM fungi which was related to specific species of AM fungi. In addition, some experiments have indicated that the activity of peroxidase (POD), phenylalanine ammonia_lyase (PAL) and polyphenol oxidase (PPO) were significantly enhanced in AM plants. In phenylpropamoid metabolism, there are two different signaling pathways in the accumulation of secondary metabolites induced by the mycorrhizal fungus: one is through the induction of PAL and chalcone synthase (CHS), and the other is through the suppression of isoflavone reductase (IFR).Although little research seldom has examined the relationship between alkaloids and AM fungi, a recent study has shown that the formation of AM is beneficial to the accumulation of alkaloids. This study also showed the species specificity in AM affected biosynthesis of alkaloids.

Differential decomposition of arbuscular mycorrhizal fungal hyphae and glomalin

Arbuscular mycorrhizal fungi (AMF) are obligate symbionts of most higher plants. In addition to being a major component of soil microbial biomass, AMF hyphae produce glomalin, a recalcitrant glycoproteinaceous substance highly correlated with soil aggregate water stability. This study addresses the lack of knowledge concerning the decomposition of hyphae and glomalin. We used an experimental design that exploited the lack of saprobic capabilities of AMF hyphae by incubating field soil samples in the dark, and hence in the absence of plant or AMF hyphal growth. In 150 days, hyphal length decreased 60%, while glomalin, quantified by the Bradford protein assay, declined only 25%. Immuno-reactive glomalin decreased 46%. This study serves as a proof-of-concept for further examination of factors that influence decomposition of AMF hyphae using similar experimental designs.

Inhibition of pre-symbiotic hyphal growth of arbuscular mycorrhizal fungus Gigaspora margarita by root exudates of Lupinus spp.

Effect of root exudates of Lupinus plants on hyphal growth of arbuscular mycorrhizal (AM) fungus was investigated in order to elucidate the failure of mycorrhizal colonization in nonhost plants. Four species of Lupinus were grown in solution culture and root exudates were collected. Spores of Gigaspora margarita were incubated with the root exudates. Hyphal length in the root exudates of L. luteus, L. aridus, and L. cosentini was shorter than that of the control treatment, irrespective of the P status of Lupinus plants. Numbers of hyphae exhibiting branching were lower in the root exudates of L. luteus and L. cosentini than in those of the controls. It is suggested that these Lupinus plants did not induce pre-symbiotic hyphal growth of AM fungi but inhibited it.

Increased allocation to external hyphae of arbuscular mycorrhizal fungi under CO2 enrichment.

Prunella vulgaris was inoculated with different arbuscular mycorrhizal fungi (AMF) and grown at two concentrations of CO2 (ambient, 350 μl l-1, and elevated, 600 μl l-1) to test whether a plants response to elevated CO2 is dependent on the species of AMF colonizing the roots. Using compartments accessible only to AMF hyphae but not to roots, we also tested whether elevated CO2 affects the growth of external AMF hyphae. Plant biomass was significantly greater at elevated than at ambient CO2; the biomass of the root system, for example, increased by a factor of 2. The colonization of AMF inside the root remained constant, indicating that the total AMF inside the root system also increased by a factor of 2. The length of external AMF hyphae at elevated CO2 was up to 5 times that at ambient CO2, indicating that elevated CO2 promoted allocation of AMF biomass to the external hyphae. The concentration and content of phosphorus in the stolons differed significantly between ambient and elevated CO2 but this resulted in either an increase or a decrease, according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO2 would result in increased P acquistion by the plant. To test this we supplied phosphorus, in a compartment only accessible to AMF hyphae. Plants did not acquire more phosphorus at elevated CO2 when phosphorus was added to this compartment. Large increases in AMF hyphal growth could, however, play a significant role in the movement of fixed carbon to the soil and increase soil aggregation.