The dual symbiosis between arbuscular mycorrhiza and nitrogen fixing bacteria benefits the growth and nutrition of the woody invasive legume Acacia cyclops under nutrient limiting conditions

Background and aims

Acacia cyclops is an invasive species within Mediterranean ecosystems, characteristically low in soil nutrients. Thus associations with nitrogen-fixing bacteria (NFB) and arbuscular mycorrhiza (AM) may provide an advantage to these legumes. This study investigated the role of AM and NFB in the growth and nutritional physiology of A. cyclops.

Methods

Seedlings were inoculated with?naturally occurring?NFB, Glomus mosseae or both, and grown under glasshouse conditions for 5 months. Plants were cultivated in sand and supplied with a 20 % strength nutrient solution.?Xylem sap nutrients, photosynthetic rates, biomass and chemical compositions, were recorded.

Results

The dual inoculation decreased the colonization of both symbionts, compared to a single symbiosis with either symbiont. Despite low colonization levels, the dual symbiosis increased host biomass and relative growth rates. This was associated with increased photosynthetic rates and enhanced nutrition. Additionally, dual symbiotic plants had enhanced N and P acquisition and utilization rates. Xylem sap analysis showed higher levels of NH ₄ ⁺ being exported from the roots to the shoots in the dual symbiotic plants compared with other treatments.

Conclusions

These findings suggest the dual symbiosis is an important factor in the growth and development of A. cyclops under nutrient limiting conditions.     

Arbuscular mycorrhizal symbiosis alleviates drought stress imposed on Knautia arvensis plants in serpentine soil

Background and Aims

Plants growing on serpentine bedrock have to cope with the unique soil chemistry and often also low water-holding capacity. As plant-soil interactions are substantially modified by arbuscular mycorrhizal (AM) symbiosis, we hypothesise that drought tolerance of serpentine plants is enhanced by AM fungi (AMF).

Methods

We conducted a pot experiment combining four levels of drought stress and three AMF inoculation treatments, using serpentine Knautia arvensis (Dipsacaceae) plants as a model.

Results

AMF inoculation improved plant growth and increased phosphorus uptake. The diminishing water supply caused a gradual decrease in plant growth, accompanied by increasing concentrations of drought stress markers (proline, abscisic acid) in root tissues. Mycorrhizal growth dependence and phosphorus uptake benefit increased with drought intensity, and the alleviating effect of AMF on plant drought stress was also indicated by lower proline accumulation.

Conclusions

We documented the role of AM symbiosis in plant drought tolerance under serpentine conditions. However, the potential of AMF to alleviate drought stress was limited beyond a certain threshold, as indicated by a steep decline in mycorrhizal growth dependence and phosphorus uptake benefit and a concomitant rise in proline concentrations in the roots of mycorrhizal plants at the highest drought intensity.     

Effects of single and mixed inoculation with two arbuscular mycorrhizal fungi in two different levels of phosphorus supply on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers

Aims

This study aimed to determine the effect of arbuscular mycorrhizal (AM) fungi and phosphorus (P) supply levels on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers.

Methods

Two commercial AM fungal isolates of Glomus intraradices (IFP Glintra) and Glomus mosseae (IFP Glm) which differ in their life cycles were used. Sweet potato plants were grown in a horizontal split-root system that consisted of two root compartments. A root-free fungal compartment that allowed the quantification of mycelial development was inserted into each root compartment. The two root compartments were inoculated either with the same or with different AM isolates, or remained free of mycorrhizal propagules. Each fungal treatment was carried out in two P supply levels.

Results

In the low P supply level, mycorrhizal colonization significantly increased β-carotene concentrations in sweet potato tubers compared with the non-mycorrhizal plants. Glomus intraradices appeared to be more efficient in increasing β-carotene concentrations than G. mosseae. Dual inoculation of the root system with the two mycorrhizal fungi did not result in a higher increase in tuber β-carotene concentrations than inoculation with the single isolates. Improved P nutrition led to higher plant tuber biomass but was not associated with increased β-carotene concentrations.

Conclusions

The results indicate a remarkable potential of mycorrhizal fungi to improve β-carotene concentrations in sweet potato tubers in low P fertilized soils. These results also suggest that β-carotene metabolism in sweet potato tubers might be specifically activated by root mycorrhizal colonization.     

Reduced mycorrhizal colonization (rmc) tomato mutant lacks expression of SymRK signaling pathway genes

Comparison of the expression of 13 genes involved in arbuscular mycorrhizal (AM) symbiosis was performed in a wild type tomato (Solanum lycopersicum cv 76R) and its reduced mycorrhizal colonization mutant rmc in response to colonization with Glomus fasiculatum. Four defense-related genes were induced to a similar extent in the mutant and wild type AM colonized plants, indicating a systemic response to AM colonization. Genes related to nutrient exchange between the symbiont partners showed higher expression in the AM roots of wild type plants than the mutant plants, which correlated with their arbuscular frequency. A symbiosis receptor kinase that is involved in both nodulation and AM symbiosis was not expressed in the rmc mutant. The fact that some colonization was observed in rmc was suggestive of the existence of an alternate colonization signaling pathway for AM symbiosis in this mutant.     

Mycorrhizal responsiveness trends in annual crop plants and their wild relatives—a meta-analysis on studies from 1981 to 2010

Background and aims

Year of release of a cultivar reflects the agricultural and breeding practices of its time; we hypothesize that there are differences in mycorrhizal responsiveness of new high yielding and old crop plants and landraces. We evaluated the importance of the year of release on mycorrhizal responsiveness, arbuscular mycorrhizal (AM) fungal root colonization and P efficiency. We also analyzed the effect of experimental treatments, P efficiency (P acquisition and P utilization efficiency) and AM fungal root colonization on a potential mycorrhizal responsiveness trend for year of release.

Methods

We conducted a meta-analysis on 39 publications working on 320 different crop plant genotypes.

Results

New cultivars were less intensely colonized but were more mycorrhiza-responsive (and possibly dependent) compared to ancestral genotypes. This trend was potentially influenced by the moderator variables density, pre-germination, plant, plant type and AMF species. AM root colonization was also important for the mycorrhizal responsiveness trend for year of release, but P efficiency was not.

Conclusions

With the data available we could find no evidence that new crop plant genotypes lost their ability to respond to mycorrhiza due to agricultural and breeding practices.     

Arbuscular mycorrhizas and their role in plant growth, nitrogen interception and soil gas efflux in an organic production system

Background and aims

Roots and mycorrhizas play an important role in not only plant nutrient acquisition, but also ecosystem nutrient cycling.

Methods

A field experiment was undertaken in which the role of arbuscular mycorrhizas (AM) in the growth and nutrient acquisition of tomato plants was investigated. A mycorrhiza defective mutant of tomato (Solanum lycopersicum L.) (named rmc) and its mycorrhizal wild type progenitor (named 76R) were used to control for the formation of AM. The role of roots and AM in soil N cycling was studied by injecting a ¹⁵N-labelled nitrate solution into surface soil at different distances from the 76R and rmc genotypes of tomato, or in plant free soil. The impacts of mycorrhizal and non-mycorrhizal root systems on soil greenhouse gas (CO₂ and ¹⁴⁺¹⁵N₂O and ¹⁵N₂O) emissions, relative to root free soils, were also studied.

Results

The formation of AM significantly enhanced plant growth and nutrient acquisition, including interception of recently applied NO ₃ ^? . Whereas roots caused a small but significant decrease in ¹⁵N₂O emissions from soils at 23?h after labeling, compared to the root-free treatment, arbuscular mycorrhizal fungi (AMF) had little effect on N₂O emissions. In contrast soil CO₂ emissions were higher in plots containing mycorrhizal root systems, where root biomass was also greater.

Conclusions

Taken together, these data indicate that roots and AMF have an important role to play in plant nutrient acquisition and ecosystem N cycling.     

Nitrogen limitation impairs plant control over the arbuscular mycorrhizal symbiosis in response to phosphorus and shading in two European sand dune species

The symbiosis of plants with arbuscular mycorrhizal fungi (AMF) may become parasitic if the cost:benefit ratio (carbon:phosphorus ratio) increases. In case of mycorrhizal parasitism, a plant may prevent growth depression through the reduction of root colonization as a form of control over the symbiosis. In this greenhouse study, we attempted to manipulate the cost:benefit ratio of the arbuscular mycorrhizal symbiosis by shading and/or phosphorus (P) fertilization in the differentially mycotrophic plant species Hieracium pilosella and Corynephorus canescens. By repeated sampling of soil cores, we assessed the temporal progress of plant investment towards mycorrhizal structures as a measure of plant control over the AMF. Unexpectedly, we found no obvious treatment effects on mycorrhizal growth dependency (MGD), most likely caused by constant N-limitation in AM plants being enhanced by P-fertilization and shade probably not exacerbating plant C-budget for AMF. This highlights the importance of N:P:C stoichiometry for the outcome of the symbiosis. Nevertheless, we found possible control mechanisms in shaded H. pilosella, with considerably higher resource investments into root than into hyphal growth, while root colonization was only marginally suppressed. This control only manifested after 4 weeks of growth under potentially detrimental conditions, emphasizing the importance of time in plant control over the arbuscular mycorrhizal symbiosis. In contrast, the less mycotrophic C. canescens did not exhibit obvious changes in mycorrhizal investments in reaction to shading and P-fertilization, possibly because the low mycotrophy and AMF colonization already imposes a functioning control mechanism in this species. Our study suggests that highly mycotrophic plants may have a stronger need to keep AMF in check than less mycotrophic plants, which may have implications for the role of mycotrophy in the outcome of symbiotic interactions in natural situations.     

Seasonal carbon allocation to arbuscular mycorrhizal fungi assessed by microscopic examination, stable isotope probing and fatty acid analysis

Background and Aim

Climate change models are limited by lack of baseline data, in particular carbon (C) allocation to – and dynamics within – soil microbial communities. We quantified seasonal C-assimilation and allocation by plants, and assessed how well this corresponds with intraradical arbuscular mycorrhizal fungal (AMF) storage and structural lipids (16:1ω5 NLFA and PLFA, respectively), as well as microscopic assessments of AMF root colonization.

Methods

Coastal Hypochoeris radicata plants were labeled with ¹³CO₂ in February, July and October, and ¹³C-allocation to fine roots and NLFA 16:1ω5, as well as overall lipid contents and AM colonization were quantified.

Results

C-allocation to fine roots and AMF storage lipids differed seasonally and mirrored plant C-assimilation, whereas AMF structural lipids and AM colonization showed no seasonal variation, and root colonization exceeded 80 % throughout the year. Molecular analyzes of the large subunit rDNA gene indicated no seasonal AMF community shifts.

Conclusions

Plants allocated C to AMF even at temperatures close to freezing, and fungal structures persisted in roots during times of low C-allocation. The lack of seasonal differences in PLFA and AM colonization indicates that NLFA analyses should be used to estimate fungal C-status. The implication of our findings for AM function is discussed.     

Is intensity of plant root mycorrhizal colonization a good proxy for plant growth rate,dominance and decomposition in nutrient poor conditions?

Questions

Mycorrhizae may be a key element of plant nutritional strategies and of carbon and nutrient cycling. Recent research suggests that in natural conditions, intensity of mycorrhizal colonization should be considered an important plant feature. How are inter‐specific variations in mycorrhizal colonization rate, plant relative growth rate (RGR ) and leaf litter decomposability related? Is (arbuscular) mycorrhizal colonization linked to the dominance of plant species in nutrient‐stressed ecosystems?

Location

Teberda State Biosphere Reserve, northwest Caucasus, Russia.

Methods

We measured plant RGR under mycorrhizal limitation and under natural nutrition conditions, together with leaf litter decomposability and field intensity of mycorrhizal colonization across a wide range of plant species, typical for alpine communities of European mountains. We applied regression analysis to test whether the intensity of mycorrhizal colonization is a good predictor of RGR and decomposition rate, and tested how these traits predict plant dominance in communities.

Results

Forb species with a high level of field mycorrhizal colonization had lower RGR under nutritional and mycorrhizal limitation, while grasses were unaffected. Litter decomposition rate was not related to the intensity of mycorrhizal colonization. Dominant species mostly had a higher level of mycorrhizal colonization and lower RGR without mycorrhizal colonization than subordinate species, implying that they were more dependent on mycorrhizal symbionts. There were no differences in litter decomposability.

Conclusions

In alpine herbaceous plant communities dominated by arbuscular mycorrhizae, nutrient dynamics are to a large extent controlled by mycorrhizal symbiosis. Intensity of mycorrhizal colonization is a negative predictor for whole plant RGR . Our study highlights the importance of mycorrhizal colonization as a key trait underpinning the role of plant species in carbon and nutrient dynamics in nutrient‐limited herbaceous plant communities.

     

Interactive influence of light intensity and soil fertility on root-associated arbuscular mycorrhizal fungi

Background and aims

Soil nutrients and light have major effects on the economics of arbuscular mycorrhizal (AM) symbioses. This study tests the main and interactive effects of soil fertility and light on AM fungal community.

Methods

We conducted a 3 year mesocosm experiment with a full two factorial design: light (full light or shade) and soil fertility (unfertilized or fertilized), on the Qinghai-Tibetan Plateau. Plant traits, soil characteristics and the AM fungal communities inside roots and in soils were measured.

Results

Shade reduced AM colonization of roots, fertilization reduced the hyphal abundance in the soil, and both factors reduced species richness of AM fungi inside plant roots. Fertilization exacerbated the negative impacts of shade on AM fungal abundance and diversity. We observed 15 phylotypes of AM fungi inside roots and ten morphotypes of AM fungal spores in the soil. Taxa responded differently to shade and fertilization and there was little congruence between the responses of fungi inside the roots and in the spore community.

Conclusions

Our findings indicate that both shade and fertilization reduce the abundance of AM fungi, but the two factors have different effects on the quality of plant roots as habitat for AM fungi.     

Ethylene Alleviates the Suppressive Effect of Phosphate on Arbuscular Mycorrhiza Formation

Phosphorus is one of the most important macronutrients required for plant growth. Plants have evolved many strategies for inorganic phosphorus (Pi) acquisition, including the symbiotic pathways, involving the formation of mycorrhiza. With regard to arbuscular mycorrhiza (AM), high Pi availability has long been known to negatively affect this association, although the underlying mechanism is unknown. In the present study, the interactive role played by ethylene and Pi in AM regulation was investigated in the tomato-Rhizophagus irregularis symbiosis. AM fungal colonization was analysed in epi, rin and NRO ethylene-responsive mutants under different Pi availability conditions, with a focus on the late stages of the interaction. Although Pi inhibited mycorrhizal parameters in the ethylene-insensitive rin mutant and wild-type cultivars, it did not alter the mycorrhization of the epi tomato mutant, which exhibits a constitutive ethylene-induced response. As with the colonization parameters, root ethylene content and the expression of AM-related and ethylene receptor 6 genes were inhibited by Pi in wild-type cultivars and rin mutants, but were unaffected or slightly activated in epi plants. The application of ethephon offsets the negative impact on the mycorrhizal development caused by the application of Pi. This compensation effect is dose dependent and was ineffective in the NRO mutant, which is more insensitive to the action of ethylene. Our results provide evidence that ethylene signalling negatively affects the suppressive effect of Pi on AM formation and suggests an overlap between this suppressive effect and the regulatory mechanism of Pi-starvation response pathway in plants mediated by ethylene.     

AM fungal communities inhabiting the roots of submerged aquatic plant <Emphasis Type="Italic">Lobelia dortmanna</Emphasis> are diverse and include a high proportion of novel taxa

While the arbuscular mycorrhizal (AM) symbiosis is known to be widespread in terrestrial ecosystems, there is growing evidence that aquatic plants also form the symbiosis. It has been suggested that symbiosis with AM fungi may represent an important adaptation for isoëtid plants growing on nutrient-poor sediments in oligotrophic lakes. In this study, we address AM fungal root colonization intensity, richness and community composition (based on small subunit (SSU) ribosomal RNA (rRNA) gene sequencing) in five populations of the isoëtid plant species Lobelia dortmanna inhabiting oligotrophic lakes in Southern Sweden. We found that the roots of L. dortmanna hosted rich AM fungal communities and about 15 % of the detected molecular taxa were previously unrecorded. AM fungal root colonization intensity and taxon richness varied along an environmental gradient, being higher in oligotrophic and lower in mesotrophic lakes. The overall phylogenetic structure of this aquatic fungal community differed from that described in terrestrial systems: The roots of L. dortmanna hosted more Archaeosporaceae and fewer Glomeraceae taxa than would be expected based on global data from terrestrial AM fungal communities.     

Role of native and exotic mycorrhizal symbiosis to develop morphological,physiological and biochemical responses coping with water drought of date palm,Phoenix dactylifera

Key Message

Arbuscular mycorrhizal (AM) symbiosis can improve date palm growth and alleviate drought-related impacts than non-mycorrhizal plants due to the ability of AMF for modifying plant metabolism and physiology.

Abstract

Date palm (Phoenix dactylifera L.) is an important agricultural and commercial crop in the North of Africa and Middle Eastern countries. During the last decade, date palm plantations were subjected to degradation due to an extensive exploitation and to drastic environmental conditions such as drought. Currently, there is a growing interest in the valorization of water due to environmental problems and economic aspects. The use of arbuscular mycorrhizal fungi (AMF) can offer a possibility to overcome these problems. The objective of this study was to study the influence of different Glomus species—Glomus intraradices, G. mosseae and Complex Aoufous (native AMF)—on the development of date palm grown under two water regimes (optimal irrigation, 75 % of field capacity or water deficit, 25 % of field capacity). Our results revealed that the beneficial effect of mycorrhizal symbiosis on plant growth depended on the fungal species and the water regime applied to the palm date seedling. While the native Complex Aoufous was the most effective in increasing the shoot height and biomass under well-watered conditions, G. intraradices was the most beneficial fungus for improving growth of plants that undergo restricted water supply. This positive effect of G. intraradices under drought conditions was not related to an enhancement of the antioxidant enzymatic activities in leaves; the association of palm date with G. intradices caused an increase in the elasticity of cell walls in leaves and allowed maintaining high water content in leaves without lowering leaf water potential under stressful conditions. The adequate selection of the AMF species is crucial for improving growth of palm date seedlings, and it must be in accordance with the water regime that will be applied to plants.     

<Emphasis Type="Italic">Gr</Emphasis> and <Emphasis Type="Italic">hp</Emphasis>-<Emphasis Type="Italic">1</Emphasis> tomato mutants unveil unprecedented interactions between arbuscular mycorrhizal symbiosis and fruit ripening

Main conclusion

Systemic responses to an arbuscular mycorrhizal fungus reveal opposite phenological patterns in two tomato ripening mutants depending whether ethylene or light reception is involved. The availability of tomato ripening mutants has revealed many aspects of the genetics behind fleshy fruit ripening, plant hormones and light signal reception. Since previous analyses revealed that arbuscular mycorrhizal symbiosis influences tomato berry ripening, we wanted to test the hypothesis that an interplay might occur between root symbiosis and fruit ripening. With this aim, we screened seven tomato mutants affected in the ripening process for their responsiveness to the arbuscular mycorrhizal fungus Funneliformis mosseae. Following their phenological responses we selected two mutants for a deeper analysis: Green ripe (Gr), deficient in fruit ethylene perception and high-pigment-1 (hp-1), displaying enhanced light signal perception throughout the plant. We investigated the putative interactions between ripening processes, mycorrhizal establishment and systemic effects using biochemical and gene expression tools. Our experiments showed that both mutants, notwithstanding a normal mycorrhizal phenotype at root level, exhibit altered arbuscule functionality. Furthermore, in contrast to wild type, mycorrhization did not lead to a higher phosphate concentration in berries of both mutants. These results suggest that the mutations considered interfere with arbuscular mycorrhiza inducing systemic changes in plant phenology and fruits metabolism. We hypothesize a cross talk mechanism between AM and ripening processes that involves genes related to ethylene and light signaling.

     

Mycorrhiza and seedling establishment in a subarctic meadow: Effects of fertilization and defoliation

Abstract. Question: How does changing resource availability induced by fertilization and defoliation affect seedling establishment and mycorrhizal symbiosis in a subarctic meadow? Location: 610 m a.s.l., Kilpisjärvi (69°03’N, 20°50’E), Finland. Methods: A short‐term full‐factorial experiment was established, with fertilization and defoliation of natural established vegetation as treatments. Seeds of two perennial herbs Solidago virgaurea and Gnaphalium norvegicum were sown in natural vegetation and their germination and growth followed. At the final harvest we measured the response in terms of arbuscular mycorrhizal (AM) colonization, biomass and nitrogen concentration of the seedlings and the established vegetation. Results: Germination rate was negatively affected by defoliation in the unfertilized plots. The shoot biomass of S. virgaurea seedlings was reduced by the defoliation and fertilization treatments, but not affected by their interaction. In G. norvegicum, the germination rate and the seedling shoot biomass were negatively correlated with moss biomass in the plots. In the established plants the arbuscular colonization rate was low and defoliation and fertilization treatments either increased or did not affect the colonization by AM fungi. In the seedlings, the colonization rate by AM fungi was high, but it was not affected by treatments. Both seedlings and established plants were colonized by dark septate fungi. Conclusions: Reduction of plant biomass by herbivores can have different effects on seedling growth in areas of high and low soil nutrient availability. The weak response of AM colonization to defoliation and fertilization suggests that AM symbiosis is not affected by altering plant resource availability under the conditions employed in this study.     

Growth response of crops to soil microbial communities from conventional monocropping and tree-based intercropping systems

Background and aims

Recent studies have shown that tree-based intercropping (TBI) systems support a more diverse soil microbial community compared to conventional agricultural systems. However, it is unclear whether differences in soil microbial diversity between these two agricultural systems have a functional effect on crop growth.

Methods

In this study, we used a series of greenhouse experiments to test whether crops respond differently to the total soil microbial community (Experiment 1) and to arbuscular mycorrhizal (AM) fungal communities alone (Experiment 2) from conventionally monocropped (CM) and TBI systems.

Results

The crops had a similar growth response to the total soil microbial communities from both cropping systems. However, when compared to sterilized controls, barley (Hordeum vulgare) and canola (Brassica napus) exhibited a negative growth response to the total soil microbial communities, while soybean (Glycine max) was unaffected. During the AM fungal establishment phase of the second experiment, ‘nurse’ plants had a strong positive growth response to AM fungal inoculation, and significantly higher biomass when inoculated with AM fungi from the CM system compared to the TBI system. Soybean was the only crop species to exhibit a significant positive growth response to AM fungal inoculation. Similar to the total soil microbial communities, AM fungi from the two cropping systems did not differ in their effect on crop growth.

Conclusion

Overall, AM fungi from both cropping systems had a positive effect on the growth of plants that formed a functional symbiosis. However, the results from these experiments suggest that negative effects of non-AM fungal microbes are stronger than the beneficial effects of AM fungi from these cropping systems.