Systemic inhibition of arbuscular mycorrhiza development by root exudates of cucumber plants colonized by Glomus mosseae

A Commercial Arbuscular Mycorrhizal Inoculum Alleviated the Effects of Acid Water on Lupinus angustifolius Grown in a Sterilized Mining Dump

Lupinus species have been sporadically reported to be colonized by arbuscular mycorrhizal fungi (AMF). The interactions between AMF and lupine plants could also be non-symbiotic, from positive to negative, as controlled by the stress conditions of the plant. The goal of the study was to reveal the existence of such positive interactions and provide preliminary data for a myco-phytoremediation technology of mining dumps using L. angustifolius as a first crop. The objective was to test the hypothesis that the AMF inoculation of an acidified dump material contaminated with heavy metals would improve the growth of L. angustifolius and decrease oxidative stress. The design consisted of a one-month bivariate pot experiment with plants grown in a mining dump soil inoculated and not inoculated with a commercial AMF inoculum sequestered in expanded clay and watered with acidic and neutral water. There was no AMF root colonization under the experimental conditions, but under neutral and acidic water conditions, the phosphorus concentrations in roots and leaves increased, and the superoxide dismutase and peroxidase activities significantly decreased due to AMF inoculation. The increase in leaf phosphorus concentration was correlated with the decrease in peroxidase activity. The fresh weight of shoots and leaves significantly increased due to the commercial inoculum (under acidic water conditions). At the end of the experiment, the ammonium concentration in the substrate was higher in the inoculated treatments than in the not inoculated ones, and the concentrations of many elements in the dump material decreased compared to the start of the experiment. A comprehensive discussion of the potential mechanisms underlying the effects of the commercial AMF inoculum on the non-host L. angustifolius is completed.

Boosting Sustainable Agriculture by Arbuscular Mycorrhiza under Stress Condition: Mechanism and Future Prospective

Global agriculture is frequently subjected to stresses from increased salt content, drought, heavy metals, and other factors, which limit plant growth and production, deteriorate soil health, and constitute a severe danger to global food security. Development of environmentally acceptable mitigation techniques against stresses and restrictions on the use of chemical fertilizers in agricultural fields is essential. Therefore, eco-friendly practises must be kept to prevent the detrimental impacts of stress on agricultural regions. The advanced metabolic machinery needed to handle this issue is not now existent in plants to deal against the stresses. Research has shown that the key role and mechanisms of arbuscular mycorrhiza fungi (AMF) to enhance plant nutrient uptake, immobilisation and translocation of heavy metals, and plant growth-promoting attributes may be suitable agents for plant growth under diversed stressed condition. The successful symbiosis and the functional relationship between the plant and AMF may build the protective regulatory mechansm against the key challenge in particular stress. AMF's compatibility with hyperaccumulator plants has also been supported by studies on gene regulation and theoretical arguments. In order to address this account, the present review included reducing the impacts of biotic and abiotic stress through AMF, the mechanisms of AMF to improve the host plant's capacity to endure stress, and the strategies employed by AM fungus to support plant survival in stressful conditions.

Arbuscular mycorrhizal fungi-induced mitigation of heavy metal phytotoxicity in metal contaminated soils: A critical review

The heavy metal pollution is a worldwide problem and has received a serious concern for the ecosystem and human health. In the last decade, remediation of the agricultural polluted soil has attracted great attention. Phytoremediation is one of the technologies that effectively alleviate heavy metal toxicity, however, this technique is limited to many factors contributing to low plant growth rate and nature of metal toxicities. Arbuscular mycorrhizal fungi (AMF) assisted alleviation of heavy metal phytotoxicity is a cost-effective and environment-friendly strategy. AMF have a symbiotic relationship with the host plant. The bidirectional exchange of resources is a hallmark and also a functional necessity in mycorrhizal symbiosis. During the last few years, a significant progress in both physiological and molecular mechanisms regarding roles of AMF in the alleviation of heavy metals (HMs) toxicities in plants, acquisition of nutrients, and improving plant performance under toxic conditions of HMs has been well studied. This review summarized the current knowledge regarding AMF assisted remediation of heavy metals and some of the strategies used by mycorrhizal fungi to cope with stressful environments. Moreover, this review provides the information of both molecular and physiological responses of mycorrhizal plants as well as AMF to heavy metal stress which could be helpful for exploring new insight into the mechanisms of HMs remediation by utilizing AMF.

Unraveling the Initial Plant Hormone Signaling, Metabolic Mechanisms and Plant Defense Triggering the Endomycorrhizal Symbiosis Behavior

Arbuscular mycorrhizal (AM) fungi establish probably one of the oldest mutualistic relationships with the roots of most plants on earth. The wide distribution of these fungi in almost all soil ecotypes and the broad range of host plant species demonstrate their strong plasticity to cope with various environmental conditions. AM fungi elaborate fine-tuned molecular interactions with plants that determine their spread within root cortical tissues. Interactions with endomycorrhizal fungi can bring various benefits to plants, such as improved nutritional status, higher photosynthesis, protection against biotic and abiotic stresses based on regulation of many physiological processes which participate in promoting plant performances. In turn, host plants provide a specific habitat as physical support and a favorable metabolic frame, allowing uptake and assimilation of compounds required for the life cycle completion of these obligate biotrophic fungi. The search for formal and direct evidences of fungal energetic needs raised strong motivated projects since decades, but the impossibility to produce AM fungi under axenic conditions remains a deep enigma and still feeds numerous debates. Here, we review and discuss the initial favorable and non-favorable metabolic plant context that may fate the mycorrhizal behavior, with a focus on hormone interplays and their links with mitochondrial respiration, carbon partitioning and plant defense system, structured according to the action of phosphorus as a main limiting factor for mycorrhizal symbiosis. Then, we provide with models and discuss their significances to propose metabolic targets that could allow to develop innovations for the production and application of AM fungal inocula.

Arbuscular Mycorrhizal Fungi for the Biocontrol of Plant-Parasitic Nematodes: A Review of the Mechanisms Involved

Arbuscular mycorrhizal fungi (AMF) are obligate root symbionts that can protect their host plant against biotic stress factors such as plant-parasitic nematode (PPN) infection. PPN consist of a wide range of species with different life styles that can cause major damage in many important crops worldwide. Various mechanisms have been proposed to play a role in the biocontrol effect of AMF against PPN. This review presents an overview of the different mechanisms that have been proposed, and discusses into more detail the plausibility of their involvement in the biocontrol against PPN specifically. The proposed mechanisms include enhanced plant tolerance, direct competition for nutrients and space, induced systemic resistance (ISR) and altered rhizosphere interactions. Recent studies have emphasized the importance of ISR in biocontrol and are increasingly placing rhizosphere effects on the foreground as well, both of which will be the focal point of this review. Though AMF are not yet widely used in conventional agriculture, recent data help to develop a better insight into the modes of action, which will eventually lead toward future field applications of AMF against PPN. The scientific community has entered an exciting era that provides the tools to actually unravel the underlying molecular mechanisms, making this a timely opportunity for a review of our current knowledge and the challenges ahead.

Quantification and role of organic acids in cucumber root exudates in Trichoderma harzianum T-E5 colonization

The ability to colonize on plant roots is recognized as one of the most important characteristics of the beneficial fungi Trichoderma spp. The aim of this study is to prove that the utilization of organic acids is a major trait of Trichoderma harzianum T-E5 for colonization of cucumber roots. A series experiments in split-root hydroponic system and in vitro were designed to demonstrate the association between the utilization of organic acids and T-E5 colonization on cucumber roots. In the split-root hydroponic system, inoculation with T-E5 (T) significantly increased the biomass of cucumber plants compared with CK (non-inoculation with T-E5). The T-E5 hyphae densely covering the cucumber root surface were observed by scanning electron microscopy (SEM). Three organic acids (oxalic acid, malic acid and citric acid) were identified from both the CK and T treatments by HPLC and LC/ESI-MS procedures. The amounts of oxalic acid and malic acid in T were significantly higher than those in CK. All the organic acids exhibited different and significant stimulation effects on the mycelial growth and conidial germination of T-E5 in vitro. An additional hydroponic experiment demonstrated the positive effects of organic acids on the T-E5 colonization of cucumber roots. In conclusion, the present study revealed that certain organic acids could be used as nutritional sources for Trichoderma harzianum T-E5 to reinforce its population on cucumber roots.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

Three sympatrically occurring species of Metarhizium show plant rhizosphere specificity

Here we tested the hypothesis that species of the soil-inhabiting insect-pathogenic fungus Metarhizium are not randomly distributed in soils but show plant-rhizosphere-specific associations. We isolated Metarhizium from plant roots at two sites in Ontario, Canada, sequenced the 5' EF-1α gene to discern Metarhizium species, and developed an RFLP test for rapid species identification. Results indicated a non-random association of three Metarhizium species (Metarhizium robertsii, Metarhizium brunneum and Metarhizium guizhouense) with the rhizosphere of certain types of plant species (identified to species and categorized as grasses, wildflowers, shrubs and trees). M. robertsii was the only species that was found associated with grass roots, suggesting a possible exclusion of M. brunneum and M. guizhouense. Supporting this, in vitro experiments showed that M. robertsii conidia germinated significantly better in Panicum virgatum (switchgrass) root exudate than did M. brunneum or M. guizhouense. M. guizhouense and M. brunneum only associated with wildflower rhizosphere when co-occurring with M. robertsii. With the exception of these co-occurrences, M. guizhouense was found to associate exclusively with the rhizosphere of tree species, predominantly Acer saccharum (sugar maple), while M. brunneum was found to associate exclusively with the rhizosphere of shrubs and trees. These associations demonstrate that different species of Metarhizium associate with specific plant types.

Long-distance transport of signals during symbiosis: are nodule formation and mycorrhization autoregulated in a similar way?

Legumes enter nodule symbioses with nitrogen-fixing bacteria (rhizobia), whereas most flowering plants establish symbiotic associations with arbuscular mycorrhizal (AM) fungi. Once first steps of symbiosis are initiated, nodule formation and mycorrhization in legumes is negatively controlled by a shoot-derived inhibitor (SDI), a phenomenon termed autoregulation. According to current views, autoregulation of nodulation and mycorrhization in legumes is regulated in a similar way. CLE peptides induced in response to rhizobial nodulation signals (Nod factors) have been proposed to represent the ascending long-distance signals to the shoot. Although not proven yet, these CLE peptides are likely perceived by leucine-rich repeat (LRR) autoregulation receptor kinases in the shoot. Autoregulation of mycorrhization in non-legumes is reminiscent to the phenomenon of "systemic acquired resistance" in plant-pathogen interactions.

Strigolactones, signals for parasitic plants and arbuscular mycorrhizal fungi

Although strigolactones play a critical role as rhizospheric signaling molecules for the establishment of arbuscular mycorrhizal (AM) symbiosis and for seed germination of parasitic weeds, scarce data are available about interactions between AM fungi and strigolactones. In the present work, we present background data on strigolactones from studies on their seed germination activity on the parasitic weeds Orobanche and Striga, the importance of nitrogen and phosphorus for this seed germination activity, and what this could mean for AM fungi. We also present results on the susceptibility of plants to AM fungi and the possible involvement of strigolactones in this AM susceptibility and discuss the role of strigolactones for the formation and the regulation of the AM symbiosis as well as the possible implication of these compounds as plant signals in other soil-borne plant-microbe interactions.

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.

Rhizosphere communication of plants, parasitic plants and AM fungi

Plants use an array of secondary metabolites to defend themselves against harmful organisms and to attract others that are beneficial. However, the attraction of beneficial organisms could also lead to abuse by malevolent organisms. An exciting example of such abuse is the relationship between plants, beneficial mutualistic arbuscular mycorrhizal fungi and harmful parasitic plants. Signalling molecules called strigolactones, which are secreted by plant roots in low concentrations, induce the growth of both obligate biotrophs. Here, we review the importance of strigolactones for these two interactions and discuss possible developments that should further clarify the role of these signalling molecules in rhizosphere processes.

An altered root exudation pattern through mycorrhization affecting microconidia germination of the highly specialized tomato pathogen Fusarium oxysporum f. sp. lycopersici (Fol) is not tomato specific but also occurs in Fol nonhost plants

The effect of root exudates from plants colonized or noncolonized by the arbuscular mycorrhizal fungus Glomus mosseae on microconidia germination of Fusarium oxysporum f. sp. lycopersici (Fol) was studied. Root exudates from the Fol-host tomato and root exudates from Fol nonhost plants were tested. Root exudates from all tested plants stimulated microconidia germination. Mycorrhization increased the stimulatory effect exhibited by the root exudates from the Fol host tomato and from all Fol nonhost plants, showing that similar changes occur in the root exudates of all plants after mycorrhization.

Colonization by Arbuscular Mycorrhizal Fungi of Sorghum Leads to Reduced Germination and Subsequent Attachment and Emergence of Striga hermonthica

TWO SORGHUM CULTIVARS: the Striga-tolerant S-35 and the Striga-sensitive CK60-B were grown with or without arbuscular mycorrhizal (AM) fungi, and with or without phosphorus addition. At 24 and 45 days after sowing (DAS) of sorghum, root exudates were collected and tested for effects on germination of preconditioned Striga hermonthica seeds. Root exudates from AM sorghum plants induced lower germination of S. hermonthica seeds than exudates from non-mycorrhizal sorghum. The magnitude of this effect depended on the cultivar and harvest time. A significantly (88-97%) lower germination of S. hermonthica seeds upon exposure to root exudates from AM S-35 plants was observed at both harvest times whereas for AM inoculated CK60-B plants a significantly (41%) lower germination was observed only at 45 DAS. The number of S. hermonthica seedlings attached to and emerged on both sorghum cultivars were also lower in mycorrhizal than in non-mycorrhizal plants. Again, this reduction was more pronounced with S-35 than with CK60-B plants. There was no effect of phosphorus addition on Striga seed germination, attachment or emergence. We hypothesize that the negative effect of mycorrhizal colonization on Striga germination and on subsequent attachment and emergence is mediated through the production of signaling molecules (strigolactones) for AM fungi and parasitic plants.

Root exudates of mycorrhizal tomato plants exhibit a different effect on microconidia germination of Fusarium oxysporum f. sp. lycopersici than root exudates from non-mycorrhizal tomato plants

The effect of root exudates from mycorrhizal and non-mycorrhizal tomato plants on microconidia germination of the tomato pathogen Fusarium oxysporum f. sp. lycopersici was tested. Microconidia germination was enhanced in the presence of root exudates from mycorrhizal tomato plants. The more tomato plants were colonized by the arbuscular mycorrhizal fungus Glomus mosseae, the more microconidia germination was increased, indicating that alterations of the exudation pattern depended on the degree of root AM colonization. Moreover, alterations of the exudation pattern of mycorrhizal plants are not only local, but also systemic. Testing the exudates from plants with a high and a low P level revealed that the alterations of the root exudates from mycorrhizal plants, resulting in a changed effect on microconidia germination, are not due to an improved P status of mycorrhizal plants.

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation

This article reviews recent developments in in situ bioremediation of trace metal contaminated soils, with particular reference to the microbial dynamics in the rhizospheres of plants growing on such soils and their significance in phytoremediation. In non-agricultural conditions, the natural role of plant growth promoting rhizobacteria (PGPR), P-solubilizing bacteria, mycorrhizal-helping bacteria (MHB) and arbuscular mycorrhizal fungi (AMF) in maintaining soil fertility is more important than in conventional agriculture, horticulture, and forestry where higher use of agrochemicals minimize their significance. These microbes initiate a concerted action when a particular population density is achieved, i.e. quorum sensing. AMF also recognize their host by signals released by host roots, allowing a functional symbiosis. AM fungi produce an insoluble glycoprotein, glomalin, which sequester trace elements and it should be considered for biostabilization leading to remediation of contaminated soils. Conclusions drawn from studies of metal uptake kinetics in solution cultures may not be valid for more complex field conditions and use of some combination of glasshouse and field experiments with organisms that occur within the same plant community is suggested. Phytoextraction strategies, such as inoculation of plants to be used for phytoremediation with appropriate heavy metal adapted rhizobial microflora, co-cropping system involving a non-mycorrhizal hyperaccumulator plant and a non-accumulator but mycorrhizal with appropriate AMF, or pre-cropping with mycotrophic crop systems to optimize phytoremediation processes, merit further field level investigations. There is also a need to improve our understanding of the mechanisms involved in transfer and mobilization of trace elements by rhizosphere microbiota and to conduct research on selection of microbial isolates from rhizosphere of plants growing on heavy metal contaminated soils for specific restoration programmes. This is necessary if we are to improve the chances of successful phytoremediation.

Plant and mycorrhizal regulation of rhizodeposition

The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil-based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474.

Regulatory mechanisms during the plant – arbuscular mycorrhizal fungus interaction

Abundant data are available on some aspects of the arbuscular mycorrhizal symbiosis, for example, plant nutrition, but because of difficulties immanent to arbuscular mycorrhizal fungi, such as the inability to culture them axenically, the relatively long time it takes to achieve root colonization, and the simultaneous presence of different morphologic stages of the fungus in the root, less information is accumulated on other aspects such as the regulation of mycorrhization. Regulatory processes in the plant – arbuscular mycorrhizal fungus interaction start before root colonization by the fungus and even before a direct physical contact between the host and the fungal symbiont. Some of the signals exchanged are still a matter of debate and will be discussed further on. After the penetration of the root by the fungus, depending on the developmental stage of the arbuscular mycorrhizal association (e.g., early or mature), a range of plant responses is activated. The possible function of several plant responses in the regulation of mycorrhization is discussed.Key words: arbuscular mycorrhiza, Glomales, autoregulation, flavonoid, recognition, root exudates.

Partner communication in the arbuscular mycorrhizal interaction

During 400 million years of genome–genome interaction, plants and arbuscular mycorrhizal (AM) fungi have become highly interdependent, both ecologically and physiologically. As a result, the differentiation of a functional mycorrhiza is a multistep process requiring the active participation of both partners. During the presymbiotic stage of the AM interaction, some active molecules present in root exudates rapidly induce several fungal genes, in addition to stimulating important cellular and metabolic functions in the fungus, such as mitochondrial biogenesis and respiration. As a result of this activation, the fungus can use its lipidic reserves and reach further developmental stages. Subsequently, the fungus produces factors that induce new gene expression in roots. The fact that the partners of the AM symbiosis exchange such "pheromonal" active molecules during the presymbiotic stage of their interaction suggests the existence of other cross-signaling molecules during the symbiotic stage. These later signals might be involved in activating fungal fatty acid synthesis and sugar uptake or be responsible for specific plant gene induction. Now the challenge is to characterize the chemical nature and the exact role of these fungal and plant regulators in the AM symbiosis.Key words: arbuscular mycorrhizal symbiosis, signaling, root exudates, Myc factor, respiration, lipid metabolism.