The arbuscular mycorrhizal symbiosis: a molecular review of the fungal dimension

Cracking the enigma: understanding strigolactone signalling in the rhizosphere

The rhizosphere is a complex physical and chemical interface between plants and their underground environment, both biotic and abiotic. Plants exude a large number of chemicals into the rhizosphere in order to manipulate these biotic and abiotic components. Among such chemicals are strigolactones, ancient signalling molecules that in flowering plants act as both internal hormones and external rhizosphere signals. Plants exude strigolactones to communicate with their preferred symbiotic partners and neighbouring plants, but at least some classes of parasitic organisms are able to 'crack' these private messages and eavesdrop on the signals. In this review, we examine the intentional consequences of strigolactone exudation, and also the unintentional consequences caused by eavesdroppers. We examine the molecular mechanisms by which strigolactones act within the rhizosphere, and attempt to understand the enigma of the strigolactone molecular diversity synthesized and exuded into the rhizosphere by plants. We conclude by looking at the prospects of using improved understanding of strigolactones in agricultural contexts.

Involvement of strigolactone hormone in root development, influence and interaction with mycorrhizal fungi in plant: Mini-review

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Arbuscular mycorrhizal fungi (AMF) and plant symbiosis.

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Role AMF in root development and plant growth promotion.

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AMF influence and plant response under strigolactone (SL) and SL-GR24 application.

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Effects and functions of SL in root development and interaction with AMF.

Arbuscular mycorrhizal fungi (AMF) and plant symbiosis is the old, fascinating and beneficial relation that exist on earth for the plants. In this review, we have elaborated that the strigolactones (SLs) are released from the roots and function with root parasite, seeds and symbiotic AMF as contact chemicals. They are transported through the xylem in the plants and can regulate plant architecture, seed germination, nodule formation, increase the primary root length, influence the root hairs and physiological reactions to non-living agents by regulating their metabolism. SLs first evolved in ancient plant lineages as regulators of the basic production processes and then took a new role to maintain the growing biological complexities of terrestrial plant. SLs belongs to a diversified category of butenolide‐bearing plant hormones related to various processes of agricultural concern. SLs also arouses the development of spores, the divergence and enlargement of hyphae of AMF, metabolism of mitochondria, reprogramming of transcription process, and generation of chitin oligosaccharides which further stimulate the early response of symbiosis in the host plant, results from better communication in plant and ability of coexistence with these fungi. The required nutrients are transferred from the roots to the shoots, which affect the physiological, biochemical, and morphological characteristics of the plant. On the other hand, the plant provides organic carbon in the form of sugars and lipids to the fungi, which they use as a source of energy and for carried out different anabolic pathways. SLs also lead to alteration in the dynamic and structure of actin in the root region as well as changes the auxin's transporter localization in the plasma membrane. Thus, this study reveals the functions that SLs play in the growth of roots, as well as their effect and interaction with AMF that promote plant growth.

Isolation and identification of beneficial orchid mycorrhizal fungi in Bletilla striata (Thunb.) Rchb.f.(Orchidaceae)

Orchid mycorrhizal fungi (OMF) are essential for orchids growth. Bletilla striata (Thunb.) Rchb.f.(Orchidaceae) has high ornamental and medicinal value. Beneficial OMF isolation is crucial to improve the survival rate of B. striata tissue culture and transplanting. In this study, we isolated and identified the beneficial OMF in B. striata from the roots of sterilized wild B. striata seedlings by culturing in four different mediums. The germination states of B. striata seeds inoculated with diverse OMF were classified and calculated. Fresh and dry weight increments of B. striata seedlings inoculated with diverse OMF were recorded after 90 d of culturing on 1/2 MS medium. ITS sequences of beneficial fungi were amplified by PCR and taxonomically identified using BLAST against the GenBank nucleotide database. Ten kinds of OMF strains were isolated from B. striata and named R1 to R10. R6 significantly promoted B. striata seeds germination (p < .01). R3 and R6 significantly promoted both the fresh and dry weight increments of B. striata seedlings (p < .05). The ITS sequence of R6 was most similar to the sequence of Serendipita. R3 was identified as Schizothecium fimbriatum by 100% ITS identity. R6 and R3 were beneficial OMF in B. striata.

Fungi and fungal interactions in the Rhynie chert: a review of the evidence, with the description of Perexiflasca tayloriana gen. et sp. nov.†

The Lower Devonian Rhynie chert is one of the most important rock deposits yielding comprehensive information on early continental plant, animal and microbial life. Fungi are especially abundant among the microbial remains, and include representatives of all major fungal lineages except Basidiomycota. This paper surveys the evidence assembled to date of fungal hyphae, mycelial cords and reproductive units (e.g. spores, sporangia, sporocarps), and presents examples of fungal associations and interactions with land plants, other fungi, algae, cyanobacteria and animals from the Rhynie chert. Moreover, a small, chytrid-like organism that occurs singly, in chain-like, linear arrangements, planar assemblages and three-dimensional aggregates of less than 10 to [Formula: see text] individuals in degrading land plant tissue in the Rhynie chert is formally described, and the name Perexiflasca tayloriana proposed for the organism. Perexiflasca tayloriana probably colonized senescent or atrophied plant parts and participated in the process of biological degradation. The fungal fossils described to date from the Rhynie chert constitute the largest body of structurally preserved evidence of fungi and fungal interactions from any rock deposit, and strongly suggest that fungi played important roles in the functioning of the Early Devonian Rhynie ecosystem.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

The Symbiosome: Legume and Rhizobia Co-evolution toward a Nitrogen-Fixing Organelle?

In legume nodules, symbiosomes containing endosymbiotic rhizobial bacteria act as temporary plant organelles that are responsible for nitrogen fixation, these bacteria develop mutual metabolic dependence with the host legume. In most legumes, the rhizobia infect post-mitotic cells that have lost their ability to divide, although in some nodules cells do maintain their mitotic capacity after infection. Here, we review what is currently known about legume symbiosomes from an evolutionary and developmental perspective, and in the context of the different interactions between diazotroph bacteria and eukaryotes. As a result, it can be concluded that the symbiosome possesses organelle-like characteristics due to its metabolic behavior, the composite origin and differentiation of its membrane, the retargeting of host cell proteins, the control of microsymbiont proliferation and differentiation by the host legume, and the cytoskeletal dynamics and symbiosome segregation during the division of rhizobia-infected cells. Different degrees of symbiosome evolution can be defined, specifically in relation to rhizobial infection and to the different types of nodule. Thus, our current understanding of the symbiosome suggests that it might be considered a nitrogen-fixing link in organelle evolution and that the distinct types of legume symbiosomes could represent different evolutionary stages toward the generation of a nitrogen-fixing organelle.

Soil microbes and plant fertilization

With respect to the adverse effects of chemical fertilization on the environment and their related expenses, especially when overused, alternative methods of fertilization have been suggested and tested. For example, the combined use of chemical fertilization with organic fertilization and/or biological fertilization is among such methods. It has been indicated that the use of organic fertilization with chemical fertilization is a suitable method of providing crop plants with adequate amount of nutrients, while environmentally and economically appropriate. In this article, the importance of soil microbes to the ecosystem is reviewed, with particular emphasis on the role of plant growth-promoting rhizobacteria, arbuscular mycorrhizal fungi, and endophytic bacteria in providing necessary nutrients for plant growth and yield production. Such microbes are beneficial to plant growth through colonizing plant roots and inducing mechanisms by which plant growth increases. Although there has been extensive research work regarding the use of microbes as a method of fertilizing plants, it is yet a question how the efficiency of such microbial fertilization to the plant can be determined and increased. In other words, how the right combination of chemical and biological fertilization can be determined. In this article, the most recent advances regarding the effects of microbial fertilization on plant growth and yield production in their combined use with chemical fertilization are reviewed. There are also some details related to the molecular mechanisms affecting the microbial performance and how the use of biological techniques may affect the efficiency of biological fertilization.

Introducing short roots in a desert perennial: anatomy and spatiotemporal foraging responses to increased precipitation

• The desert flora possesses diverse root architectures that result in fast growth in response to precipitation. We introduce the short root, a previously undescribed second-order root in the aridland chamaephyte Cryptantha flava, and explore fine root production. • We describe the short root anatomy and associated fine roots, correlate standing fine root crop with soil moisture, and explore the architectural level - the short root, third-order lateral roots, or the whole root system - at which fine roots are induced by watering and the amount of water required. • We show that short roots are borne at intervals on lateral roots and produce fine roots at their tips; new fine roots are white and have root hairs, while brown and black fine roots are apparently dead; and fine root production is triggered at the level of lateral roots and with relatively low precipitation (≤ 2 cm). • Short roots are suberized and thus are probably not capable of water uptake themselves, but serve as initiation sites for fine roots that grow rapidly in response to rainfall. Thus, C. flava should be a beneficiary of projected precipitation increases in habitats where rainfall is pulsed.

Leaf nitrogen dioxide uptake coupling apoplastic chemistry, carbon/sulfur assimilation, and plant nitrogen status

Emission and plant uptake of atmospheric nitrogen oxides (NO + NO(2)) significantly influence regional climate change by regulating the oxidative chemistry of the lower atmosphere, species composition and the recycling of carbon and nutrients, etc. Plant uptake of nitrogen dioxide (NO(2)) is concentration-dependent and species-specific, and covaries with environmental factors. An important factor determining NO(2) influx into leaves is the replenishment of the substomatal cavity. The apoplastic chemistry of the substomatal cavity plays crucial roles in NO(2) deposition rates and the tolerance to NO(2), involving the reactions between NO(2) and apoplastic antioxidants, NO(2)-responsive germin-like proteins, apoplastic acidification, and nitrite-dependent NO synthesis, etc. Moreover, leaf apoplast is a favorable site for the colonization by microbes, which disturbs nitrogen metabolism of host plants. For most plant species, NO(2) assimilation in a leaf primarily depends on the nitrate (NO(3) (-)) assimilation pathway. NO(2)-N assimilation is coupled with carbon and sulfur (sulfate and SO(2)) assimilation as indicated by the mutual needs for metabolic intermediates (or metabolites) and the NO(2)-caused changes of key metabolic enzymes such as phosphoenolpyruvate carboxylase (PEPc) and adenosine 5'-phosphosulfate sulfotransferase, organic acids, and photorespiration. Moreover, arbuscular mycorrhizal (AM) colonization improves the tolerance of host plants to NO(2) by enhancing the efficiency of nutrient absorption and translocation and influencing foliar chemistry. Further progress is proposed to gain a better understanding of the coordination between NO(2)-N, S and C assimilation, especially the investigation of metabolic checkpoints, and the effects of photorespiratory nitrogen cycle, diverse PEPc and the metabolites such as cysteine, O-acetylserine (OAS) and glutathione.

Plant growth and metal distribution in tissues of Prosopis juliflora-velutina grown on chromium contaminated soil in the presence of Glomus deserticola

Arbuscular mycorrhizal fungi have been known to increase metal uptake in plants. In this study, mesquite (Prosopis juliflora-velutina) inoculated with Glomus deserticola or amended with EDTA were grown for 30 days in soil containing Cr(III) or Cr(VI) at 0, 40, 80, and 160 mg kg(-1). Total amylase activity (TAA) was monitored as a stress indicator. Element concentrations and distribution in tissue were determined using ICP-OES, electron scanning microprobe, and TEM. Inoculated Cr(VI) treated plants had 21% and 30% more Cr than uninoculated and EDTA treated roots, respectively, at 80 mg Cr kg(-1) treatment. In the case of Cr(III), EDTA produced the highest Cr accumulation in roots. TAA was higher in inoculated plants grown with Cr(III) at 80 and 160 mg kg(-1) and Cr(VI) at 40 and 160 mg kg(-1). The X-ray mapping showed higher metal concentrations in the vascular system of inoculated plants and the TEM micrographs demonstrated the presence of G. deserticola in roots.

Phylogenetic affinity of arbuscular mycorrhizal symbionts in Psilotum nudum

Many lineages of land plants (from lycopsids to angiosperms) have non-photosynthetic life cycle phases that involve obligate mycoheterotrophic arbuscular mycorrhizal (AM) associations where the plant host gains organic carbon through glomalean symbionts. Our goal was to isolate and phylogenetically identify the AM fungi associated with both the autotrophic and underground mycoheterotrophic life cycle phases of Psilotum nudum. Phylogenetic analyses recovered 11 fungal phylotypes in four diverse clades of Glomus A that form AM associations with P. nudum mycoheterotrophic gametophytes and autotrophic sporophytes, and angiosperm roots found in the same greenhouse pots. The correspondence of identities of AM symbionts in P. nudum sporophytes, gametophytes and neighboring angiosperms provides compelling evidence that photosynthetic heterospecific and conspecific plants can serve as the ultimate sources of fixed carbon for mycoheterotrophic gametophytes of P. nudum, and that the transfer of carbon occurs via shared fungal networks. Moreover, broader phylogenetic analyses suggest greenhouse Psilotum populations, like field-surveyed populations of mycoheterotrophic plants, form AM associations with restricted clades of Glomus A. The phylogenetic affinities and distribution of Glomus A symbionts indicate that P. nudum greenhouse populations have the potential to be exploited as an experimental system to further study the physiology, ecology and evolution of mycoheterotrophic AM associations.

A novel nuclear protein interacts with the symbiotic DMI3 calcium- and calmodulin-dependent protein kinase of Medicago truncatula

Many higher plants establish symbiotic relationships with arbuscular mycorrhizal (AM) fungi that improve their ability to acquire nutrients from the soil. In addition to establishing AM symbiosis, legumes also enter into a nitrogen-fixing symbiosis with bacteria known as rhizobia that results in the formation of root nodules. Several genes involved in the perception and transduction of bacterial symbiotic signals named "Nod factors" have been cloned recently in model legumes through forward genetic approaches. Among them, DMI3 (Doesn't Make Infections 3) is a calcium- and calmodulin-dependent kinase required for the establishment of both nodulation and AM symbiosis. We have identified, by a yeast two-hybrid system, a novel protein interacting with DMI3 named IPD3 (Interacting Protein of DMI3). IPD3 is predicted to interact with DMI3 through a C-terminal coiled-coil domain. Chimeric IPD3::GFP is localized to the nucleus of transformed Medicago truncatula root cells, in which split yellow fluorescent protein assays suggest that IPD3 and DMI3 physically interact in Nicotiana benthamiana. Like DMI3, IPD3 is extremely well conserved among the angiosperms and is absent from Arabidopsis. Despite this high level of conservation, none of the homologous proteins have a demonstrated biological or biochemical function. This work provides the first evidence of the involvement of IPD3 in a nuclear interaction with DMI3.

Nested multiplex PCR--a feasible technique to study partial community of arbuscular mycorrhizal fungi in field-growing plant root

Plant can be infected by different arbuscular mycorrhizal fungi, but little is known about the interaction between them within root tissues mainly because different species cannot be distinguished on the basis of fungal structure. Accurate species identification of Arbuscular mycorrhizal fungi (AMF) colonized in plant roots is the cornerstone of mycorrhizal study, yet this fundamental step is impossible through its morphological character alone. For accurate, rapid and inexpensive detection of partial mycorrhizal fungal community in plant roots, a nested multiplex polymerase chain reaction (PCR) was developed in this study. Five discriminating primers designed based on the variable region of the 5' end of the large ribosomal subunit were used in the experiment for testing their specificity and the sensitivity in nested PCR by using spores from Glomus mosseae (BEG12), Glomus intraradices (BEG141), Scutellospora castaneae (BEG1) and two unidentified Glomus sp. HAUO3 and HAUO4. The feasibility assay of nested multiplex PCR was conducted by use of spore mixture, Astragalus sinicum roots co-inoculated with 4 species of arbuscular mycorrhizal fungi from pot cultures and 15 different field-growing plant roots respectively after analyses of the compatibility of primers. The result indicated that the sensitivity was in the same range as that of the corresponding single PCR reaction. Overall accuracy was 95%. The efficiency and sensitivity of this multiplex PCR procedure provided a rapid and easy way to simultaneously detect several of arbuscular mycorrhiza fungal species in a same plant root system.

Expression profiling of up-regulated plant and fungal genes in early and late stages of Medicago truncatula-Glomus mosseae interactions

Suppression subtractive hybridization (SSH), expression profiling and EST sequencing identified 12 plant genes and six fungal genes that are expressed in the arbuscular mycorrhizal symbiosis between Medicago truncatula and Glomus mosseae. All the plant genes and three of the fungal genes were up-regulated in symbiotic tissues. Expression of 15 of the genes is described for the first time in mycorrhizal roots and two are novel sequences. Six M. truncatula genes were also activated during appressorium formation at the root surface, suggesting a role in this early stage of mycorrhiza establishment, whilst the other six plant genes were only induced in the late stages of mycorrhization and could be involved in the development or functioning of the symbiosis. Phosphate fertilization had no significant influence on expression of any of the plant genes. Expression profiling of G. mosseae genes indicated that two of them may be associated with appressorium development on roots and one with arbuscule formation or function. The other three fungal genes were expressed throughout the life-cycle of G. mosseae.

Differential expression of Glomus intraradices genes in external mycelium and mycorrhizal roots of tomato and barley

Relative quantitative RT-PCR and western blotting were used to investigate the expression of three genes with potentially regulatory functions from the arbuscular mycorrhizal fungus Glomus intraradices in symbiosis with tomato and barley. Standardisation of total RNA per sample and determination of different ratios of plant and fungal RNA in roots as colonisation proceeded were achieved by relative quantitative RT-PCR using universal (NS1/NS21) and organism-specific rRNA primers. In addition, generic primers were designed for amplification of plant or fungal beta-tubulin genes and for plant glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes as these have been suggested as useful controls in symbiotic systems. The fungal genes Ginmyc1 and Ginhb1 were expressed only in the external mycelium and not in colonised roots at both mRNA and protein levels, with the proteins detected almost exclusively in the insoluble fractions. In contrast, mRNA of Ginmyc2 was identified in both external and intraradical mycelium. In mycorrhizal roots, Ginmyc2 and fungal beta-tubulin mRNAs increased in proportion to fungal rRNA as colonisation proceeded, suggesting that accumulation reflected intraradical fungal growth. Fungal alpha-tubulin protein and beta-tubulin mRNA both appeared to be more abundantly accumulated in AM hyphae within heavily colonised roots than in external hyphae, relative to fungal rRNA. Tomato GAPDH mRNA accumulation was proportional to tomato rRNA, but accumulation of tomato beta-tubulin mRNA was reduced in colonised roots compared to non-mycorrhizal roots. These results provide novel evidence of differential spatial and temporal regulation of AM fungal genes, indicate that the expression of tubulin genes of both plant and fungus may be regulated during colonisation and validate the use of multiple 'control' genes in analysis of mycorrhizal gene expression.

Use of bromodeoxyuridine immunocapture to identify active bacteria associated with arbuscular mycorrhizal hyphae

Arbuscular mycorrhizae are beneficial for crops grown under low-till management systems. Increasingly, it is becoming apparent that bacteria associated with mycorrhizae can enhance the beneficial relationship between mycorrhizae and plants. However, it has been difficult to study these relationships by conventional techniques. In this study actively growing bacteria were identified in soil from an undisturbed fallow field known to contain arbuscular mycorrhizae by using molecular tools to eliminate the need for cultivation. A thymidine analog, bromodeoxyuridine (BrdU), was added to the soil and incubated for 2 days. DNA was extracted, and the newly synthesized DNA was isolated by immunocapture of the BrdU-containing DNA. The active bacteria in the community were identified by 16S rRNA gene PCR amplification and DNA sequence analysis. Based on 16S rRNA gene sequence information, a selective medium was chosen to isolate the corresponding active bacteria. Bacillus cereus strain VA1, one of the bacteria identified by the BrdU method, was isolated from the soil and tagged with green fluorescent protein. By using confocal microscopy, this bacterium was shown to clearly attach to arbuscular mycorrhizal hyphae. This study was the first to use this combination of molecular and traditional approaches to isolate, identify, and visualize a specific bacterium that is active in fallow soil and associates with arbuscular mycorrhizal hyphae.

Identification of a cDNA from the arbuscular mycorrhizal fungus Glomus intraradices that is expressed during mycorrhizal symbiosis and up-regulated by N fertilization

A cDNA library was constructed with RNA from Glomus intraradices-colonized lettuce roots and used for differential screening. This allowed the identification of a cDNA (Gi-1) that was expressed only in mycorrhizal roots and was of fungal origin. The function of the gene product is unknown, because Gi-1 contained a complete open reading frame that was predicted to encode a protein of 157 amino acids which only showed little homology with glutamine synthetase from Helicobacter pylori. The time-course analysis of gene expression during the fungal life cycle showed that Gi-1 was expressed only during the mycorrhizal symbiosis and was not detected in dormant or germinating spores of G. intraradices. P fertilization did not significantly change the pattern of Gi-1 expression compared with that in the unfertilized treatment, whereas N fertilization (alone or in combination with P) considerably enhanced the Gi-1 transcript accumulation. This increase in gene expression correlated with plant N status and growth under such conditions. The possible role of the Gi-1 gene product in intermediary N metabolism of arbuscular mycorrhizal symbiosis is further discussed.