Mycorrhiza Establishment Research Articles

OsEXO70L2 is required for large lateral root formation and arbuscular mycorrhiza establishment in rice

As a major subunit of the exocyst complex, members of the EXO70 family have mainly been shown to play roles in cell polarity and morphogenesis in Arabidopsis, but their roles in plant endosymbiosis, such as with arbuscular mycorrhizal fungi (AMF), have rarely been reported. Here, using knockout and overexpression lines, we show that OsEXO70L2, which encodes a divergent EXO70 protein in rice, controls the number of primary roots and is essential for large lateral root formation. Furthermore, the OsEXO70L2 mutant sr1 displayed rare internal AMF hyphae and no arbuscules. We also found that AMF sporulation can occur in roots despite low colonization and that AMF colonization and sporulation are modulated by photoperiod and co-culture with clover. Finally, genes related to auxin homeostasis were found to be affected in the OsEXO70L2 knockout or overexpression lines, suggesting that auxin is at least partly responsible for the phenotypes. This study provides new perspectives on the role of the exocyst complex during root development and AM in rice.

Structural analyzes suggest that MiSSP13 and MiSSP16.5 may act as proteases inhibitors during ectomycorrhiza establishment in Laccaria bicolor

•The signaling process during mycorrhiza establishment involves intense molecular communication between symbionts. It has been suggested that a group of protein effectors, the so-called MiSSPs, plays a broader function in the symbiosis metabolism, however, many of these remain uncharacterized structurally and functionally.•Herein we used three-dimensional protein structure modeling methods, ligand analysis, and molecular docking to structurally characterize and describe two protein effectors, MiSSP13 and MiSSP16.5, with enhanced expression during the mycorrhizal process in Laccaria bicolor.•MiSSP13 and MiSSP16.5 show structural homology with the cysteine and aspartate protease inhibitor, cocaprin (CCP1). Through structural analysis, it was observed that MiSSP13 and MiSSP16.5 have an active site similar to that observed in CCP1. The protein-protein docking data showed that MiSSP13 and MiSSP16.5 interact with the papain and pepsin proteases at sites that are near to where CCP1 interacts with these same targets, suggesting a function as inhibitor of cysteine and aspartate proteases. The interaction of MiSSP13 with papain and MiSSP16.5 with pepsin was stronger than the interaction of CCP1 with these proteases, suggesting that the MiSSPs had a greater activity in inhibiting these classes of proteases. Based on the data supplied, a model is proposed for the function of MiSSPs 13 and 16.5 during the symbiosis establishment. Our findings, while derived from in silico analyses, enable us formulate intriguing hypothesis on the function of MiSSPs in ectomycorrhization, which will require experimental validation.

Frequency of ectomycorrhiza in black cherry (<i>Prunus serotina</i> Ehrh.) and common cherry (<i>Prunus padus</i> L.) growing in the immediate vicinity

Although alien tree species have been studied for many years, it is still unclear why some of them become invasive. The establishment of mycorrhizae is considered a factor influencing invasion success. This study aimed to compare the frequency of mycorrhizae in the roots of the foreign and invasive black cherry and the native common cherry, which are related. The frequency of ectomycorrhizae was assumed to be higher in common cherry (<i>Prunus padus</i>) than in black cherry (<i>Prunus serotina</i>). Thirty 2–3-year-old trees (15 <i>P. serotina</i> and 15 <i>P. padus</i>) were collected from stands where they grew next to each other. Samples were collected in June 2022 from the Forest Experimental Station of Siemianice, Złotówek, and Mrowino. The presence of mycorrhizal fungi was assessed. Morphotypes of ectomycorrhizae were determined with a stereoscopic microscope. The average mycorrhizal frequency was calculated. The roots were scanned and determined with the WinRhizo image analysis system. The length, surface area, diameter, volume, and number of root tips were determined. Statistical analysis of the obtained results was performed with Statistica v.13. The laboratory analyses indicated the dominance of morphotype 3 in all the examined roots of common cherry and black cherry. The highest proportion of type 3 mycorrhizae was found in common cherry at each location. This study showed that black cherry can form ectomycorrhizal associations, which probably positively influenced the success of its invasion.

The shaping of onion seedlings performance through substrate formulation and co-inoculation with beneficial microorganism consortia.

Smart management in crop cultivation is increasingly supported by application of arbuscular mycorrhizal fungi (AMF) and plant growth-promoting microorganisms (PGPM), which sustain soil fertility and plant performance. The aim of this study was the evaluation of the effects of consortia composed of (Claroideoglomus claroideum BEG96, Claroideoglomus etunicatum BEG92, Funneliformis geosporum BEG199, Funneliformis mosseae BEG 95, and Rhizophagus irregularis BEG140) and PGPM (Azospirillum brasilense - AZ, or Saccharothrix sp. - S) on onion cultivated in growing media with a composition corresponding to a degraded soil. Three types of substrate formulations were used, with peat:sand ratios of 50:50, 70:30, 100:0 (v:v). The analysis of substrate parameters crucial for its fertility (pH, salinity, sorption complex capacity, and elements' content) and characteristics reflecting onion seedlings' performance (fresh weight, stress biomarkers, and elements' content) was performed. AMF colonized onion roots in all treatments, showing increasing potential to form intercellular structures in the substrates rich in organic matter. Additionally, co-inoculation with PGPM microorganisms accelerated arbuscular mycorrhiza establishment. Increased antioxidant activity and glutathione peroxidase (GPOX) activity of onion roots sampled from the formulations composed of peat and sand in the ratio of 100:0, inoculated with AMF+S, and positive correlation between GPOX, fresh weight and antioxidant activity of onion roots reflected the successful induction of plant acclimatization response. Total phenols content was the highest in roots and leaves of onion grown in substrates with 70:30 peat:sand ratio, and, in the case of roots, it was correlated with AMF colonization parameters but not with antioxidant activity. AMF and PGPM efficiency in supporting onion growth should be linked to the increased onion root system capacity in mineral salts absorption, resulting in more efficient aboveground biomass production. AMF and PGPM consortia were effective in releasing minerals to soluble fraction in substrates rich in organic matter, making elements available for uptake by onion root system, though this phenomenon depended on the PGPM species. Microorganism consortia enhanced onion seedlings' performance also in substrates with lower content of organic carbon through plant biofertilization and phytostimulation.

AMF species do matter: Rhizophagus irregularis and Funneliformis mosseae affect healthy and PVY-infected Solanum tuberosum L. in a different way.

Arbuscular mycorrhizal fungi (AMF) were documented to positively influence plant growth and yield, which is extremely important for the production of many crops including potato. However, the nature of the interaction between arbuscular mycorrhiza and plant virus that share the same host is not well characterized. In this study, we examined the effect of different AMF, Rhizophagus irregularis and Funneliformis mosseae, on healthy and potato virus Y (PVY)-infected Solanum tuberosum L. The analyses conducted included the measurement of potato growth parameters, oxidative stress indicators, and photosynthetic capacity. Additionally, we evaluated both the development of AMF in plant roots and the virus level in mycorrhizal plants. We found that two AMF species colonized plant roots to varying degrees (ca. 38% for R. irregularis vs. 20% for F. mosseae). Rhizophagus irregularis had a more positive effect on potato growth parameters, causing a significant increase in the total fresh and dry weight of tubers, along with virus-challenged plants. Furthermore, this species lowered hydrogen peroxide levels in PVY-infected leaves and positively modulated the levels of nonenzymatic antioxidants, i.e., ascorbate and glutathione in leaves and roots. Finally, both fungal species contributed to reduced lipid peroxidation and alleviation of virus-induced oxidative damage in plant organs. We also confirmed an indirect interaction between AMF and PVY inhabiting the same host. The two AMF species seemed to have different abilities to colonize the roots of virus-infected hosts, as R. irregularis showed a stronger drop in mycorrhizal development in the presence of PVY. At the same time, arbuscular mycorrhiza exerted an effect on virus multiplication, causing increased PVY accumulation in plant leaves and a decreased concentration of virus in roots. In conclusion, the effect of AMF-plant interactions may differ depending on the genotypes of both symbiotic partners. Additionally, indirect AMF-PVY interactions occur in host plants, diminishing the establishment of arbuscular mycorrhiza while changing the distribution of viral particles in plants.

Long-lasting impact of chito-oligosaccharide application on strigolactone biosynthesis and fungal accommodation promotes arbuscular mycorrhiza in Medicago truncatula.

• The establishment of arbuscular mycorrhiza (AM) between plants and Glomeromycotina fungi is preceded by the exchange of chemical signals: fungal released Myc-factors, including chitoligosaccharides (CO) and lipo-chitooligosaccharides (LCO), activate plant symbiotic responses, while root exuded strigolactones stimulate hyphal branching and boost CO release. Furthermore, fungal signaling reinforcement through CO application was shown to promote AM development in Medicago truncatula, but the cellular and molecular bases of this effect remained unclear. • Here we focused on long-term M. truncatula responses to CO treatment, demonstrating its impact on the transcriptome of both mycorrhizal and non-mycorrhizal roots over several weeks and providing an insight into the mechanistic bases of the CO-dependent promotion of AM colonization. • CO treatment caused the long-lasting regulation of strigolactone biosynthesis and fungal accommodation related genes. This was mirrored by an increase in root didehydro-orobanchol content, and the promotion of accommodation responses to AM fungi in root epidermal cells. Lastly, an advanced down-regulation of AM symbiosis marker genes was observed at the latest time point in CO-treated plants, in line with an increased number of senescent arbuscules. • Overall, CO treatment triggered molecular, metabolic and cellular responses underpinning a protracted acceleration of AM development.

Mycorrhizae helper bacteria for managing the mycorrhizal soil infectivity

Arbuscular mycorrhizal fungi are major components of soil microbiota and mainly interact with other microorganisms in the rhizosphere. Mycorrhiza establishment impacts the plant physiology and some nutritional and physical properties of the rhizospheric soil. These effects alter the development of the root or mycorrhizas resulting from the activity of soil microorganisms. The rhizosphere of mycorrhizal plants (mycorrhizosphere), is inhabited by large microbial activities responsible for several key ecosystem processes. This review is focused on the microbial interactions between mycorrhizal fungi and components of rhizosphere microbiota and highlight the agronomic potentialities of the Mycorrhiza Helper Bacteria on mycorrhiza formation. The main conclusion is that this MHB effect in the rhizosphere of mycorrhizal plants, enhance plant fitness and soil quality and are of great interest to ensure sustainable agricultural development and ecosystem functioning.

Alkaline extract of the seaweed Ascophyllum nodosum stimulates arbuscular mycorrhizal fungi and their endomycorrhization of plant roots

Ascophyllum nodosum extracts (ANE) are well-established plant biostimulants that improve stress tolerance and crop vigour, while also having been shown to stimulate soil microbes. The intersection of these two stimulatory activities, and how they combine to enhance plant health, however, remains poorly understood. In the present study, we aimed to evaluate: (1) the direct effect of ANE on the arbuscular mycorrhizal fungus Rhizophagus irregularis, and (2) whether ANE influences endomycorrhization in plants. ANE enhanced development of R. irregularis in vitro, showing greater spore germination, germ tube length, and hyphal branching. Greenhouse-grown Medicago truncatula drench-treated with ANE formed mycorrhizal associations faster (3.1-fold higher mycorrhization at week 4) and grew larger (29% greater leaf area by week 8) than control plants. Foliar applications of ANE also increased root colonization and arbuscular maturity, but did not appear to enhance plant growth. Nonetheless, following either foliar or drench application, M. truncatula genes associated with establishment of mycorrhizae were expressed at significantly higher levels compared to controls. These results suggest that ANE enhances mycorrhization through both direct stimulation of arbuscular mycorrhizal fungus growth and through stimulation of the plant’s accommodation of the symbiont, together promoting the establishment of this agriculturally vital plant–microbe symbiosis.

Mycorrhiza-Induced Resistance against Foliar Pathogens Is Uncoupled of Nutritional Effects under Different Light Intensities.

The use of microbial inoculants, particularly arbuscular mycorrhizal fungi, has great potential for sustainable crop management, which aims to reduce the use of chemical fertilizers and pesticides. However, one of the major challenges of their use in agriculture is the variability of the inoculation effects in the field, partly because of the varying environmental conditions. Light intensity and quality affect plant growth and defense, but little is known about their impacts on the benefits of mycorrhizal symbioses. We tested the effects of five different light intensities on plant nutrition and resistance to the necrotrophic foliar pathogen Botrytis cinerea in mycorrhizal and non-mycorrhizal lettuce plants. Our results evidence that mycorrhiza establishment is strongly influenced by light intensity, both regarding the extension of root colonization and the abundance of fungal vesicles within the roots. Light intensity also had significant effects on plant growth, nutrient content, and resistance to the pathogen. The effect of the mycorrhizal symbiosis on plant growth and nutrient content depended on the light intensity, and mycorrhiza efficiently reduced disease incidence and severity under all light intensities. Thus, mycorrhiza-induced resistance can be uncoupled from mycorrhizal effects on plant nutrition. Therefore, mycorrhizal symbioses can be beneficial by providing biotic stress protection even in the absence of nutritional or growth benefits.

Phytohormones and volatile organic compounds, like geosmin, in the ectomycorrhiza of Tricholoma vaccinum and Norway spruce (Picea abies)

The ectomycorrhizospheric habitat contains a diverse pool of organisms, including the host plant, mycorrhizal fungi, and other rhizospheric microorganisms. Different signaling molecules may influence the ectomycorrhizal symbiosis. Here, we investigated the potential of the basidiomycete Tricholoma vaccinum to produce communication molecules for the interaction with its coniferous host, Norway spruce (Picea abies). We focused on the production of volatile organic compounds and phytohormones in axenic T. vaccinum cultures, identified the potential biosynthesis genes, and investigated their expression by RNA-Seq analyses. T. vaccinum released volatiles not usually associated with fungi, like limonene and β-barbatene, and geosmin. Using stable isotope labeling, the biosynthesis of geosmin was elucidated. The geosmin biosynthesis gene ges1 of T. vaccinum was identified, and up-regulation was scored during mycorrhiza, while a different regulation was seen with mycorrhizosphere bacteria. The fungus also released the volatile phytohormone ethylene and excreted salicylic and abscisic acid as well as jasmonates into the medium. The tree excreted the auxin, indole-3-acetic acid, and its biosynthesis intermediate, indole-3-acetamide, as well as salicylic acid with its root exudates. These compounds could be shown for the first time in exudates as well as in soil of a natural ectomycorrhizospheric habitat. The effects of phytohormones present in the mycorrhizosphere on hyphal branching of T. vaccinum were assessed. Salicylic and abscisic acid changed hyphal branching in a concentration-dependent manner. Since extensive branching is important for mycorrhiza establishment, a well-balanced level of mycorrhizospheric phytohormones is necessary. The regulation thus can be expected to contribute to an interkingdom language.

Gene expression in Rhizoglomus irregulare at two different time points of mycorrhiza establishment in Helianthus annuus roots, as revealed by RNA-seq analysis.

Arbuscular mycorrhizal fungi (AMF) play a fundamental role in plant growth and nutrition in natural and agricultural ecosystems. Despite the importance of such symbionts, the different developmental changes occurring during the AMF life cycle have not been fully elucidated at the molecular level. Here, the RNA-seq approach was used to investigate Rhizoglomus irregulare specific and common transcripts at two different time points of mycorrhizal establishment in Helianthus annuus in vivo. Four days after inoculation, transcripts related to cellular remodeling (actin and tubulin), cellular signaling (calmodulin, serine/threonine protein kinase, 14-3-3 protein, and calcium transporting ATPase), lipid metabolism (fatty acid desaturation, steroid hormone, and glycerophospholipid biosynthesis), and biosynthetic processes were detected. In addition to such transcripts, 16days after inoculation, expressed genes linked to binding and catalytic activities; ion (K+, Ca2+, Fe2+, Zn2+, Mn2+, Pi, ammonia), sugar, and lipid transport; and those involved in vacuolar polyphosphate accumulation were found. Knowledge of transcriptomic changes required for symbiosis establishment and performance is of great importance to understand the functional role of AMF symbionts in food crop nutrition and health, and in plant diversity in natural ecosystems.

Advances in mechanisms of nutrient exchange between mycorrhizal fungi and host plants

Mycorrhizae, formed through the colonization of soil mycorrhizal fungi into the roots of host plants, are common symbiosis in the terrestrial ecosystems. The establishment of mycorrhizae is mainly based on the bidirectional nutrient exchanges between the symbiotic partners. Mycorrhizal fungi can absorb mineral nutrients, such as nitrogen and phosphorus, from soil and transport them to the host plants for their growth. As an exchange, host plants supply mycorrhizal fungi with the carbohydrates in the form of lipids or sugars, which are essential for fungal growth. In recent years, the mechanism of nutrient exchange between the mycorrhizal fungi and host plants has been a hot research topic. Important progresses have been achieved in mechanisms of host plants nutrient uptake and transport mediated by the mycorrhizal fungi. In this review, recent advances in nutrient exchange between arbuscular mycorrhizal fungi, ectomycorrhizal fungi and host plants were summarized, especially in the absorption and bidirectional transfer mechanisms of important nutrients, such as carbon, nitrogen and phosphorus. The potential regulatory effects of nutrient exchange in the mycorrhizal development were also reviewed. In addition, key problems and prospects of related researches were analyzed. This paper would be meaningful for the establishment of mycorrhizal model and the optimization of mycorrhizal effects.

Underground communication – the new elements of signalling pathways of arbuscular mycorrhizal symbiosis

Abstract Mycorrhiza is a symbiotic relationship between living cells of the roots of higher plants and non-pathogenic fungi which inhabit soil and belong to Glomeromycota (endomycorrhizae) and Basidiomycota, Ascomycota (ectomycorrhizae). Although the phenomenon of mycorrhiza was discovered by a Polish botanist F.D. Kamieński already in 1881, various stages of establishing the symbiotic relationship between the partners are still not fully understood and explained. According to the current knowledge, the roots of host plants release strigolactones, which stimulate germination and branching of spores of arbuscular fungi. As a result, the fungi synthesize molecular signals, i.e. chitooligosaccharides (COs) and lipochitooligosaccharides (LCOS), called MycF factors. Thanks to the development of molecular biology techniques the probable cascade of events during the recognition of fungal MycF factor by the host-plant has been outlined. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase 1 (HMGR1) and also its product, mevalonic acid (MVA), play an essential role in the biosynthesis of sterols and isoprenoids in a plant cell. The recent studies indicate that these compounds may also play a very important role during establishing of the symbiotic mycorrhizal relationship. It is believed that MVA detects and transmits MycF factor to a cell nucleus of a host-plant triggering numerous necessary mechanisms in the plant cell to activate next steps of the mycorrhizal symbiosis. The discovery of HMGR1 and MVA sheds new light on symbiotic nature of mycorrhiza. This paper is a review of the current knowledge on the signal exchange during symbiotic interactions between mycorrhizal fungi and host plants. 1. Introduction. 2. Symbiotic nature of arbuscular fungi. 3. Arbuscular mycorrhiza in early stages. 4. Exchange of signaling molecules during arbuscular mycorrhiza formation. 5. Mevalonic acid – secondary signaling molecule messengers in the arbuscular mycorrhiza. 6. Protein kinase CCaMK as a key element in the establishment of arbuscular mycorrhiza. 7. Summary

A method to minimize the time needed for formation of mycorrhizas in sweet corn seedlings for outplanting using AM fungus inoculum produced on-farm

A preliminary greenhouse growth phase prior to outplanting allows for earlier stand establishment and the greater profits that early-harvested crops can receive. Inoculation of these crops with arbuscular mycorrhizal [AM] fungi would help combat the effects of low soil temperatures upon phosphorus uptake and establishment of mycorrhizas in the field. Significant establishment of mycorrhizas on roots of sweet corn (Zea mays L.) in the greenhouse is constrained by the approximate two week residence time. Experiments were conducted to examine the benefits of a one week incubation of inoculated potting media at greenhouse temperatures prior to sowing Z. mays seeds upon subsequent development of mycorrhizas. Resulting levels of colonization of roots were contrasted to those of seedlings grown in media that received a 4°C incubation to inhibit spore germination prior to sowing seeds. Levels of AM fungus colonization were compared to those of other seedlings grown in media with a range of inoculum densities. Pre-incubation of inoculated potting media in the greenhouse prior to sowing significantly increased both percentage root length colonized and total root length colonized by AM fungi 2–5 fold at the 9–11day sampling period over that in media incubated in the cold room prior to sowing. Comparison to colonization levels in plants exposed to a range of inoculum densities showed that warm incubation increased the effective inoculum density an average of 80%. Pre-incubation in the greenhouse of AM fungus inoculated compartmented flats of potting media proved to be an effective way to enhance formation of mycorrhizas and should prove to be an effective tool to enhance the effective potency of inoculated potting media.

Tricholoma vaccinum host communication during ectomycorrhiza formation.

The genome sequence of Tricholoma vaccinum was obtained to predict its secretome in order to elucidate communication of T. vaccinum with its host tree spruce (Picea abies) in interkingdom signaling. The most prominent protein domains within the 206 predicted secreted proteins belong to energy and nutrition (52%), cell wall degradation (19%) and mycorrhiza establishment (9%). Additionally, we found small secreted proteins that show typical features of effectors potentially involved in host communication. From the secretome, 22 proteins could be identified, two of which showed higher protein abundances after spruce root exudate exposure, while five were downregulated in this treatment. The changes in T. vaccinum protein excretion with first recognition of the partner were used to identify small secreted proteins with the potential to act as effectors in the mutually beneficial symbiosis. Our observations support the hypothesis of a complex communication network including a cocktail of communication molecules induced long before physical contact of the partners.

Nutrient cycling in the mycorrhizosphere

Optimizing the turnover and recycling of nutrients, a fundamental issue for the sustainability and productivity of agro-ecosystems is depending on the functionality of a framework of plant-soil interactions where microbial populations are involved. Both mutualistic symbionts and saprophytic microorganisms living at the root-soil interfaces, the rhizosphere, or in the plant-associated soil, are recognized as essential drivers of nutrient cycling, availability and capture. Among the mutualistic symbionts, arbuscular mycorrhizal (AM) fungi are one of the most influential groups of soil biota because after establishing the AM symbiosis with most plant species they enhance plant nutrient uptake properties. Saprophytic microorganisms are recognized for their abilities to propel nitrogen (N) fixation and/or phosphorus (P) mobilization, two fundamental processes for sustain plant productivity. Mycorrhiza establishment changes the biological and physical-chemical properties of the rhizosphere, developing the so-called mycorrhizosphere. Particularly relevant is the mycorrhizosphere of legume plants since it also involves the symbiosis with N2-fixing nodulating rhizobial bacteria. In this overview of mycorrhizosphere interactions related to nutrient cycling, after describing the protagonist microorganisms, the mechanisms responsible for nutrient acquisition by AM-plants are first analyzed. Then, the processes involved in mycorrhizosphere establishment and functions are described. Finally, the achievements derived from managing selected AM fungi and beneficial bacteria interactions (mycorrhizosphere tailoring) are discussed. The use of 15N and 32P to elucidate the contribution of the mycorrhizosphere components to plant nutrient acquisition is detailed.

Pineapple (Ananas comosus) cv. pérola ex vitro growth and mycorrhizal colonization affected by in vitro sucrose concentration

The aim of this study was to evaluate the effect of sucrose concentration in the culture medium on growth and on the establishment of mycorrhizas during the acclimatization of pineapple cv. Pérola. The plantlets were micropropagated in MS culture medium with 0, 10, 20 and 30 g L-1 of sucrose and then they were acclimatized during 12 weeks under greenhouse conditions, in a sandy soil - compost mixture, uninoculated or inoculated with a Rhizophagus clarus isolate. Plantlets from the culture medium with 20 g and 30 g of sucrose L-1 showed higher shoot and root biomass than those from sugar-free medium. Mycorrhizal colonization was lower in plantlets micropropagated in sucrose-free medium, but the intensity of arbuscules did not differ among treatments. In the 12-week period of acclimatization, mycorrhizal colonization had no effect on plant biomass.

Characterization of expressed genes in the establishment of arbuscular mycorrhiza between Amorpha fruticosa and Glomus mosseae

Arbuscular mycorrhiza (AM) formed between plant roots and fungi is one of the most widespread symbiotic associations in nature. To understand the molecular mechanisms of AM formation, we profiled 30 symbiosis-related genes expressed in Amorpha fruticosa roots colonized by Glomus mosseae and in non-mycorrhizal roots at different stages using differential-display RT-PCR (DDRT-PCR). The expressed genes were confirmed by reverse Northern blotting. Eleven fragments were sequenced and putatively identified by homologous alignment. Of the eleven AM-related genes, five were obtained at the early-stage of plant-fungus interaction and six at the later stage. Three expressed sequence tag (ESTs) sequences were found to originate from the fungi and eight from the host plant by use of PCR evaluation of gDNA of both plant and fungi. The target genes included an ATP-binding cassette sub-family transporter gene, a transposon-insertion display band, and a photosynthesis-related gene. The results provided information on the molecular mechanisms underlying the development of mycorrhizal symbiosis between woody plants and AM fungi.

Arbuscular mycorrhizal fungi and aphids interact by changing host plant quality and volatile emission

Summary Most plants interact with both arbuscular mycorrhizal (AM) fungi, which increase nutrient acquisition, and herbivores such as aphids, which drain nutrients from plants. Both AM fungi and aphids can affect plant metabolic pathways and may influence each other by altering the condition of the shared host plant. This study tests simultaneously the effects of AM fungi on interactions with aphids (bottom‐up effects) and the effects of aphids on interactions with AM fungi (top‐down effects). We hypothesized that: (i) attractiveness of plants to aphids is regulated by induced changes in production of plant volatile organic compounds (VOCs) triggered by AM fungi or aphids; (ii) aphids reduce AM fungal colonization; and (iii) AM fungal colonization affects aphid development. Broad beans were exposed to AM fungi, aphids and a combination of both. To test for the strength of bottom‐up and top‐down effects, separate treatments enabled establishment of mycorrhizas either before or after aphids were added to plants. VOCs produced by plants were used to (i) test their attractiveness to aphids and (ii) identify the semiochemicals causing attraction. We also measured plant growth and nutrition, AM fungal colonization and aphid reproduction. AM fungi increased the attractiveness of plants to aphids, and this effect tended to prevail even for aphid‐infested plants. However, both attractiveness and aphid population growth depended on the timing of AM fungal inoculation. AM fungi suppressed emission of the sesquiterpenes (E)‐caryophyllene and (E)‐β‐farnesene, and aphid attractiveness to VOCs was negatively associated with the proportion of sesquiterpenes in the sample. Emission of (Z)‐3‐hexenyl acetate, naphthalene and (R)‐germacrene D was regulated by an interaction between aphids and AM fungi. Aphids had a negative effect on mycorrhizal colonization, plant biomass and nutrition. Our data show that below‐ and above‐ground organisms can interact by altering the quality of their shared host plant even though there is no direct contact between them. Plant interactions with herbivores and AM fungi operate in both directions: AM fungi have a key bottom‐up role in insect host location by increasing the attractiveness of plant VOCs to aphids, whereas aphids inhibit formation of AM symbioses.

Analyzing the soybean transcriptome during autoregulation of mycorrhization identifies the transcription factors GmNF-YA1a/b as positive regulators of arbuscular mycorrhization.

BackgroundSimilarly to the legume-rhizobia symbiosis, the arbuscular mycorrhiza interaction is controlled by autoregulation representing a feedback inhibition involving the CLAVATA1-like receptor kinase NARK in shoots. However, little is known about signals and targets down-stream of NARK. To find NARK-related transcriptional changes in mycorrhizal soybean (Glycine max) plants, we analyzed wild-type and two nark mutant lines interacting with the arbuscular mycorrhiza fungus Rhizophagus irregularis.ResultsAffymetrix GeneChip analysis of non-inoculated and partially inoculated plants in a split-root system identified genes with potential regulation by arbuscular mycorrhiza or NARK. Most transcriptional changes occur locally during arbuscular mycorrhiza symbiosis and independently of NARK. RT-qPCR analysis verified nine genes as NARK-dependently regulated. Most of them have lower expression in roots or shoots of wild type compared to nark mutants, including genes encoding the receptor kinase GmSIK1, proteins with putative function as ornithine acetyl transferase, and a DEAD box RNA helicase. A predicted annexin named GmAnnx1a is differentially regulated by NARK and arbuscular mycorrhiza in distinct plant organs. Two putative CCAAT-binding transcription factor genes named GmNF-YA1a and GmNF-YA1b are down-regulated NARK-dependently in non-infected roots of mycorrhizal wild-type plants and functional gene analysis confirmed a positive role for these genes in the development of an arbuscular mycorrhiza symbiosis.ConclusionsOur results indicate GmNF-YA1a/b as positive regulators in arbuscular mycorrhiza establishment, whose expression is down-regulated by NARK in the autoregulated root tissue thereby diminishing subsequent infections. Genes regulated independently of arbuscular mycorrhization by NARK support an additional function of NARK in symbioses-independent mechanisms.