Mycorrhizal Establishment Research Articles

Microbial contribution to the carbon flux in the soil: A literature review

ABSTRACT Carbon flows into and out of the soil are important processes that contribute to controlling the global climate. The relationship between soil organisms and the climate is interdependent since the organisms that contribute to carbon and greenhouse gas fluxes are simultaneously affected by climate change and soil management. Temperature, soil moisture, pH, nutrient level, redox potential and organic matter quality are key elements affecting the microorganisms involved in organic carbon flows in the soil. Climate, topography (slope and position in the landscape), soil texture, soil mineralogy and land-use regulate those key elements and, thus, the C fluxes in the pedosphere. Soil microbes can increase carbon influx and storage by promoting plant growth, mycorrhizal establishment, and particle aggregation. Conversely, microorganisms contribute to carbon efflux from the soil via methanogenesis, rhizospheric activity, and organic carbon mineralization. Nevertheless, strategies and management practices could be used to balance out carbon emissions to the atmosphere. For example, carbon influx and storage in the soil can be stimulated by plant growth promoting microorganisms, greater plant diversity via crop rotation and cover crops, cultivating mycotrophic plants, avoiding or reducing the use of fungicides and adopting organic farming, no-tillage crop systems and conservative soil management strategies. Therefore, this review aimed to shed light on how soil microorganisms can contribute to increase C influxes to the soil, and its significance for climate change. Then, we also seek to gather the practical actions proposed in the scientific literature to improve carbon sequestration and storage in the soil. In summary, the review provides a comprehensive basis on soil microorganisms as key to carbon fluxes and helpers to lessen climate change by increasing carbon fixation and storage in agroecosystems via stimulation or application of beneficial microorganisms.

The Effects of Rice Husk Biochar Rate on Arbuscular Mycorrhizal Fungi and Growth of Soursop (Annona muricata L.) Seedlings

Soursop (Annona muricata L.) is increasingly planted in Malaysia for its agronomical importance as a multipurpose plant with medicinal and health benefits. However, there is limited information on soursop cultivation in Malaysia. Soursop can be inoculated with arbuscular mycorrhizal fungi (AMF) as a sustainable approach for successful seedlings growth before transplanting to field. As a beneficial symbiotic soil microorganism, AMF has been known as biofertilizer which can improve soil conditions and plant growth. Following the need to understand the soil management using soil beneficial microorganisms for soursop cultivation, this experiment aimed to determine the effects of AMF and biochar on the growth of soursop seedlings. The polybag experiment was conducted under glasshouse conditions at Farm 15, Faculty of Agriculture, UPM Serdang, Selangor. The study was arranged in Randomised Complete Block Design (RCBD) with eight treatments comprising different rates of rice husk biochar (RHB), with and without AMF and four replications. The treatments were; T1 (Control), T2 (AMF only), T3 (−AMF with 10 t/ha RHB), T4 (+AMF with 10 t/ha RHB), T5 (−AMF with 15 t/ha RHB), T6 (+AMF with 15 t/ha RHB), T7 (−AMF with 20 t/ha RHB) and T8 (+AMF with 20 t/ha RHB). Plant growth measurement were determined upon harvesting after four months of planting. Shoot and root biomass, leaf area meter, root surface area and root length were found significantly highest in soursop seedlings grown in soils amended with 10 t/ha RHB with or without AMF as compared to control. It was concluded that RHB, either individually or when in combination with AMF, had significantly improved the growth performance of soursop seedlings at nursery stage. Therefore, earliest inoculation of soursop seedlings with AMF at nursery stage could ensure successful mycorrhizal establishment and growth performance of soursop seedlings before transplanted under field condition.

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.

A combination of chitooligosaccharide and lipochitooligosaccharide recognition promotes arbuscular mycorrhizal associations in Medicago truncatula

Plants associate with beneficial arbuscular mycorrhizal fungi facilitating nutrient acquisition. Arbuscular mycorrhizal fungi produce chitooligosaccharides (COs) and lipo-chitooligosaccharides (LCOs), that promote symbiosis signalling with resultant oscillations in nuclear-associated calcium. The activation of symbiosis signalling must be balanced with activation of immunity signalling, which in fungal interactions is promoted by COs resulting from the chitinaceous fungal cell wall. Here we demonstrate that COs ranging from CO4-CO8 can induce symbiosis signalling in Medicago truncatula. CO perception is a function of the receptor-like kinases MtCERK1 and LYR4, that activate both immunity and symbiosis signalling. A combination of LCOs and COs act synergistically to enhance symbiosis signalling and suppress immunity signalling and receptors involved in both CO and LCO perception are necessary for mycorrhizal establishment. We conclude that LCOs, when present in a mix with COs, drive a symbiotic outcome and this mix of signals is essential for arbuscular mycorrhizal establishment.

Arbuscular Mycorrhiza Alleviates Restrictions to Substrate Water Flow and Delays Transpiration Limitation to Stronger Drought in Tomato.

Arbuscular mycorrhizal fungi (AMF) proliferate in soil pores, on the surface of soil particles and affect soil structure. Although modifications in substrate moisture retention depend on structure and could influence plant water extraction, mycorrhizal impacts on water retention and hydraulic conductivity were rarely quantified. Hence, we asked whether inoculation with AMF affects substrate water retention, water transport properties and at which drought intensity those factors become limiting for plant transpiration. Solanum lycopersicum plants were set up in the glasshouse, inoculated or not with Funneliformis mosseae, and grown for 35 days under ample water supply. After mycorrhizal establishment, we harvested three sets of plants, one before (36 days after inoculation) and the second (day 42) and third (day 47) within a sequential drying episode. Sampling cores were introduced into pots before planting. After harvest, moisture retention and substrate conductivity properties were assessed and water retention and hydraulic conductivity models were fitted. A root water uptake model was adopted in order to identify the critical substrate moisture that induces soil derived transpiration limitation. Neither substrate porosity nor saturated water contents were affected by inoculation, but both declined after substrates dried. Drying also caused a decline in pot water capacity and hydraulic conductivity. Plant available water contents under wet (pF 1.8–4.2) and dry (pF 2.5–4.2) conditions increased in mycorrhizal substrates and were conserved after drying. Substrate hydraulic conductivity was higher in mycorrhizal pots before and during drought exposure. After withholding water from pots, higher substrate drying rates and lower substrate water potentials were found in mycorrhizal substrates. Mycorrhiza neither affected leaf area nor root weight or length. Consistently with higher substrate drying rates, AMF restored the plant hydraulic status, and increased plant transpiration when soil moisture declined. The water potential at the root surface and the resistance to water flow in the rhizosphere were restored in mycorrhizal pots although the bulk substrate dried more. Finally, substrates colonized by AMF can be more desiccated before substrate water flux quantitatively limits transpiration. This is most pronounced under high transpiration demands and complies with a difference of over 1,000 hPa in substrate water potential.

Transcriptome changes induced by arbuscular mycorrhizal fungi in sunflower (Helianthus annuus L.) roots

Arbuscular mycorrhizal (AM) fungi are essential elements of soil fertility, plant nutrition and productivity, facilitating soil mineral nutrient uptake. Helianthus annuus is a non-model, widely cultivated species. Here we used an RNA-seq approach for evaluating gene expression variation at early and late stages of mycorrhizal establishment in sunflower roots colonized by the arbuscular fungus Rhizoglomus irregulare. mRNA was isolated from roots of plantlets at 4 and 16 days after inoculation with the fungus. cDNA libraries were built and sequenced with Illumina technology. Differential expression analysis was performed between control and inoculated plants. Overall 726 differentially expressed genes (DEGs) between inoculated and control plants were retrieved. The number of up-regulated DEGs greatly exceeded the number of down-regulated DEGs and this difference increased in later stages of colonization. Several DEGs were specifically involved in known mycorrhizal processes, such as membrane transport, cell wall shaping, and other. We also found previously unidentified mycorrhizal-induced transcripts. The most important DEGs were carefully described in order to hypothesize their roles in AM symbiosis. Our data add a valuable contribution for deciphering biological processes related to beneficial fungi and plant symbiosis, adding an Asteraceae, non-model species for future comparative functional genomics studies.

Symbiotic association between golden berry (Physalis peruviana) and arbuscular mycorrhizal fungi in heavy metal-contaminated soil

AbstractPhysalis peruvianais one of the most promising tropical fruit plants because of its rapid growth, high yield, and nutritional quality. This study was designed to investigate plant development under heavy metal contamination (Cd, Pb) and responsiveness to arbuscular mycorrhizal fungi (AMF) colonization byRhizophagus clarumandClaroideoglomus claroideum. The antioxidant capacity, total lipid content and fatty acid profile in fruits, accumulation of Cd and Pb in different plant parts, plant dry biomass, and mycorrhizal colonization were determined. As a result of inoculation, a considerable reduction in Cd and Pb in the fruits was observed, compared with non-inoculated plants. The fruit number and dry weight increased in plants associated withC. claroideum.These plants also showed higher acid phosphatase activity, root protein accumulation and glomalin production. The type of antioxidant defense was AMF strain-dependent. Antioxidant activity and H2O2neutralization were enzymatic rather than non-enzymatic processes in the fruits ofC. claroideumplants compared with those forming an association withR. clarum. Mycorrhizal establishment changed the composition and concentration of fruits’ fatty acids. The ratio of unsaturated fatty acids was increased. With respect to the accumulation of bioactive compounds in golden berry the present findings are important for obtaining the optimum benefits of mycorrhizal association under unfavorable conditions.

Species richness and composition of arbuscular mycorrhizal fungi occurring on eucalypt trees (Eucalyptus camaldulensis Dehnh.) in rainy and dry season

River red gum (Eucalyptus camaldulensis Dehnh.), the most commonly planted eucalypt species globally, has several advantages and is widely used for many purposes, which makes the tree important. Mycorrhizal establishment in eucalyptus has been known for many years, and the benefits of this symbiosis have been commercially explored. The main goal of this research was to assess the diversity and distribution of arbuscular mycorrhizal fungi (AMF) on eucalyptus planted in agricultural fields in the rainy and dry season. Fields were chosen in ten different sites located in four provinces in the northeast of Thailand. Rhizosphere soil and root samples were collected and the number of AMF spores and AMF root colonization were assessed. The number of AMF spores was higher in the rainy season than in the dry season, while AMF root colonization was higher in the dry season than in the rainy season. On the basis of morphological identification of AMF, a total of 35 AMF fungal taxa in eight genera were identified, ten belonging to Acaulospora, one to Dentiscutata, one to Entrophospora, 16 to Glomus, three to Gigaspora, one to Racocetra, two to Scutellospora, and one to Septoglomus. Glomus was the dominant genus followed by Acaulospora. Relative abundance, and frequency of occurrence were higher in the rainy season than in the dry season. Racocetra fulgida was the most common species with a frequency of occurrence of 90% in rainy season, and 80% in dry season. Species richness, Simpson's index of dominance and Shannon-Wiener index of diversity were not significantly different between both seasons.

Rhizophagus irregularis as an elicitor of rosmarinic acid and antioxidant production by transformed roots of Ocimum basilicum in an in vitro co-culture system.

Arbuscular mycorrhiza is a symbiotic association formed between plant roots and soil borne fungi that alter and at times improve the production of secondary metabolites. Detailed information is available on mycorrhizal development and its influence on plants grown under various edapho-climatic conditions, however, very little is known about their influence on transformed roots that are rich reserves of secondary metabolites. This raises the question of how mycorrhizal colonization progresses in transformed roots grown in vitro and whether the mycorrhizal fungus presence influences the production of secondary metabolites. To fully understand mycorrhizal ontogenesis and its effect on root morphology, root biomass, total phenolics, rosmarinic acid, caffeic acid and antioxidant production under in vitro conditions, a co-culture was developed between three Agrobacterium rhizogenes-derived, elite-transformed root lines of Ocimum basilicum and Rhizophagus irregularis. We found that mycorrhizal ontogenesis in transformed roots was similar to mycorrhizal roots obtained from an in planta system. Mycorrhizal establishment was also found to be transformed root line-specific. Colonization of transformed roots increased the concentration of rosmarinic acid, caffeic acid and antioxidant production while no effect was observed on root morphological traits and biomass. Enhancement of total phenolics and rosmarinic acid in the three mycorrhizal transformed root lines was found to be transformed root line-specific and age dependent. We reveal the potential of R. irregularis as a biotic elicitor in vitro and propose its incorporation into commercial in vitro secondary metabolite production via transformed roots.

Gr and hp-1 tomato mutants unveil unprecedented interactions between arbuscular mycorrhizal symbiosis and fruit ripening.

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.

Effect of volatiles versus exudates released by germinating spores of Gigaspora margarita on lateral root formation

Arbuscular mycorrhizal (AM) fungi influence the root system architecture of their hosts; however, the underlying mechanisms have not been fully elucidated. Ectomycorrhizal fungi influence root architecture via volatiles. To determine whether volatiles also play a role in root system changes in response to AM fungi, spores of the AM fungus Gigaspora margarita were inoculated on the same plate as either wild type (WT) Lotus japonicus, the L. japonicus mutant Ljcastor (which lacks the symbiotic cation channel CASTOR, which is required for inducing nuclear calcium spiking, which is necessary for symbiotic partner recognition), or Arabidopsis thaliana, separated by cellophane membranes (fungal exudates experiment), or on different media but with a shared head space (fungal volatiles experiment). Root development was monitored over time. Both germinating spore exudates (GSEs) and geminated-spore-emitted volatile organic compounds (GVCs) significantly promoted lateral root formation (LRF) in WT L. japonicus. LRF in Ljcastor was significantly enhanced in the presence of GVCs. GVCs stimulated LRF in A. thaliana, whereas GSEs showed an inhibitory effect. The expression profile of the genes involved in mycorrhizal establishment and root development were investigated using quantitative reverse transcription-PCR analysis. Only the expression of the LjCCD7 gene, an important component of the strigolactone synthesis pathway, was differentially expressed following exposure to GVCs. We conclude that volatile organic compounds released by the germinating AM fungal spores may stimulate LRF in a symbiosis signaling pathway (SYM)- and host-independent way, whereas GSEs stimulate LRF in a SYM- and host-dependent way.

Role of the arbuscular mycorrhizal symbiosis in tolerance response against Armillaria mellea in lavender

Lavender species form the arbuscular mycorrhizal symbiosis and are at the same time highly susceptible to white root rot. In an attempt to evaluate the response of mycorrhizal Lavandula angustifolia L. to Armillaria mellea (Vahl:Fr) P. Kumm in a greenhouse experiment, plants were previously inoculated with an isolate of the arbuscular mycorrhizal fungus Rhizophagus irregularis (former Glomus intraradices BEG 72) and the influence of the pH growing medium on the plant-symbiont-pathogen interaction was tested in gnotobiotic autotrophic growth systems in which mycorrhizal inoculum was obtained from root organ cultures. After ten months growth dual-inoculated lavender plants grown in containers with a pasteurized substrate mixture produced a similar number of spikes than healthy plants and achieved equivalent plant diameter coverage. When the growing medium in the autotrophic systems was supplemented with calcium carbonate, the inoculation of lavender plantlets with R. irregularis at higher pH (7.0 and 8.5) media caused a significant decrease of A. mellea presence in plant roots, as detected by qPCR. Moreover, the observation of internal root mycorrhizal infection showed that the extent of mycorrhizal colonization increasedin plant rootsgrown at higher pH, indicating that tolerance to white root rot in lavender plants inoculated with R. irregularis could be associated to mycorrhizal establishment.

Sources of inocula influence mycorrhizal colonization of plants in restoration projects: a meta‐analysis

Inoculation may influence mycorrhizal colonization and provide benefits to plants in restoration projects. However, it is unclear whether inoculation has consistent effects across ecosystem types, if it has long‐term effects on colonization, and whether sources of inocula differ in their effectiveness. To address these issues, we performed a meta‐analysis of published restoration studies across a variety of ecosystems to examine the effects of mycorrhizal inoculation on mycorrhizal establishment and plant growth under field conditions. Although we included trials from a variety of geographic locations, disturbance types, and ecosystem types, the majority were based in temperate ecosystems in the Northern Hemisphere, and fewer trials were from tropical ecosystems. Across ecosystem types, we found that inoculation consistently increased the abundance of mycorrhizal fungi in degraded ecosystems, and thus improved the establishment of plants. These benefits did not significantly attenuate over time. Moreover, inocula from different sources varied in their effects on mycorrhizal colonization. Inocula sourced from reference ecosystems and inocula with specific fungal species yielded higher increases in mycorrhizal colonization than did inocula from commercial sources. These results suggest that inocula source matters, and that an initial investment into mycorrhizal inoculation could provide lasting benefits for facilitating the establishment of the below‐ and aboveground components of restored ecosystems.

Host compatibility and fertilization level modulate mycorrhizal establishment and growth of two ornamental shrubs

Purchase online the PDF of Host compatibility and fertilization level modulate mycorrhizallishment and growth of two ornamental shrubs, Battini, F.,Avio, L.,Incrocci, L.,Carmassi, G.,Agnolucci, M. - Pisa University Press - Article

The membrane proteome of Medicago truncatula roots displays qualitative and quantitative changes in response to arbuscular mycorrhizal symbiosis.

During arbuscular mycorrhizal symbiosis, one of the most widespread mutualistic associations in nature, the endomembrane system of plant roots is believed to undergo qualitative and quantitative changes in order to sustain both the accommodation process of the AM fungus within cortical cells and the exchange of nutrients between symbionts. Large-scale GeLC-MS/MS proteomic analysis of the membrane fractions from mycorrhizal and nonmycorrhizal roots of M. truncatula coupled to spectral counting retrieved around one hundred proteins that displayed changes in abundance upon mycorrhizal establishment. The symbiosis-related membrane proteins that were identified mostly function in signaling/membrane trafficking and nutrient uptake regulation. Besides extending the coverage of the root membrane proteome of M. truncatula, new candidates involved in the symbiotic program emerged from the current study, which pointed out a dynamic reorganization of microsomal proteins during the accommodation of AM fungi within cortical cells.

Protein actors sustaining arbuscular mycorrhizal symbiosis: underground artists break the silence

The roots of most land plants can enter a relationship with soil-borne fungi belonging to the phylum Glomeromycota. This symbiosis with arbuscular mycorrhizal (AM) fungi belongs to the so-called biotrophic interactions, involving the intracellular accommodation of a microorganism by a living plant cell without causing the death of the host. Although profiling technologies have generated an increasing depository of plant and fungal proteins eligible for sustaining AM accommodation and functioning, a bottleneck exists for their functional analysis as these experiments are difficult to carry out with mycorrhiza. Nonetheless, the expansion of gene-to-phenotype reverse genetic tools, including RNA interference and transposon silencing, have recently succeeded in elucidating some of the plant-related protein candidates. Likewise, despite the ongoing absence of transformation tools for AM fungi, host-induced gene silencing has allowed knockdown of fungal gene expression in planta for the first time, thus unlocking a technological limitation in deciphering the functional pertinence of glomeromycotan proteins during mycorrhizal establishment. This review is thus intended to draw a picture of our current knowledge about the plant and fungal protein actors that have been demonstrated to be functionally implicated in sustaining AM symbiosis mostly on the basis of silencing approaches.

A new process to promote the use of controlled mycorrhization practice in forest nurseries

The aims of this study were to test a new mycorrhizal inoculation process using a “catalyser” of the mycorrhizal establishment (termite mounds of Macrotermes subhyalinus) to minimize the requested volume of fungal inoculum added to the cultural substrate. The effects of the termite mound were explored on mycorrhiza formation between an Australian Acacia, Acacia holosericea and an ectomycorrhizal fungus or an arbuscular mycorrhizal fungus using a two-step cultural system. The first step of this cultural practice was the inoculation of A. holosericea seedlings in small soil volumes (5 L plastic containers planted with 100 pre-germinated seeds) whereas the second one allowed the development of these mycorrhized plants in larger soil volumes (1 L pots planted with one seedling). Termite mound amendment significantly enhanced the mycorrhizal formation from both types of fungal isolates. This stimulating effect could probably be attributed to the introduction via the termite mound of a bacterial group (that is, fluorescent pseudomonads) that could act as Mycorrhiza Helper Bacteria (MHB). Since it is possible to reduce the requested fungal inoculum in controlled mycorrhization practice using M. subhyalinus mound powders, this biotechnological process could be useful in re-afforestation of tropical regions by lowering the requested fungal inoculum quantities and reducing the financial costs of controlled mycorrhization in forest nurseries.

What we know about arbuscular mycorhizal fungi and associated soil bacteria

Mycorrhizal fungi are common soil microorganisms and are well known for their symbiotic association with the roots of host plants. The soil is a complex environment harbouring a wide diversity of microorganisms. The interaction between soil bacteria and arbuscular mycorrhizal fungi has been shown in several studies to be both beneficial in terms of mycorrhizal establishment and as well as plant growth promotion. This has resulted in groups of bacteria being functionally termed Mycorrhizal Helper Bacteria, Phosphate Solubilising Bacteria and Plant Growth Promoting Rhizobacteria. Several of these groups overlap and in such a complex environment, it is likely that the combinations of microorganisms interacting with arbuscular mycorrhizal fungi enhance the benefits that are attributed to the relationship. Many different microorganisms inhabit the soil. This review will focus on the bacterial interactions and their potential use in agricultural biotechnology. Key words: Mycorrhizal fungi, bacteria, soil, microorganisms, growth.

Transcriptional regulation of defence genes and involvement of the WRKY transcription factor in arbuscular mycorrhizal potato root colonization

The establishment of arbuscular mycorrhizal associations causes major changes in plant roots and affects significantly the host in term of plant nutrition and resistance against biotic and abiotic stresses. As a consequence, major changes in root transcriptome, especially in plant genes related to biotic stresses, are expected. Potato microarray analysis, followed by real-time quantitative PCR, was performed to detect the wide transcriptome changes induced during the pre-, early and late stages of potato root colonization by Glomus sp. MUCL 41833. The microarray analysis revealed 526 up-regulated and 132 down-regulated genes during the pre-stage, 272 up-regulated and 109 down-regulated genes during the early stage and 734 up-regulated and 122 down-regulated genes during the late stage of root colonization. The most important class of regulated genes was associated to plant stress and in particular to the WRKY transcription factors genes during the pre-stage of root colonization. The expression profiling clearly demonstrated a wide transcriptional change during the pre-, early and late stages of root colonization. It further suggested that the WRKY transcription factor genes are involved in the mechanisms controlling the arbuscular mycorrhizal establishment by the regulation of plant defence genes.

Changes in soil aggregation and glomalin-related soil protein content as affected by the arbuscular mycorrhizal fungal species Glomus mosseae and Glomus intraradices

Arbuscular mycorrhizal (AM) fungi are key organisms of the soil/plant system, influencing soil fertility and plant nutrition, and contributing to soil aggregation and soil structure stability by the combined action of extraradical hyphae and of an insoluble, hydrophobic proteinaceous substance named glomalin-related soil protein (GRSP). Since the GRSP extraction procedures have recently revealed problems related to co-extracting substances, the relationship between GRSP and AM fungi still remains to be verified. In this work the hypothesis that GRSP concentration is positively correlated with the occurrence of AM fungi was tested by using Medicago sativa plants inoculated with different isolates of Glomus mosseae and Glomus intraradices in a microcosm experiment. Our results show that (i) mycorrhizal establishment produced an increase in GRSP concentration – compared to initial values – in contrast with non-mycorrhizal plants, which did not produce any change; (ii) aggregate stability, evaluated as mean weight diameter (MWD) of macroaggregates of 1–2 mm diameter, was significantly higher in mycorrhizal soils compared to non-mycorrhizal soil; (iii) GRSP concentration and soil aggregate stability were positively correlated with mycorrhizal root volume and weakly correlated with total root volume; (iv) MWD values of soil aggregates were positively correlated with values of total hyphal length and hyphal density of the AM fungi utilized. The different ability of AM fungal isolates to affect GRSP concentration and to form extensive and dense mycelial networks, which may directly affect soil aggregates stability by hyphal enmeshment of soil particles, suggests the possibility of selecting the most efficient isolates to be utilized for soil quality improvement and land restoration programs.