Higher Arbuscular Mycorrhizal Research Articles

Gene expression controlling signalling molecules within mutualistic associations of an invasive plant: An evolutionary perspective

Abstract Chemical signals are crucial in mediating ecological and evolutionary adaptation of plants to their environments. Indeed, invasive plants may produce greater amounts of chemical metabolites in their new ranges. Some of these chemicals can enhance their mutualistic interactions and improve invasive plant performance, but genetic mechanisms of such adaptations are unexplored. We used Triadica sebifera plants as a model to investigate evolutionary changes in chemical signals that enhance arbuscular mycorrhizal (AM) fungal associations. Previous studies found that T. sebifera plants from invasive populations had higher root exudate concentrations of the flavonoid quercetin and elevated AM fungal colonization compared with those from native populations. Here, we explored genetic variation in strigolactone concentrations in root exudates and their contribution to AM fungal colonization with strigolactone analogue GR24 additions. In addition, we studied how gene expression patterns related to flavonoid and strigolactone biosynthesis varied among invasive and native populations using comparative genomics, transcriptomics and fluorescent real‐time quantitative PCR. We found that plants from invasive populations had higher concentrations of the strigolactone 5‐deoxystrigol in root exudates that were correlated with higher AM fungal colonization rates, relative to those from native populations. Exogenous applications of GR24, a synthetic analogue of 5‐deoxystrigol, increased AM fungal colonization. We found higher expression of a single gene in flavonoid (FLS) and strigolactone (DAD1) biosynthesis pathways increased levels of quercetin and 5‐deoxystrigol in plants from invasive populations, respectively. Synthesis. Understanding the role of genetic evolution in enhancing mutualisms of invasive plants provides new insights into the underlying mechanisms of plant invasion. This study suggests that an invasive plant enhanced its mutualisms by upregulating the expression of key genes related to secondary metabolites in root exudates which stimulate symbiotic relationships with AM fungi. Thus, these findings provide insights into genetic mechanisms that underlie higher AM fungal colonization and enhanced invasive plant performance.

Tradeoffs among root functional traits for phosphorus acquisition in 13 soybean genotypes contrasting in mycorrhizal colonization.

Plants have adapted to acquire phosphorus (P) primarily through advantageous root morphologies, responsive physiological pathways, and associations with mycorrhizal fungi. Yet, to date, little information exists on how variation in arbuscular mycorrhizal (AM) colonization is coordinated with root morphological and physiological traits to enhance P acquisition. Thirteen root functional traits associated with P acquisition were characterized at full bloom stage in pot cultures under low soil P availability conditions for 13 soybean genotypes contrasting in AM colonization. Significant variation in root functional traits was observed in response to low P stress among the 13 tested soybean genotypes contrasting in AM colonization. Genotypes with low AM colonization exhibited greater root proliferation but with less advantageous root physiological characteristics for P acquisition. In contrast, genotypes with high AM colonization exhibited less root growth but higher phosphatase activities and carboxylate content in the rhizosheath. Root dry weights, and contents of carbon and P were positively correlated with root morphological traits of different root orders and whole root systems, and were negatively correlated with AM colonization of fine roots and whole root systems, as well as, rhizosheath phosphatase activities and carboxylate contents. These results taken in combination with significant positive correlation between plant P content and root morphological traits indicate that root morphological traits play a primary role in soybean P acquisition. The results suggest that efficient P acquisition involves tradeoffs among carbon allocation to root proliferation, mycorrhizal symbiosis, or P-mobilizing exudation. Complementarity and complexity in the selection of P acquisition strategies was notable among soybean genotypes contrasting in AM colonization, which is closely related to plant C budgeting.

Contrasted root trait responses between saplings of an arbuscular and an ectomycorrhizal tree species in open field compared to forest conditions

Abstract Plant nutrient acquisition strategies range along a spectrum from autonomous foraging to investment in cooperative foraging through mycorrhizal associations. However, in temperate ecosystems, many plant species encounter contrasted levels of symbiont availability in open fields versus closed forests. Little is known about how fungal partner availability may be associated with intraspecific variation in other root foraging traits in natural settings. Here, we addressed this issue by sampling saplings from two tree species: the arbuscular mycorrhizal (AM) Acer rubrum and the ectomycorrhizal (ECM) Quercus rubra from open fields (AM dominated) and adjacent forest plots (ECM dominated). For each species and environment, we measured morphological, architectural and symbiotic root traits. For the open field, Quercus had greater specific root length (SRL) while Acer had higher AM colonization and root diameter. In the closed forest, the opposite pattern was observed, namely Quercus had higher ECM colonization and Acer greater SRL. Both species showed evidence of a shift towards autonomous root foraging in the habitat with low expected symbiont abundance (open field for Quercus and forest for Acer). Although the confounding effects of site abiotic properties could not be strictly controlled in this study, these results suggest that plants might adjust root foraging traits according to local habitat conditions. Synthesis: Our results shed new light on the intraspecific variation in plant position along the so‐called ‘collaboration gradient’, and suggest that mycorrhizal symbiont availability, along with other factors such as competition and site properties, may contribute to this variation.

Fine-resolution global maps of root biomass carbon colonized by arbuscular and ectomycorrhizal fungi

Despite the recognized importance of mycorrhizal associations in ecosystem functioning, the actual abundance patterns of mycorrhizal fungi belowground are still unknown. This information is key for better quantification of mycorrhizal impacts on ecosystem processes and for incorporating mycorrhizal pathways into global biogeochemical models. Here we present the first high-resolution maps of fine root stocks colonized by arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) fungi (MgC ha−1). The maps were assembled by combining multiple open-source databases holding information on root biomass carbon, the proportion of AM and EcM tree biomass, plot-level relative abundance of plant species and intensity of AM and EcM root colonization. We calculated root-associated AM and EcM abundance in 881 spatial units, defined as the combination of ecoregions and land cover types across six continents. The highest AM abundances are observed in the (sub-)tropics, while the highest EcM abundances occur in the taiga regions. These maps serve as a basis for future research where continuous spatial estimates of root mycorrhizal stocks are needed.

Mycorrhizal Fungi Associated With Juniper and Oak Seedlings Along a Disturbance Gradient in Central Mexico

Competition for resources between arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) plants can alter belowground mycorrhizal communities, but few studies have investigated host effects on both AM and ECM communities. In Central Mexico, the AM plant Juniperus deppeana is frequently used for reforesting areas affected by soil erosion, while the surrounding native forests are dominated by ECM oak trees. Oaks are capable of associating with both AM and ECM fungi during part of their life cycle (a feature known as dual mycorrhization) but it is unclear whether junipers possess such ability. To assess how juniper planting may affect belowground fungal interactions with oaks, we investigated mycorrhizal associations in J. deppeana and Quercus rugosa seedlings along a disturbance gradient: a native oak forest, a mixed Juniperus-Quercus population in secondary vegetation and a juniper site severely degraded by mining extraction. We measured root colonization and identified fungal communities using soil and root meta-barcoding of the ITS2 rDNA region. ECM fungal community composition was strongly affected by disturbance (regardless of host), while the community composition of AM fungi was mostly host-dependent, with a higher AM fungal richness in J. deppeana. Importantly, the fungal communities associated with Q. rugosa seedlings significantly changed in the vicinity of juniper trees, while those of J. deppeana seedlings were not affected by the presence of oak trees. Even though ECM fungal richness was higher in Q. rugosa and in the native forest, we detected a variety of ECM fungi associated exclusively with J. deppeana seedlings, suggesting that this plant species may be colonized by ECM fungi. Our results indicate that J. deppeana can alter ECM native fungal communities, with implications for its use in reforestation of mixed oak forests.

Environmental Conditions and Grazing Exerted Effects on Arbuscular Mycorrhizal in Plants at Southern Patagonia Rangelands

Arbuscular mycorrhizal (AM) fungi play a major role in maintaining ecosystem functions. AM fungi are found in most ecosystems including rangelands currently under increasing pressures from human activities. Southern Patagonia (Argentina) is a semiarid region influenced by extensive livestock production in rangelands. There is a lack of information about the environmental conditions and functionality of AM fungi in plant species of Patagonia ecosystems associated with livestock grazing. We assessed how soil properties, climatic conditions, and grazing intensities influence the response of AM fungi colonization. We studied most palatable and representative plant species (Poa dusenii, Rytidosperma virescens, Festuca gracillima, Nardophylum bryoides, Mulguraea tridens, and Carex argentina) growing in main ecological areas of Southern Patagonia. Most of the studied plant species (except C. argentina) presented AM symbiosis. AM colonization showed a negative relationship with soil organic carbon and nitrogen and a positive relationship with soil bulk density and pH. Results suggest that plants promoted a higher root AM colonization when soil nutrients and water availability (rainfall) are limiting. Sheep stocking rates had a differential impact depending on the ecological areas and plant life forms. High grazing decreased the AM colonization in the ecological areas with palatable plant dominance, suggesting that the impacts of grazing could lead to further negative effects on the ecosystem. Moderate grazing allows to maintain higher AM colonization, which would probably benefit the aboveground production of palatable plant and, consequently, to herbivores, particularly in degraded rangelands like Patagonian steppes. This study improves the knowledge of AM association in Patagonian semiarid rangelands by increasing our understanding of the impacts of grazing on belowground ecology. This information becomes relevant for grazing sustainable management, which may contribute to food security.

Enhancement of sorghum grain yield and nutrition: A role for arbuscular mycorrhizal fungi regardless of soil phosphorus availability

Societal Impact StatementSorghum is an important cereal crop that provides calories and nutrients for much of the world's population, and it is often grown with low fertiliser input. Optimising the yield, nutritive content and bioavailability of sorghum grain with minimal input is of importance for human nutrition, and arbuscular mycorrhizal (AM) fungi have previously shown potential to assist in this. Across sorghum genetic diversity, AM fungi improved the yield, nutrition and zinc and iron bioavailability of grain in a low phosphorus soil. Thus, food production systems that effectively manage AM fungi may improve consumer outcomes.Summary Sorghum is a C4 cereal crop that is an important source of calories and nutrition across the world, predominantly cultivated and consumed in low‐ and middle‐income countries. Sorghum can be highly colonised by arbuscular mycorrhizal (AM) fungi, and the plant‐fungal association can lead to improvements in biomass and nutrient uptake. High‐throughput phenotyping allows us to non‐destructively interrogate the ‘hidden’ effects of AM fungi on sorghum growth and phenology. Eight genetically diverse sorghum genotypes were grown in a soil amended with 2 or 20 mg P kg−1 and inoculated with an AM fungal culture of Rhizophagus irregularis. High‐throughput phenotyping uncovered the ‘hidden’ effects of AM fungi on growth and phenology, while grain biomass, nutrition, Zn and Fe bioavailability and root AM colonisation was determined after destructive harvest. Sorghum plants colonised by AM fungi generally performed better than non‐AM control plants, with greater yield, harvest indices, and grain P, Zn and Fe contents. During the early growth stages, AM colonisation led to temporary growth depressions. There were also AM fungal and P fertilisation effects on sorghum time‐of‐flowering. The sorghum genotype with the highest AM colonisation could barely produce grain when non‐inoculated. The two genotypes that failed to mature had very low AM colonisation. Generally, the genetically diverse sorghum genotypes were highly responsive to AM colonisation and produced more grain of greater nutritive quality when colonised, without adverse consequences for micronutrient bioavailability.

Fertilization integrated with microbial inoculants improves bell pepper production

ABSTRACT Increased use of fertilizers under intensive agriculture has the possibility of environmental contamination. The use of microbial inoculants may be effective in sustaining crop productivity, soil health, and supplementing chemical fertilizers. The investigation evaluated responses of arbuscular mycorrhizal (AM) fungi, Glomus mosseae (G) and Acaulospora laevis (A) and the bacterium Pseudomonas fluorescens (Pf) on bell pepper (Capsicum annuum L.) when provided superphosphate at F1 = 60 kg ha−1 (0.45 g/pot), F2 = 120 kg ha−1 (0.90 g/pot), or F3 = 240 kg ha−1 (1.80 g/pot). Plant growth, nutrient content, and yield 120 days after transplanting (DAT) were determined. Plants responded best to F1 followed by F2, while F3 produced inhibitory effects. The highest AM colonization and spore numbers were with G + A+ Pf, resulting in an increase in chlorophyll content, root phosphatase activity, shoot fresh weight, early fruit formation, number of fruit/plant, fruit size, weight, and early ripening. Inoculation with G. mosseae alone increased leaf area, photosynthesis, plant height, root freshness and dry weight. The G+ Pf commonly produced highest shoot fresh weight and fruit nutrient content. The effectiveness of these inoculants on bell pepper growth and yield was highest; 60 and 120 kg ha−1 P-fertilizer level when amended with G + A+ Pf. Use of AM or micro-inoculants may reduce fertilizer input and enhance nutrient utilization efficiency, which positively affected qualitative characteristics, nutrient uptake, and resistance to suboptimal conditions, but has few or little effect on quantitative characteristics. More work is needed to determine optimum rates of the materials tested to produce marketable size fruit.

Development of inoculum production of arbuscular mycorrhizal fungi for using as biofertilizer in Thai Jasmine rice (Oryza sativa) organic cultivation

Thai Jasmine rice, a high-quality variety of rice, can further be enhanced for the economic values through organic cultivation using arbuscular mycorrhizae fungi. Therefore, a study was conducted during 2017 to 2019 at Chiang Mai University and Tin Nee Yom Farm in Chiang Mai province, Thailand. This study investigated the optimum substrate formulas and the host plants for inoculum production of arbuscular mycorrhizal (AM) fungi, and to evaluate the efficiency of the inoculum of AM fungi for the growth and yield of Thai Jasmine rice in paddy fields of organic farming. The substrates used for in oculum production were soil, sand, leaf compost and vermiculite. AM fungi were cultured in pots of eight substrate formulas and used Brachiaria ruziziensis and Crotalaria juncea as the host plants. The results showed that the best formula which had the highest AM spore density was the formula containing compost mixed with vermiculite (1:2 v/v) and used B. ruziziensis as the host plant. For using C. juncea as the host plant, the optimum formula for spore production of AM fungi the formula containing soil mixed with vermiculite at the ratio of 2:1 (v/v). The dominant species of AM fungi associated with B. ruziziensis in the optimum formula were found to be four species of Aaulospora scrobiculata, A. tuberculata, Funnelifomis coronatum and Rhizophagus intreradices. Whereas C. juncea was used as the host plant in the optimum formula, there were also found to be three species of the AM fungi except R. intreradices. Using of AM fungi from the highest inoculum product as a biofertilizer in the organic farming in Chiang Dao district, Chiang Mai province in northern Thailand, increased growth of Thai Jasmine rice by about 12% in the paddy field without compost application and about 13% with compost application to the rice plants. N, P and K contents in the rice grains were about 1.3, 1.5 and 6.9 times higher than those of the uninoculated treatment. Therefore, the AM inoculum product of this study can be used as an efficient biofertilizer for increasing grain yields of Thai Jasmine rice in organic paddy fields. Both of optimum formulas can be used for commercial inoculum production of AM fungi for using as biofertilizer in organic farming.

English

Growth response of two clones of Manihot esculenta Crantz (Cassava), TMS 30555 and TMS 30572 to Bentex T soil treatment was studied. Mycorrhizal root colonization in relation to growth parameters such as stomata size, plant water content, plant foliation, as well as height and stem circumference was examined. Bentex T, a fungicide which could be used to limit the growth of Arbuscular Mycorrhizal (AM) fungi was added to soil at the concentrations of zero 0 (control), 50, 100, 500 and 1000 µg a.i. /g soil. Growth parameters had minimal variations (pEƒ 0.05) between treatments in both clones of the plant. However, clonal differences at (pE‚ 0.01) occurred in some of the growth parameters. The level of root colonization by the AM fungi affected the growth response of the plant. The untreated soil (control) with the highest AM fungi root colonization (84%) had the least plant foliation (15 and 16) and height (34.1 and 28.5 cm) for TMS 30572 and TMS 30555, respectively. The highest values obtained for stomata size (width and length) were at 50 µg/g bentex concentration; 0.040 and 0.019mm for TMS 30572 and 0.017 and 0.007 for TMS 30555, respectively. The least value obtained for the stomata size was at the zero (0) µg/g bentex concentration. Plants from soil treated with 100 µg/g bentex T concentration had the highest amount of water; 75% for TMS 30572 and 76% for TMS 30555. The untreated soil had plants with the least amount of water. Implications of Bentex T soil treatment of cassava plants was discussed in relation to mycorrhizal colonization rating and some growth parameters of the test plant. Key words: AM fungal colonization rating, growth response, cassava, Bentex T Soil Treatment

Plant functional groups associate with distinct arbuscular mycorrhizal fungal communities.

The benefits of the arbuscular mycorrhizal (AM) symbiosis between plants and fungi are modulated by the functional characteristics of both partners. However, it is unknown to what extent functionally distinct groups of plants naturally associate with different AM fungi. We reanalysed 14 high-throughput sequencing data sets describing AM fungal communities associating with plant individuals (2427) belonging to 297 species. We examined how root-associating AM fungal communities varied between plants with different growth forms, photosynthetic pathways, CSR (competitor, stress-tolerator, ruderal) strategies, mycorrhizal statuses and N-fixing statuses. AM fungal community composition differed in relation to all studied plant functional groups. Grasses, C4 and nonruderal plants were characterised by high AM fungal alpha diversity, while C4 , ruderal and obligately mycorrhizal plants were characterised by high beta diversity. The phylogenetic diversity of AM fungi, a potential surrogate for functional diversity, was higher among forbs than other plant growth forms. Putatively ruderal (previously cultured) AM fungi were disproportionately associated with forbs and ruderal plants. There was phylogenetic correlation among AM fungi in the degree of association with different plant growth forms and photosynthetic pathways. Associated AM fungal communities constitute an important component of plant ecological strategies. Functionally different plants associate with distinct AM fungal communities, linking mycorrhizal associations with functional diversity in ecosystems.

Composition of the arbuscular mycorrhizal fungal community and changes in diversity of the rhizosphere of Clematis fruticosa over three seasons across different elevations

AbstractClematis fruticosa Turcz. is a dominant native shrub with the potential for spontaneous vegetation succession for restoration of the ecologically fragile and arid Daqing Mountains of Inner Mongolia, which represent an important ecological transition zone in northern China. However, knowledge regarding the interactions between native species and their root‐associated arbuscular mycorrhizal (AM) fungi across different seasons and elevation gradients remains obscure. High AM fungal spore density, root colonization and arbuscular richness suggested that C. fruticosa could establish intimate symbiotic associations with AM fungi. A total of 174 AM fungal virtual taxa were identified from rhizosphere soil samples, and Glomus was dominant. The most distinct AM fungal community composition and the greatest richness and diversity were observed in the spring and at the highest elevation. Seasons • elevation interactions significantly affected Simpson diversity but not AM fungal community composition, observed richness and Shannon index. Seasons had a significant direct on AM fungal observed richness and Shannon index, and elevation had a significant direct on AM fungal observed richness. Soil available phosphorus was the most important driving factor in AM fungal richness and diversity, followed by seasons and soil moisture content. These results provied new insights suggesting that there are active and diverse “functional ingredients”, for example root‐associated AM fungi during the growing season, particularly with an earlier onset in the spring. They are potentially major contributors to the re‐vegetation of degraded lands and the maintenance of soil structure in arid and semiarid ecosystems, which is attributable to seasonal variation in host physiology in stressful environments.Highlights We studied the effects of seasons and elevation on AM fungal richness, diversity and community in ecologically fragile mountain areas. Distinct community composition and greater richness and diversity of AM fungi were detected in the spring and at the highest elevation. AM fungal richness and diversity depended on certain soil variation related to seasons and elevation.

AM fungal diversity and its impact across three types of mid-temperate steppe in Inner Mongolia, China.

Arbuscular mycorrhizal (AM) fungal diversity was measured in three different natural mid-temperate steppe types: the meadow steppe, typical steppe, and desert steppe. In these steppe soils, 24 AM fungal species from eight genera were identified, in which Glomus had the highest relative abundance. Funneliformis geosporus, Glomus microaggregatum, and Septoglomus constrictum had high relative abundance and were found widely across varying soil depth and steppe type. Meadow steppes had significantly higher AM fungal species richness compared to typical steppes and desert steppes, but there was no significant difference between typical steppes and desert steppes. AM fungal spore density, two Bradford-reactive soil protein (BRSP) fractions, and extraradical hyphal length densities (HLDs) were significantly different among the three steppe types. Alkaline phosphatase and acid phosphatase activity, urease activity, and soil bacterial and actinomycotic quantity were significantly related to the AM fungal spore density and species richness in these arid and semi-arid steppes. Therefore, steppe types could influence the distribution pattern of AM fungal diversity and the content of glomalin-related soil protein (GRSP).

Arbuscular mycorrhizal fungal diversity increases growth and phosphorus uptake in C3 and C4 crop plants

Most plants associate with arbuscular mycorrhizal (AM) fungi which can enhance their growth and nutrient uptake. Outcomes of the AM symbiosis can be highly variable, depending on soil fertility, plant functional group (C3, C4) and AM fungal diversity. This study assessed the growth and nutrient (C, N, P) responses of two C3 (Triticum aestivum and Hordeum vulgare) and two C4 (Sorghum bicolor and Zea mays) plants to different AM fungal inocula (no AM fungi, single AM fungal species, and four AM fungal species) under high and low P conditions. Higher AM fungal diversity resulted in greater P concentration and aboveground biomass of H. vulgare and S. bicolor. Triticum aestivum did not respond to AM fungi, while Z. mays responded positively but a similar positive response of Z. mays growth and nutrition occured when it was colonised with single or multiple AM fungal species. These findings suggest that, although C3 crop plants are less responsive to AM fungi than C4, some C3 and C4 species can benefit from higher AM fungal diversity in the soil.

Arbuscular Mycorrhizal and Dark Septate Endophyte Fungal Association in Cassava (Manihot esculenta Crantz) Varieties, Southern India

The study examined the root characteristics, type of arbuscular mycorrhizal (AM) and dark septate endophyte (DSE) fungal associations and their extent of colonization in 13 widely cultivated cassava (Manihot esculenta Crantz) varieties in southern India. Root and associated soil samples of each cassava variety were collected from Salem and Kolli Hills of the Eastern Ghats in south India. In addition, we also investigated the relationship between the extent of colonization of AM, DSE fungal and AM fungal spore communities and soil characteristics for each cassava variety. Roots of all the cassava varieties were colonized by AM fungi and 12 varieties had co-colonization of DSE fungi. The AM morphology in all the cassava varieties was of an intermediate type with intercellular and intracellular linear hyphae and arbusculate coils. In addition, we found high AM colonization rates (73-89%) but only moderate rates by DSE (22-68%) fungi. We also found 17 AM fungal spore morphotypes and their abundance varied significantly among cassava varieties. In addition, different physicochemical properties of soils may possibly increase AMF diversity, while different varieties of cassava are commonly associated with differing levels of AM and DSE fungi colonization with soil factors potentially influencing the symbiosis.

Detection of arbuscular mycorrhizal fungi associated with pecan (Caryaillinoinensis) trees by molecular and morphological approaches.

Arbuscular mycorrhizal (AM) fungal community associated with pecan (Caryaillinoinensis) roots and rhizospheric soils was assessed by spore isolation and morphological characterisation and by pyrosequencing of AM molecular markers. The AM fungal community associated with pecan growing in the field, was always more diverse than that associated with pecan growing in containers. This was not observed when AM richness was studied, suggesting that soil disturbance by a reduction in host plant richness leads to a less equitable distribution of AM fungal species, in contrast to natural soils. The chosen primers (AMV4.5F/AMDGR) for pyrosequencing showed high AM fungal specificity. Based on 97% sequence similarity, 49 operational taxonomic units (MOTUs) were obtained and, amongst these, 41 MOTUs corresponded to the Glomeromycotaphylum. The number of obtained AM sequences ranged from 2164, associated with field samples, to 5572 obtained from pecan trap pot culture samples, defining 30 and 29 MOTUs, respectively. Richness estimated by conventional species identification was 6 and 9 AM fungal species in soil and pot samples, respectively. Claroideoglomuslamellosum, Funneliformismosseae and Entrophosporainfrequens were the only taxa detected using both techniques. Predominant sequences in the pecan rhizosphere samples, such as Rhizoglomusirregulare and other less abundant (Dominikiairanica, Dominikiaindica, Sclerocystissinuosa, Paraglomuslaccatum), were detected only by pyrosequencing. Detection of AM fungal species based on spore morphology, in combination with molecular approaches, provides a more comprehensive estimate of fungal community composition.

Transcriptomic analysis reveals the possible roles of sugar metabolism and export for positive mycorrhizal growth responses in soybean.

To elucidate molecular mechanisms controlling differential growth responses to root colonization by arbuscular mycorrhizal (AM) fungi varying in colonization and cooperative behavior, a pot experiment was carried out using two soybean genotypes and three AM inocula. The results showed that inoculation by cooperative Rhizophagus irregularis (Ri) or less cooperative Glomus aggregatum with high AM colonization (Ga-H) significantly promoted plant growth compared with inoculation by G. aggregatum with low AM colonization (Ga-L). A comparative RNA sequencing analysis of the root transcriptomes showed that fatty acid synthesis pathway was significantly enriched in all three AM inoculation roots. However, sugar metabolism and transport were significantly enriched only in Ri and Ga-H inoculation, which was consistent with positive growth responses in these two inoculation treatments. Accordingly, the expression levels of the key genes related to sugar metabolism and transport were also upregulated in Ri and Ga-H roots compared with Ga-L roots. Of them, two sugars will eventually be exported transporters (SWEET) transporter genes, GmSWEET6 (Glyma.04G198600) and GmSWEET15 (Glyma.06G166800), and one invertase (Glyma.17G227900) gene were exclusively induced only in Ri and Ga-H roots. Promoter analyses in transgenic soybean roots further demonstrated that GUS driven by the GmSWEET6 promoter was highly expressed in arbuscule-containing cortical cells. Additionally, Ri and Ga-H inoculation increased the contents of sucrose, glucose and fructose in both shoots and roots compared with those of Ga-L and non-mycorrhizal. These results imply that positive mycorrhizal growth responses in plants might mostly be due to the stimulation of photosynthate metabolism and transport by AM fungal inoculum with high colonization capabilities.

Mycorrhizal fungal community structure in tropical humid soils under fallow and cropping conditions

Little is known to what extent soil biota, in particular, the mycorrhizae are altered through different fallow durations/types in tropical soils. We found that soil-N, -C, -Al, -K and -Ca contents significantly differed due to the fallow durations/types. Subsequently, the effects of fallow types and soil depths on the diversity, species richness and community structure of arbuscular mycorrhizal (AM) fungi were examined. A higher AM species richness was identified in the cropping than in forest fallow fields, suggesting a positive cropping feedback on the AM community composition. Distribution of the AM species was positively related to soil properties, specifically soil-pH, and soil-Pi, -Ca and -Mg contents. The soil properties conjointly accounted for 78.5% of explained variation in the AM community composition, signifying that the main factors altering the community structure under different fallow and cropping systems were the soil properties. Among the soil chemical characteristics, the soil-pH disclosed a significant explained variation in the AM community composition in the topsoil layer under the short fallow. Structural modeling equation to understand multiple predictive pathways that connect soil properties, fallow practices and AM community structures indicated that soil-C, -N and -Ca contents were highlighted as important factors influencing the AM community compositions.

Land-use change impact on mycorrhizal symbiosis in female and male plants of wild Carica papaya (Caricaceae)

It has been acknowledged that land-use change has negative effects on genetic diversity and sex ratio in dioecious species, but less attention has been paid on the influence that land-use change has on the biotic interactions, especially between dioecious species and arbuscular mycorrhizal (AM) fungi. AM mutualism involves reciprocal transfer of carbohydrates and mineral nutrients between the host plant’s roots and these fungi. Here, we report spatial and temporal variation in AM colonization in dioecious wild Carica papaya plants growing in sites with different land use intensity. We tagged, recorded the basal stem circumference and collected roots of reproductive female and male Carica papaya plants in three wild sites during dry and rainy seasons of western Mexico. We also collected soil samples in each site to conduct soil chemical analyses. The sexes of C. papaya did not show significant differences in the frequency (percentage of root colonized by intraradical fungal structures) and abundance (length of intraradical hyphae) of AM fungi but the higher AM colonization was observed during the dry season, and in the site with the lowest disturbance. There was no relationship between soil chemistry and AM colonization. Overall, our findings suggest that land-use intensity has a negative effect on AM colonization and we discuss the consequences of habitat loss for the reproductive female and male plants, the implications of decreasing AM colonization for wild Carica papaya plants an important species that provides a source of genetic variation for the C. papaya varieties.

Arbuscular mycorrhizal fungi promote silicon accumulation in plant roots, reducing the impacts of root herbivory

Studies have shown that arbuscular mycorrhizal (AM) fungi can reduce the performance of typically detrimental root feeding insects, yet the mechanisms remain unclear. This study aimed to investigate the effects of different sources of AM inocula on plant resistance to a root feeding insect in two different soils with different silicon (Si) concentrations. Sugarcane (Saccharum spp. hybrid) was grown in high or low Si soil; plants were treated with either an inoculum comprising the native AM fungi, a commercial AM fungal inoculum or with no AM fungi. Root herbivore (Dermolepida albohirtum) performance was measured in a feeding assay. In the low Si soil AM fungi increased root Si concentrations and reduced root herbivore performance. Both commercial and native AM treatments increased root Si and also reduced root herbivore growth rates by 107% and 81%, respectively. AM colonisation positively correlated with root Si concentrations. Distinct from this, in the high Si soil AM fungi had no impact on root Si or root herbivore growth. However, root consumption was reduced; a response independent of Si concentrations. Our study suggests AM fungi can enhance Si based plant defences against root herbivores, but also highlights that interactions between AM fungi and root herbivores involves multiple mechanisms requiring further research.