Fungal Root Colonization Research Articles

Aboveground Competition and Herbivory Overpower Plant-Soil Feedback Contributions to Succession in a Remediated Grassland

Plant-soil feedback (PSF) can provide a driving force during ecological succession by altering soil properties in ways that benefit or disadvantage other species in the successional sequence. Succession may be inevitable in disturbed sites remediated by planting early successional species, but information on PSF in such settings is lacking. We investigated whether gray birch (Betula populifolia), a native pioneer tree, alters succession from grassland to deciduous forest at a site contaminated with zinc, lead, and cadmium. We investigated PSF within the context of competition, herbivory, and soil contaminants, and evaluated whether gray birch, as a metal accumulator, engages in elemental allelopathy, poisoning other species through its metal-contaminated leaf litter. We assessed effects of gray birch on neighboring plant community structure, soil chemistry, fungal root symbionts, and germination, growth, and herbivory of seedlings of black oak (Quercus velutina) and sugar maple (Acer saccharum), two tree species expected to follow gray birch in succession. Gray birch was associated with increased diversity in its neighborhood grassland community, increased herbivory on black oak seedlings, and influenced root fungal colonization in both species. Seedling biomass was correlated with colonization by ectomycorrhizal fungi in black oak, but not with arbuscular mycorrhizal or dark septate fungal colonization in sugar maple. Gray birch had no effect on maple seedling performance or soil chemistry, and a small effect on oak performance in the absence of aboveground competition. We found little evidence consistent with elemental allelopathy. Oak and maple seedlings responded more strongly to variation in soil nutrients than heavy metals, and maintained leaf metal concentration profiles markedly different from those in soils. We conclude that gray birch alters its environment in ways that could promote establishment and succession of woody species, potentially favoring ectomycorrhizal species, but most effects of PSF are overpowered by aboveground competition, herbivory, and/or existing soil factors. This study well illustrates why potential PSF effects on plant performance and community structure must be examined within the context of other ecological processes. Such a broad understanding can inform decisions made in remediation and management of disturbed sites, and our understanding of plant succession and coexistence in general.

Fungal-Induced Formation of Auxin Maxima in Arabidopsis thaliana Roots

Phytohormones are crucial molecules for plant development and the interaction with microbes. This study focused on the phytohormone auxin (indole-3-acetic acid, IAA) and its role in the interaction of Arabidopsis thaliana (L.) Heynh. roots with two beneficial (Piriformospora indica and Mortierella hyalina) and two pathogenic (Alternaria brassicicola and Verticillium dahliae) root-colonizing fungi. Arabidopsis plants expressing the dual reporter construct DR5::EGFP-DR5v2::tdTomato allow visualization of auxin maxima during early stages of the plant–fungus interaction. Fluorescence microscopy was used to monitor changes in auxin levels and distribution patterns. We hereby demonstrate that only the beneficial P. indica activates the IAA reporter system. M. hyalina- but not P. indica-colonized roots accumulate jasmonates which might prevent the activation of the IAA reporter system. Additionally, both the necrotrophic fungus A. brassicicola and the biotrophic fungus V. dahliae completely inhibit the fluorescence emission from the IAA reporter system within 3–6 h. The results indicate that the reporter system responds to IAA accumulation in symbiotic roots, but the activation process might be controlled by a crosstalk with other phytohormones, such as jasmonates.

Contrasting common measures of arbuscular mycorrhizal fungal root colonization

Estimating the abundance of arbuscular mycorrhizal fungi relies entirely on indirect methods, meaning all measures are associated with some variability. The most common methods use microscopic estimates of the relative proportion of root length colonized by fungal structures. These methods typically examine root subsamples. While such methods are inexpensive and relatively simple, significant variation within single root system means there is opportunity for sampling bias. We evaluated the two most common methods of percent root length colonization for AM fungi both as a subsample and for the entire root system of flax plants. We compared these measures to a novel technique that returns projected fungal surface area (fungal coverage), by using microphotography and imaging analysis. Both microscopic methods overestimated the colonization intensity compared to image analysis. Among the microscopic methods, the method which incorporated colonization intensity (Trouvelot) was significantly more similar to imaging method results, than the method that is based on the presence/absence of the fungus (McGonigle).

Tall Fescue and E. coenophiala Genetics Influence Root-Associated Soil Fungi in a Temperate Grassland.

A constitutive, host-specific symbiosis exists between the aboveground fungal endophyte Epichloë coenophiala (Morgan-Jones & W. Gams) and the cool-season grass tall fescue (Lolium arundinaceum (Schreb.) Darbysh.), which is a common forage grass in the United States, Australia, New Zealand, and temperate European grasslands. New cultivars of tall fescue are continually developed to improve pasture productivity and animal health by manipulating both grass and E. coenophiala genetics, yet how these selected grass-endophyte combinations impact other microbial symbionts such as mycorrhizal and dark septate fungi remains unclear. Without better characterizing how genetically distinct grass-endophyte combinations interact with belowground microorganisms, we cannot determine how adoption of new E. coenophiala-symbiotic cultivars in pasture systems will influence long-term soil characteristics and ecosystem function. Here, we examined how E. coenophiala presence and host × endophyte genetic combinations control root colonization by belowground symbiotic fungi and associated plant nutrient concentrations and soil properties in a 2-year manipulative field experiment. We used four vegetative clone pairs of tall fescue that consisted of one endophyte-free (E−) and one E. coenophiala-symbiotic (E+) clone each, where E+ clones within each pair contained one of four endophyte genotypes: CTE14, CTE45, NTE16, or NTE19. After 2 years of growth in field plots, we measured root colonization of arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE), extraradical AMF hyphae in soil, total C, N, and P in root and shoot samples, as well as C and N in associated soils. Although we observed no effects of E. coenophiala presence or symbiotic genotype on total AMF or DSE colonization rates in roots, different grass-endophyte combinations altered AMF arbuscule presence and extraradical hyphal length in soil. The CTE45 genotype hosted the fewest AMF arbuscules regardless of endophyte presence, and E+ clones within NTE19 supported significantly greater soil extraradical hyphae compared to E− clones. Because AMF are often associated with improved soil physical characteristics and C sequestration, our results suggest that development and use of unique grass-endophyte combinations may cause divergent effects on long-term ecosystem properties.

Differential interaction of the dark septate endophyte Cadophora sp. and fungal pathogens in vitro and in planta.

ABSTRACTDark septate endophytes (DSEs) present a group of widespread root-colonizing fungi. The role of these endophytes in ecosystems and their interactions with plant pathogens are not well understood. In the current study, we assessed the antagonistic potential of the model DSE Cadophora sp. against the tomato soilborne pathogens Rhizoctonia solani, Pythium aphanidermatum and Verticillium dahliae. To investigate their interactions, we conducted in vitro assays followed by a greenhouse experiments in which tomato plants were inoculated with different combinations of the DSE and pathogens. RNA accumulation of selected tomato pathogenesis-related genes and of Cadophora sp. genes with putative antifungal function was analyzed. Cadophora sp. inhibited the growth of the fungal pathogens in vitro and vice versa; a negative impact of the pathogens on the growth of the DSE was also detected. In roots, however, this mutual negative interaction could not be observed. Expression analyses of plant genes could not explain this differential effect, but among the Cadophora sp. genes analyzed, a gene coding for a chalcone synthase was downregulated in planta. The data indicate that plants can change the interaction between fungi and, therefore, in vitro detected antagonism does not necessarily reflect the situation inside the plant.

Soil microbes that may accompany climate warming increase alpine plant production.

Climate change is causing species with non-overlapping ranges to come in contact, and a key challenge is to predict the consequences of such species re-shuffling. Experiments on plants have focused largely on novel competitive interactions; other species interactions, such as plant-microbe symbioses, while less studied, may also influence plant responses to climate change. In this greenhouse study, we evaluated interactions between soil microbes and alpine-restricted plant species, simulating a warming scenario in which low-elevation microbes migrate upslope into the distribution of alpine plants. We examined three alpine grasses from the Rocky Mountains, CO, USA (Poa alpina, Festuca brachyphylla, and Elymus scribneri). We used soil inocula from within (resident) or below (novel) the plants' current elevation range and examined responses in plant biomass, plant traits, and fungal colonization of roots. Resident soil inocula from the species' home range decreased biomass to a greater extent than novel soil inocula. The depressed growth in resident soils suggested that these soils harbor more carbon-demanding microbes, as plant biomass generally declined with greater fungal colonization of roots, especially in resident soil inocula. Although plant traits did not respond to the provenance of soil inocula, specific leaf area declined and root:shoot ratio increased when soil inocula were sterilized, indicating microbial mediation of plant trait expression. Contrary to current predictions, our findings suggest that if upwardly migrating microbes were to displace current soil microbes, alpine plants may benefit from this warming-induced microbial re-shuffling.

NADPH oxidase regulates chemotropic growth of the fungal pathogen Fusarium oxysporum towards the host plant.

Soil-inhabiting fungal pathogens use chemical signals to locate and colonise the host plant. In the vascular wilt fungus Fusarium oxysporum, hyphal chemotropism towards tomato roots is triggered by secreted plant peroxidases (Prx), which catalyse the reductive cleavage of reactive oxygen species (ROS). Here we show that this chemotropic response requires the regulated synthesis of ROS by the conserved fungal NADPH oxidase B (NoxB) complex, and their transformation into hydrogen peroxide (H2 O2 ) by superoxide dismutase (SOD). Deletion of NoxB or the regulatory subunit NoxR, or pharmacological inhibition of SOD, specifically abolished chemotropism of F.oxysporum towards Prx gradients. Addition of isotropic concentrations of H2 O2 rescued chemotropic growth in the noxBΔ and noxRΔ mutants, but not in a mutant lacking the G protein-coupled receptor Ste2. Prx-triggered rapid Nox- and Ste2-dependent phosphorylation of the cell wall integrity mitogen-activated protein kinase (CWI MAPK) Mpk1, an essential component of the chemotropic response. These results suggest that Ste2 and the CWI MAPK cascade function downstream of NoxB in Prx chemosensing. Our findings reveal a new role for Nox enzymes in directed hyphal growth of a filamentous pathogen towards its host and might be of broad interest for chemotropic interactions between plants and root-colonising fungi.

Relationship between Arbuscular Mycorrhizal Association and Edaphic Variables in Mangroves of the Coast of Yucatán, Mexico

Arbuscular mycorrhizal fungi (AMF) are present in a great variety of ecosystems, including wetlands. However, little is known regarding the environmental factors that affect their abundance, diversity and development within the rhizosphere of their hosts in flooded soils. This study had the objective of describing the edaphic environment and the mycorrhizal status of two mangrove species: Avicennia germinans (L.) L. and Conocarpus erectus L., and to identify the edaphic variables that determine the percentage of mycorrhizal colonization, spore density and AMF species richness on their rhizospheres. To accomplish this, roots and rhizospheric soil of A. germinans and C. erectus were collected on the coastal lagoon “La Carbonera”, Yucatan, Mexico. Soil analyses were conducted, the percentage of root fungal colonization was determined and the spores isolated, identified and quantified. Structural equation models were elaborated for each mangrove species to determine the relationship between the edaphic environment and the arbuscular mycorrhizal association. The rhizospheres of the mangroves differ physico-chemically and, while mycorrhizal colonization occurs in both species and spores are present in their rhizospheres, the results indicate higher values in C. erectus compared to A. germinans. Substrate water content and pH are the edaphic variables that determine the AMF variation in the analyzed mangrove rhizospheres.

Crop yield, weed cover and ecosystem multifunctionality are not affected by the duration of organic management

Organic farming is gaining importance in view of its beneficial effects on soil quality, environmental performance and biodiversity. However, it is still unclear how organic management performs over time and whether the duration of organic management influences crop yield and ecosystem functioning. Here we compared 34 fields in Swiss farms assigned to four groups: 1) conventionally managed farms; 2) farms in transition to organic farming (in the 1st – 3rd year); 3) farms converted moderately long ago (9–13 years); and 4) farms subjected to long-term organic farming (15–32 years). We selected one field per farm and examined in two subsequent years whether management practices (conventional vs. organic farming) and the duration of organic management affected crop yield, weed cover, soil fertility and biodiversity as well as the overall system performance, assessed as ecosystem multifunctionality. Maize yield (-6.0%) and wheat yield (-22.2%) decreased in organic compared to conventional fields. However, the duration of organic management did not affect crop yield. There was also no effect of the duration of organic management on weed cover but it was much higher under organic management, with mean values of 33.0% in organic compared to 2.0% in conventional fields in maize, and 13.4% compared to 1.2% in wheat, respectively. Soil fertility and microbial activities were not significantly different between management practices, which might be due to the large variation among fields. Root colonization of arbuscular mycorrhizal fungi increased (+19.7%) under organic management in wheat. Overall, this study demonstrates a rapid shift of agro-ecological functions after conversion to organic farming and that the duration of organic management has no impact on crop yield, weed cover and soil fertility.

Implications of mycoremediated dry olive residue application and arbuscular mycorrhizal fungi inoculation on the microbial community composition and functionality in a metal-polluted soil

Metal-polluted soils represent hostile environments affecting the composition and functions of soil microbial communities. This study evaluated the implication of combining the mycoremediated dry olive residue (MDOR) amendment application with the inoculation of the arbuscular mycorrhizal fungi (AMF) Funneliformis mosseae in restoring the quality, composition, and functionality of soil microbial communities. To achieve this aim, a mesocosms experiment was set up that included three variations: i) with and without application of Penicillium chrysogenum-10-transformed MDOR (MDOR_Pc), and Chondrosterum purpureum-transformed MDOR (MDOR_Cp) amendments; ii) with and without F. mosseae inoculation; and iii) 30-day and 60-day soil treatment time. As a result of this combined treatment, changes in the soil labile organic C and N fractions were observed throughout the experiment. Increases in the abundance of phospholipid fatty acids (PLFAs) for bacteria, actinobacteria, and Gram− and Gram+ bacteria were also recorded at the end of the experiment. The addition of MDOR amendments boosted fungal and AM fungi communities. AM fungi root and soil colonization was also enhanced as the result of improvement nutrient turnover and spatial conditions caused by adding MDOR in combination with an inoculation of F. mosseae. The composition and functionality of microbial communities seemed to be an important ecological attribute indicating an apparently fully functional restoration of this metal-polluted soil and therefore suggesting the suitability of the combined MDOR and AM fungus treatment as a reclamation practice.

ASSOCIATION OF ARBUSCULAR MYCORRHIZAL FUNGAL SPECIES IN THE PLANT SPECIES OF BARGUR HILLS, ERODE DISTRICT, TAMIL NADU, INDIA

The present study was carried out the arbuscular mycorrhizal fungal root colonization and spore population diversity some medicinal plants species at Bargur hills Western Ghats of (Anthyur taluk), Erode district, Tamil Nadu, India. Root and rhizosphere soil samples were collected during the month of August,2017-March, 2018 from the surface to 20 cm depth as well as pH were also measured. Totally 25 plant species belonging to 19 families recovered Arbuscular mycorrhizal fungal spore and root colonization. The results of the present study arbuscular mycorrhizal spore population in the rhizosphere soil and rootcolonization of all the plant species. A total of 22 AM fungal species belonging to 7 genera and 2 different orders were recorded from the rhizosphere soil samples of this study region. The Glomus was dominant had seen in rhizosphere soil samples in all the medicinal plant species. The maximum spore population was found in the rhizosphere soil samples of Leucas aspera (470 /100 g soil) which belongs to the family Lamiaceae and lowest spore population was observed in the Tephorosia purpurea (123 /100g soil) belongs to Fabaceae. The highest 83 % AM fungal infection was found in roots of Achyranthus aspera belongs to the family Amaranthaceae, while the lowest 23 % AM fungal association was found in the root of Mimosa pudica belongs to the family Mimosaceae.

The reproductive phenology of Acaena elongata and its relation with arbuscular mycorrhizal fungi

Plant sexual reproduction is an energetic process since it requires large amounts of resource allocation for flower and fruit production. Therefore, the establishment of a mutualism with arbuscular mycorrhizal (AM) fungi can increase the plant’s reproductive success. Acaena elongata is a native weed and a plant species indicator of anthropogenic perturbations in the understory of Abies religiosa forests of the Magdalena river basin in Mexico City. The aim of this work was to assess whether: i) there are temporal and spatial variations in AM fungal root colonization in A. elongata, ii) AM fungal root colonization is linked to A. elongata flower and fruit production patterns, and iii) AM fungal root colonization, spore abundance and richness, and A. elongata reproductive phenology are related to soil variables. Eight plots were established with five randomly selected individuals of A. elongata. Over the course of 1 year, the A. elongata reproductive phenology, environmental temperature, and relative soil moisture were registered monthly. Additionally, root samplings for AM fungal colonization and soil for determining the AM fungal spore abundance and richness were carried out during the rainy and dry seasons. The mean total AM fungal root colonization percentage was higher in the dry season (78 ± S.E. 11.09%) than in the rainy season (71 ± S.E. 13.2%); A. elongata showed reproductive structures throughout the year, with negative significant correlations between colonization and some phenological phases during the rainy season. Flower and fruit production as well as AM fungal root colonization were related to variations in the abiotic variables in each season and population. This study contributes to the understanding of the relationship between AM fungi and a plant species indicator of anthropogenic perturbations.

Context‐dependent biotic interactions control plant abundance across altitudinal environmental gradients

Many biotic interactions influence community structure, yet most distribution models for plants have focused on plant competition or used only abiotic variables to predict plant abundance. Furthermore, biotic interactions are commonly context‐dependent across abiotic gradients. For example, plant–plant interactions can grade from competition to facilitation over temperature gradients. We used a hierarchical Bayesian framework to predict the abundances of 12 plant species across a mountain landscape and test hypotheses on the context‐dependency of biotic interactions over abiotic gradients. We combined field‐based estimates of six biotic interactions (foliar herbivory and pathogen damage, fungal root colonization, fossorial mammal disturbance, plant cover and plant diversity) with abiotic data on climate and soil depth, nutrients and moisture. All biotic interactions were significantly context‐dependent along temperature gradients. Results supported the stress gradient hypothesis: as abiotic stress increased, the strength or direction of the relationship between biotic variables and plant abundance generally switched from negative (suggesting suppressed plant abundance) to positive (suggesting facilitation/mutualism). For half of the species, plant cover was the best predictor of abundance, suggesting that the prior focus on plant–plant interactions is well‐justified. Explicitly incorporating the context‐dependency of biotic interactions generated novel hypotheses about drivers of plant abundance across abiotic gradients and may improve the accuracy of niche models.

Monitoring of fungal root colonisation, arbuscular mycorrhizal fungi diversity and soil microbial processes to assess the success of ecosystem translocation.

To compensate for an airport expansion, including construction on valuable wet meadows of a Natura 2000 system, 1.3 ha of turf, cut into blocks, were transferred to artificial basins in a habitat garden. To evaluate the impact of translocation on this ecosystem, and thus the success of its preservation, we monitored fungal root colonisation of Molinia caerulea, the diagnostic plant species for wet meadows, along with arbuscular mycorrhizal fungi (AMF) species richness and composition in soils and soil microbial processes for three seasons: prior to the transfer (2013) and the two following years (2014-15). We observed few changes in the fungal colonisation of M. caerulea, suggesting that the fungal associations of this species were unaffected. The number of AMF species declined directly after the translocation; however, in 2015, an increased number of species was recorded. There were no differences in AMF species composition, nor did soil basal respiration rate, substrate-induced respiration (SIR), or substrate-active biomass (Cmic) change over the years. Only metabolic coefficient (qCO2) decreased after the transfer. The small number of effects in fungal root colonisation, AMF diversity, and microbial processes following the translocation of the ecosystem prove its success. This can be attributed to the deep turf translocation that kept the relevant microbial communities almost unaffected.

Effect of drought and season on arbuscular mycorrhizal fungi in a subtropical secondary forest

Arbuscular mycorrhizal (AM) fungi in both soil and roots were examined in May (summer) and December (winter) under a 4-y drought experiment in a Chinese subtropical secondary forest. Drought significantly decreased AM fungal extra-radical hyphal density, spore density, and root colonization rate in both seasons. These AM parameters were significantly higher in summer than in winter in the control treatment, but only AM fungal extra-radical hyphal density exhibited the same seasonal trend in the drought treatment. In total, 45 AM fungal operational taxonomic units (OTUs) were obtained at a 97% sequence similarity level using Illumina sequencing of 18S rDNA. Drought and season had no significant effects on AM fungal OTU richness in soil and roots. AM fungal community composition in soil and roots was significantly affected by season but not by drought. This finding enhances our understanding of the response of AM fungi to global climate change in subtropical forest ecosystems. • AM fungal abundance was decreased by drought in both seasons. • AM fungal richness was not affected by drought or season. • AM fungal community composition was influenced by season but not by drought.

Sewage sludge biochar alters root colonization of mycorrhizal fungi in a soil cultivated with corn

Sewage sludge biochar (SSB) has demonstrated a large potential to improve soil quality, availability of nutrients, particularly phosphorus (P), and increase crop productivity. There is a lack of information on the effect of P from SSB on the soil microbiota, especially on arbuscular mycorrhizal fungi. Six treatments were arranged in randomized blocks, with four replicates: 1) Control - without any amendments; 2) Mineral fertilization with NPK; 3) Biochar prepared at 300 °C (BC300); 4) Biochar prepared at 500 °C (BC500); 5) BC300 + NPK; 6) BC500 + NPK. Amendments were applied in two consecutive harvests. Microbial biomass carbon (MBC), percentage of mycorrhizal colonization (MC) and easily extractable glomalin-related soil protein (EEG) of soil mycorrhizal fungi were evaluated in all treatments. Amendment with BC300 resulted in an increase in MBC, while all treatments resulted in an increase in the colonization of corn roots by AMF. Consequently, SSB stimulated the synthesis and excretion of EEG from the soil. Biochar, NPK mineral fertilizer and their combinations increased MC in corn compared to the control. However, no additive effects of SSB and NPK addition were observed.

Impact of Crude Oil on Functional Groups of Culturable Bacteria and Colonization of Symbiotic Microorganisms in the <i>Clitoria-Brachiaria</i> Rhizosphere Grown in Mesocosms

This research evaluated the changes on populations of culturable N-fixing free bacteria (NFFB) and P-solubilizing bacteria (PSB), as well as on the root nodulation by native rhizobia, the root colonization and spore number of arbuscular mycorrhizal fungi (AMF), in the rhizosphere of Clitoria ternatea and Brachiaria brizantha grown in mesocosms contaminated with crude oil (0, 3000, 6000, 9000, and 12000 mg kg-1), for 240 days. After 24 h of soil contamination, the highest populations of NFFB and PSB (5.5 and 4.9 LogUFC, respectively) were found in control, and the lowest populations were obtained at 12000 mg kg-1 (5.1 and 4.2 LogUFC, respectively). In contrast, at 60 and 240 days, the control showed lower populations of NFFB and PSB (5.4 and 4.8 LogUFC, respectively) than contaminated treatments. The highest number or root nodules in C. ternatea was quantified in control at 60 and 240 days (25 and 27 nodules, respectively) in comparison to those observed at the treatment with 12000 mg kg-1 (7 and 1 nodule, respectively). At 60 days, AMF colonization in both plant species, and the number of spores significantly decreased as the crude oil concentration increased; however, at 240 days, the highest number of AMF spores was recorded at treatments with 6000 and 12000 mg kg-1. The dry weight of both plant species significantly decreased as crude oil concentrations increased. Although C. ternatea was more susceptible to the toxic effects of crude oil, this plant species showed greater content of total chlorophyll than B. brizantha.

Effect of salinity on spore germination, hyphal length and root colonization of the Arbuscular Mycorrhizal Fungi

Various environmental factors influence the growth and development of Arbuscular Mycorrhizal Fungi (AMF), one of which is salinity. Salinity can affect several stages of AMF growth. Theresearch aims to study the influence of salinity towards germination, growth, and performance of AMF, namely Gi. margarita and G. etunicatum. The level of salinity given ranges from 0 - 10,000 ppm with an interval of 2,000 ppm. Spore germination experiments were carried out in petri dish culture while evaluation was carried out in open pot culture with sorghum as host. The results showed that increasing salinity decreased the initial day of germination, the percentage of germination and hyphal growth. The effect is greater on Gi. margaritathanG. etunicatum on all parameters measured. An increase of salinity up to 10,000 ppm decreased the percentage of Gi. margarita root colonization by 69.07% and G. etunicatumby 37.78%. Nevertheless, the effect of salinity observed in this study towards germination and growth of AMF wasmore categorized as delaying rather than inhibiting or stopping.

Identification and expression analysis of the arbuscular mycorrhiza-inducible Rieske non-heme oxygenase Ptc52 gene from tomato

Arbuscular mycorrhizal (AM) formation enhances plant growth and fitness through improved uptake of water and mineral nutrients in exchange for carbon compounds to the AM fungus. The fungal structure for the reciprocal exchange of nutrients in the symbiosis is the arbuscule, and defence genes expressed in cells containing arbuscules could play a role in the control of hyphal spread and arbuscule formation in the root. We characterized and analyzed the Ptc52 gene from tomato (SlPtc52), a member of the gene family of non-heme oxygenases, whose function has been related to the lethal leaf spot 1 (Lls1) lesion mimic phenotype in plants which is sometimes associated with enhanced disease resistance. Sequence analysis of the SlPTC52 protein revealed conserved typical motifs from non-heme oxygenases, including a Rieske [2Fe-2S] motif, a mononuclear non-heme iron-binding motif and a C-terminal CxxC motif. The level of transcript accumulation was low in stem, flower and green fruits, and high in leaves. Although SlPtc52 expression was perceptible at low levels in roots, its expression increased concomitantly with AM fungus root colonization. Tomato non-mycorrhizal hairy roots expressing the GUS protein under the control of promoter SlPtc52 exhibited GUS activity in the endodermis, the apical meristem of the root tip and in the lateral root primordium. AM fungal colonization also resulted in intensive GUS activity that clearly corresponds to cortical cells containing arbuscules. SlPtc52 gene silencing led to a delay in root colonization and a decrease in arbuscular abundance, suggesting that SlPTC52 plays a regulatory role during AM symbiosis.

Differences in arbuscular mycorrhizal colonization and P acquisition between genotypes of the tropical Brachiaria grasses: is there a relation with BNI activity?

In a field experiment in Palmira, Colombia, we studied mycorrhizal root colonization, phosphomonoesterase activities and P and N foliar content before and after N fertilization among different Brachiaria genotypes with demonstrated biological nitrification inhibition (BNI) capacity. Furthermore, we tested the potential nitrification rate (PNR) in soil in order to confirm the inhibition of nitrification of the selected genotypes and relate the BNI performance with P acquisition. We hypothesized that (i) genotypes will differ in key variables related to P acquisition, and that there will be a positive correlation between (ii) arbuscular mycorrhizal fungi (AMF) root colonization, P uptake, and BNI activity, and (iii) between the activity of acid and alkaline phosphomonoesterase and BNI performance. Higher N immobilization 1 week after application of synthetic fertilizer (ammonium sulfate) and low PNR of Brachiaria humidicola CIAT 679 and CIAT 16888 confirmed that these genotypes have high-BNI activity. Despite the relatively high soil P status, high affinity of Brachiaria grasses for AMF was observed at the study site: more than 60% of root length was colonized by AMF in high-BNI genotypes, versus 45% in low-BNI Brachiaria cv. Mulato. The N content of high-BNI genotypes was positively correlated with mycorrhizal root colonization suggesting the uptake of NH4+ by AMF and its transfer to high-BNI genotypes and/or regulation of AMF colonization by P demand. Furthermore, increased activity of acid phosphomonoesterase (6.98 and 7.68 μmol g−1 h−1 in high-BNI versus 5.20 μmol g−1 h−1 in low-BNI genotypes) and the depletion of the most labile available P fractions in the rhizosphere of high-BNI genotypes (by 21–32%) suggest enhanced P uptake and P-use efficiency. To the best of our knowledge, this is the first study that explored relations between BNI and biotic factors affecting P acquisition. Our results highlight the importance of AMF in Brachiaria grasses even under high P availability and warrant further studies including a larger number of different BNI genotypes that can elucidate biotic plant-soil interactions affecting nutrient-use efficiencies in improved pastures under low and high P status.