Mycorrhizal Response Research Articles

Exogenous Phytohormones Modulate Mycorrhiza-Induced Changes in Root Hair Configuration of Trifoliate Orange

Mycorrhizas alter root hair profile, but it is not clear whether exogenous phytohormones regulate the mycorrhizal effects on root hair. Studies were carried out in a two-chambered rootbox separated by 37-μm nylon mesh to establish root+hyphae chamber carrying trifoliate orange [Poncirus trifoliata (L.) Raf.] as the test plant inoculated with Diversispora versiformis and hyphae chamber (without roots). Indole butyric acid (IBA), abscisic acid (ABA), and jasmonic acid (JA) (each at 0.1 μM concentration) were weekly applied into hyphae chamber, in total of six times before plant harvest. Mycorrhization strongly stimulated plant growth performance, and exogenous phytohormones, especially IBA, further magnified the mycorrhizal-stimulated growth responses. Three exogenous phytohormones decreased mycorrhizal colonization in taproot and first-order lateral roots, but increased in second- and third-order lateral roots. These phytohormones also increased hyphal length in nylon mesh and soil, irrespective of root+hyphae or hyphae chamber. Mycorrhizal inoculation significantly increased root hair density of different root classes, and exogenous hormones further strengthened the mycorrhizal effect. Average root hair length was stimulated by mycorrhization, but all exogenous phytohormones weakened the mycorrhizal response. Mycorrhization in combination with exogenous phytohormones showed no response on root hair diameter. Our studies, hence, suggested that application of exogenous phytohormones in hyphae chamber strengthened the D. versiformis-induced changes in average root hair density but weakened in average root hair length in trifofliate orange grown in root+hyphae chamber.

Meta-Analysis of Crop and Weed Growth Responses to Arbuscular Mycorrhizal Fungi: Implications for Integrated Weed Management

Integrated weed management (IWM) relies upon multiple chemical, physical, or biological weed management techniques to achieve an acceptable level of weed control. Agents that selectively suppress weeds but not crops and that can be manipulated in agriculture will be promising components for inclusion in IWM. We used a meta-analytic approach to investigate the potential of arbuscular mycorrhizal fungi (AMF) to contribute to IWM. We quantified the effect of crop and weed host status (strong and weak AMF hosts are divided in this study by a 10% root length colonization threshold), AMF diversity (single vs. mixed), and soil N and P fertility management on plant mycorrhizal growth responses (MGRs). Our results indicated that weak host weeds had consistently lower MGRs than strong host crops in both controlled and field conditions. Moreover, these differences in MGRs between weak host weeds and strong host crops were more pronounced under mixed AMF inoculum and low N and P nutrient availability. In contrast, MGR of strong host weeds was not different from strong host crops in general. However, we observed a wide range of MGRs among strong host weeds, some of which had much lower MGRs than strong host crops. In addition, in the presence of N and P fertilizers, strong host crops had a stronger positive response to AMF than strong host weeds. Thus, our meta-analysis indicates that AMF have potential to contribute to weed control by direct and indirect pathways: directly suppress weak host weeds, and indirectly suppress some strong host weeds mediating by competitive effects exerted by strong host crops. We suggest that management practices affecting AMF diversity and crop and weed mycorrhizal responses could be chosen to improve the contribution of AMF to IWM. Better understanding is needed of crop–weed–AMF interactions and management practices that enhance this form of weed management.

Enhancement of growth in mangrove plant (Ceriops tagal) by Rhizophagus clarus

ABSTRACTThe present study was conducted to investigate the growth response of Ceriops tagal Perr.) C.B. Robinson a mangrove species belonging to the family Rhizophoraceae. Three dominant native arbuscular mycorrhizal (AM) fungal species viz., Rhizophagus clarus, R. intraradices, and Acaulospora laevis were selected. The results revealed that the Mycorrhizal Plant Responsiveness (MGR) was greater in AM inoculated plants compared to control plants. Among AM fungal treatments, plants inoculated with R. clarus recorded significant increase in growth responses compared to remaining AM treatments. The study suggests that R. clarus was the most efficient AM species exhibiting greater influence on growth and biomass. Differences in growth parameters between AM inoculated treatments and control are reasonably linked to colonization levels. The result of the present study indicates that potential native AM fungal species can be use in mangrove reforestation programs as bio-inoculum.

Trap Culture Technique for Propagation of Arbuscular Mycorrhizal Fungi using Different Host Plants

Arbuscular mycorrhizal fungi (AMF) spore propagation and long term maintenance is still a complicated technique for farmers. The use of AMF for their ability to promote plant growth and protect plants against pathogen attack and environmental stresses demands AMF propagation for large scale application. This study aimed to propagate AMF spores by trap culture technique and assess their ability to propagate with different host plants in a continuous plant cycle. Mycorrhizal inoculation by trap culture in maize resulted in longer shoots and roots than sudangrass plants. Increase in dry weight with higher percentage also was observed for maize plants. After first and second plant cycle, maize plants had the higher percentage of mycorrhizal response in terms of colonization and arbuscules than sudangrass. Maximum in spore count also achieved in the pots of maize plants. The results show that maize plant is more suitable host plant for AMF spore propagation and trap culture technique can be used effectively to maintain the AMF culture for long time.

Reduced mycorrhizal responsiveness leads to increased competitive tolerance in an invasive exotic plant

Summary Arbuscular mycorrhizal (AM) fungi can exert a powerful influence on the outcome of plant–plant competition. Since some exotic plants interact differently with soil biota such as AM fungi in their new range, range‐based shifts in AM responsiveness could shift competitive interactions between exotic and resident plants, although this remains poorly studied. We explored whether genotypes of the annual exotic Centaurea solstitialis (yellow starthistle), collected from populations across the native and non‐native ranges, differed in responsiveness to AM fungi in the introduced range and whether range‐based differences in mycorrhizal responsiveness affected how strongly C. solstitialis tolerated competition with the North American native bunchgrass, Stipa pulchra. Grown alone, C. solstitialis from both ranges derived only weak benefits from AM fungi. However, association with AM fungi was costly to plants when grown in competition with S. pulchra. The magnitude of the suppressive effect of AM fungi was greater for genotypes from native versus introduced populations. Synthesis. Many exotic invasive species are known to associate weakly with AM fungi, which may be beneficial in disturbed habitats where competition for resources is low. Our results indicate that reduced mycorrhizal associations may also benefit invaders in a competitive environment. Centaurea solstitialis were more strongly suppressed by established S. pulchra plants in the presence versus absence of AM fungi, but exotic genotypes were less suppressed than native genotypes. This suggests that AM fungi may contribute to invasion resistance in established native communities, but range‐based shifts in the way exotic genotypes respond to AM fungal partners may counter such biotic resistance.

Host Plant Physiology and Mycorrhizal Functioning Shift across a Glacial through Future [CO2] Gradient.

Rising atmospheric carbon dioxide concentration ([CO2]) may modulate the functioning of mycorrhizal associations by altering the relative degree of nutrient and carbohydrate limitations in plants. To test this, we grew Taraxacum ceratophorum and Taraxacum officinale (native and exotic dandelions) with and without mycorrhizal fungi across a broad [CO2] gradient (180-1,000 µL L-1). Differential plant growth rates and vegetative plasticity were hypothesized to drive species-specific responses to [CO2] and arbuscular mycorrhizal fungi. To evaluate [CO2] effects on mycorrhizal functioning, we calculated response ratios based on the relative biomass of mycorrhizal (MBio) and nonmycorrhizal (NMBio) plants (RBio = [MBio - NMBio]/NMBio). We then assessed linkages between RBio and host physiology, fungal growth, and biomass allocation using structural equation modeling. For T. officinale, RBio increased with rising [CO2], shifting from negative to positive values at 700 µL L-1 [CO2] and mycorrhizal effects on photosynthesis and leaf growth rates drove shifts in RBio in this species. For T. ceratophorum, RBio increased from 180 to 390 µL L-1 and further increases in [CO2] caused RBio to shift from positive to negative values. [CO2] and fungal effects on plant growth and carbon sink strength were correlated with shifts in RBio in this species. Overall, we show that rising [CO2] significantly altered the functioning of mycorrhizal associations. These symbioses became more beneficial with rising [CO2], but nonlinear effects may limit plant responses to mycorrhizal fungi under future [CO2]. The magnitude and mechanisms driving mycorrhizal-CO2 responses reflected species-specific differences in growth rate and vegetative plasticity, indicating that these traits may provide a framework for predicting mycorrhizal responses to global change.

Mycorrhizal Symbiotic Efficiency on C3 and C4 Plants under Salinity Stress - A Meta-Analysis.

A wide range of C3 and C4 plant species could acclimatize and grow under the impact of salinity stress. Symbiotic relationship between plant roots and arbuscular mycorrhizal fungi (AMF) are widespread and are well known to ameliorate the influence of salinity stress on agro-ecosystem. In the present study, we sought to understand the phenomenon of variability on AMF symbiotic relationship on saline stress amelioration in C3 and C4 plants. Thus, the objective was to compare varied mycorrhizal symbiotic relationship between C3 and C4 plants in saline conditions. To accomplish the above mentioned objective, we conducted a random effects models meta-analysis across 60 published studies. An effect size was calculated as the difference in mycorrhizal responses between the AMF inoculated plants and its corresponding control under saline conditions. Responses were compared between (i) identity of AMF species and AMF inoculation, (ii) identity of host plants (C3 vs. C4) and plant functional groups, (iii) soil texture and level of salinity and (iv) experimental condition (greenhouse vs. field). Results indicate that both C3 and C4 plants under saline condition responded positively to AMF inoculation, thereby overcoming the predicted effects of symbiotic efficiency. Although C3 and C4 plants showed positive effects under low (EC < 4 ds/m) and high (>8 ds/m) saline conditions, C3 plants showed significant effects for mycorrhizal inoculation over C4 plants. Among the plant types, C4 annual and perennial plants, C4 herbs and C4 dicot had a significant effect over other counterparts. Between single and mixed AMF inoculants, single inoculants Rhizophagus irregularis had a positive effect on C3 plants whereas Funneliformis mosseae had a positive effect on C4 plants than other species. In all of the observed studies, mycorrhizal inoculation showed positive effects on shoot, root and total biomass, and in nitrogen, phosphorous and potassium (K) uptake. However, it showed negative effects in sodium (Na) uptake in both C3 and C4 plants. This influence, owing to mycorrhizal inoculation, was significantly higher in K uptake in C4 plants. For our analysis, we concluded that AMF-inoculated C4 plants showed more competitive K+ ions uptake than C3 plants. Therefore, maintenance of high cytosolic K+/Na+ ratio is a key feature of plant salt tolerance. Studies on the detailed mechanism for the selective transport of K in C3 and C4 mycorrhizal plants under salt stress is lacking, and this needs to be explored.

Soil moisture legacy effects: Impacts on soil nutrients, plants and mycorrhizal responsiveness

Although most land-plants form associations with arbuscular mycorrhizal fungi (AMF) as a means of optimising nutrient capture, legacy effects of altered soil moisture regimes on plant responses to arbuscular mycorrhizas (AM) have not been studied. As rainfall patters change with climate change, soil moisture legacy effects, and their impact on plants, soil and microbes may become increasingly important. Results of an experiment are presented in which soil was subjected to a range of different soil moisture regimes prior to planting a mycorrhiza-defective tomato mutant and its mycorrhizal wild-type progenitor. There were clear legacy effects of the soil moisture regime prior to planting on soil physicochemical properties, plant growth and nutrition, the formation of AM and mycorrhizal responsiveness. For example, in the Dry treatment the plants were well colonized by AM, there was a clear benefit to the plants in terms of mycorrhizal growth responses and mycorrhizal P responses. In contrast, in the Intermediate treatment AM colonisation was lower, there was little benefit in terms of mycorrhizal responses. Finally, in the Wet and Wet/Dry treatments AM colonization levels were similar (albeit lower) to those in the Dry treatment, but mycorrhizal growth responses were lower and more variable. Together, these results clearly indicate that soil nutrients, plant growth and nutrition and mycorrhizal responsiveness are affected by soil moisture legacy effect. Consequently, as we move into a period where more variable and intense rainfall amounts and patterns have been projected, we need to consider soil moisture legacy effects.

Assessment of silver nanoparticles contamination on faba bean-Rhizobium leguminosarum bv. viciae-Glomus aggregatum symbiosis: Implications for induction of autophagy process in root nodule

The current study sought to explore the response of application of purified silver nanoparticles (Ag NPs) to soil grown with faba bean (Vicia faba L.) and inoculated either with Rhizobium leguminosarum bv. viciae ASU (KF670819) or Glomus aggregatum 14G or in combination. Ag NPs was synthesized and stabilized using PVP and characterized by UV–vis spectroscopy and the characteristic surface plasmon resonance band centered at 430nm. Also, the morphologies and structures of Ag NPs were characterized by X-ray diffraction and transmission electron microscopy and the size distribution ranged from 5 to 50nm. In the first experiment, the germination and seedling growth of faba bean plants were tested under different concentrations of Ag+ as AgNO3 and Ag NPs (100–900μgkg−1 soil). The germination declined by 40% when exposed to Ag NPs at concentration 800μgkg−1 soil, while the same level from Ag+ completely inhibited seed germination. In the second experiment the effect of a high concentration of Ag NPs (800μgkg−1 soil) on R. leguminosarum bv. viciae growth and G. aggregatum activity in soil and their symbiosis with faba bean was investigated. It was proved that Ag NPs considerably retarded the process of nodulation, nitrogenase activity, mycorrhizal colonization, mycorrhizal responsiveness and glomalin content. High concentration of Ag NPs (800μgkg−1 soil) resulted in detectable alterations including the intracellular deterioration of cytoplasmic components by means of autophagy and disintegration of bacteroids. These findings elucidate the mechanism of toxic action of Ag NPs which resulted in early senescence of root nodules.

Mycorrhizal response trades off with plant growth rate and increases with plant successional status.

Early-successional plant species invest in rapid growth and reproduction in contrast to slow growing late-successional species. We test the consistency of "trade-offs between plant life history and responsiveness on arbuscular mycorrhizal fungi. We selected four very early-, seven early-, 11 middle-, and eight late-successional plant species from six different families and functional groups and grew them with and without a mixed fungal inoculum and compared root architecture, mycorrhizal responsiveness, and plant growth rate. Our results indicate mycorrhizal responsiveness increases with plant successional stage and that this effect explains more variation in mycorrhizal response than is explained by phylogenetic relatedness. The mycorrhizal responsiveness of individual plant species was positively correlated with mycorrhizal root infection and negatively correlated with average plant mass and the number of root tips per unit mass, indicating that both plant growth rate and root architecture trade off with investment in mycorrhizal mutualisms. Because late-successional plants are very responsive to mycorrhizal fungi, our results suggest that fungal community dynamics may be an important driver of plant succession.

Arbuscular mycorrhizal fungal effects on plant competition and community structure

Summary Arbuscular mycorrhizal fungi (AMF) mediate plant interspecific competition and community structure. However, the magnitude and direction of AMF effects and underlying mechanisms are not clear. Here, we synthesized the results of 304 studies to evaluate how AMF affect plant competition and community structure and which abiotic and biotic conditions in experimental design modify these AMF effects. The magnitude and direction of AMF effects on plant competitive ability (in terms of competitive response) differed markedly among plant functional groups. When AMF inoculum was added, competitive ability was strongly enhanced in N‐fixing forbs and was significantly suppressed in C3 grasses, whereas no effect was observed in C4 grasses, non‐N‐fixing forbs and woody species. Furthermore, AMF inoculation increased competitive ability of perennial species when their competitors were annual species. AMF inoculation differentially influenced separate aspects of plant community structure and species composition. AMF inoculation significantly increased plant diversity but had no effects on plant productivity. Response of dominant plant species to AMF inoculation was the determining factor in explaining variations in how and to what degree plant diversity was influenced by AMF inoculation. When dominant species derived strong benefits from AMF, their dominance level was increased by AMF inoculation, which consequently decreased plant diversity. We did not find stronger AMF effects on plant diversity and productivity when greater numbers of AMF species were used in the inoculation. Synthesis. Despite large variations in AMF effects among studies, a unifying mechanism was observed that the mycorrhizal responsiveness (differences in plant growth between AMF and non‐AMF colonization treatments) of target and neighbouring plant species can determine AMF effects on the competitive outcome among plant species, which in turn influenced plant species diversity and community composition. Given that plant traits, soil nutrient conditions and probably mycorrhizal fungal traits are all factors determining the degree of mycorrhizal response of plant species, future studies should explicitly consider each of these factors in experimental design to better understand AMF effects on plant coexistence, plant community dynamics and ecosystem processes.

Evidence for functional divergence in AM fungal communities from different montane altitudes

Abstract A considerable amount of symbiotic functional variability has been demonstrated among arbuscular mycorrhizal fungal (AMF) species. However, little is known about the functional divergence in AMF communities from contrasting climates. Ecotypes of Pennisetum flaccidum were grown in growth chambers with AMF communities originating from two elevations (3 105 and 4 176 m asl) on Mount Segrila in Tibet. The combinations were grown under two temperature regimes (7–12 °C and 14–19 °C), equivalent to the mean temperatures during the growing season at the two elevations. AMF communities from different elevations showed different fungal performance at both temperatures. In most cases, AMF from cold habitats developed higher intraradical and extraradical colonization at 7–12 °C than AMF from the warmer habitat, and vice versa. For mycorrhizal growth responses (MGR), there was no consistent local versus foreign effect, but AMF inoculum from colder habitats consistently led to a higher MGR than the other inoculum. These results provide strong evidence for functional differences among AMF communities from contrasting climates.

The importance of arbuscular mycorrhiza for Cyclamen purpurascens subsp. immaculatum endemic in Slovakia.

At present, there is no relevant information on arbuscular mycorrhiza and the effect of the symbiosis on the growth of wild populations of cyclamens. To fill this gap, two populations of Cyclamen purpurascens subsp. immaculatum, endemic in Nízke Tatry (NT) mountains and Veľká Fatra (VF) mountains, Slovakia, were studied in situ as well as in a greenhouse pot experiment. For both populations, mycorrhizal root colonization of native plants was assessed, and mycorrhizal inoculation potential (MIP) of the soils at the two sites was determined in 3 consecutive years. In the greenhouse experiment, the growth response of cyclamens to cross-inoculation with arbuscular mycorrhizal fungi (AMF) was tested: plants from both sites were grown in their native soils and inoculated with a Septoglomus constrictum isolate originating either from the same or from the other plant locality. Although the MIP of soil at the NT site was significantly higher than at the VF site, the level of AMF root colonization of C. purpurascens subsp. immaculatum plants in the field did not significantly differ between the two localities. In the greenhouse experiment, inoculation with AMF generally accelerated cyclamen growth and significantly increased all growth parameters (shoot dry weight, leaf number and area, number of flowers, tuber, and root dry weight) and P uptake. The two populations of C. purpurascens subsp. immaculatum grown in their native soils, however, differed in their response to inoculation. The mycorrhizal growth response of NT plants was one-order higher compared to VF plants, and all their measured growth parameters were stimulated regardless of the fungal isolates' origin. In the VF plants, only the non-native (NT originating) isolate showed a significant positive effect on several growth traits. It can be concluded that mycorrhiza significantly increased fitness of C. purpurascens subsp. immaculatum, despite the differences between plant populations, implying that AMF symbionts should be taken into account in conservation programs of this endemic plant.

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

Effects of arbuscular mycorrhizal fungi on plant growth depend on root system: a meta-analysis

Our aim was to explore the way that root system type affects mycorrhizal growth response of plants. An extensive meta-analysis with 943 peer-review publications was conducted to test the difference in mycorrhizal responses between taproot plants and plants with a fibrous root system. We found that taproot plants showed greater growth response (biomass, P and N uptake) to colonization by arbuscular mycorrhizal fungi (AMF) than do plant species with fibrous root systems. This response pattern was dependent on stress types, AMF identity and species richness, and particularly the type of stress (abiotic vs. biotic). Taproot plants respond more to AMF than plants with a fibrous root system; but no difference was shown under biotic stress. The interaction effect seen for AMF and biotic stress was significantly higher for plants with fibrous root system, but was not significant between taproot plants and abiotic stress. Difference in biomass response was only found for Glomeraceae and Gigasporaceae between the two types of plants, while difference was found in P uptake response for Glomeraceae and Claroideoglomeraceae. However, plants with fibrous root system showed higher growth response than taproot plants under nematode stress. Taproot plants might be more dependent on mycorrhiza than plants with fibrous root system. This indicates that environmental conditions can modify the relative abundance of taproot plants and plants with fibrous root system through mycorrhizal functioning, which will regulate plant community dynamics and processes.

Screening of biomass production of cultivated forage grasses in response to mycorrhizal symbiosis under nutritional deficit conditions

AbstractArbuscular mycorrhizal fungi (AMF) colonize the root systems of most natural grassland species and usually increase plant growth by enhancing nutrients provision. This effect on growth responses of cultivated forage grasses is scarcely known, particularly under nutritional deficit conditions. We examined total biomass production, aboveground and belowground biomass and tillering of three temperate and three tropical cultivated forage grasses. Seedlings of each species were inoculated with a mixture of mycorrhizal fungi and later grown for 5 months under nutritional deficit conditions. The mycorrhizal symbiosis promoted aboveground and belowground biomass production in five out of six grass species. Grass species differed in their mycorrhizal responsiveness: tropical grasses (Panicum coloratum cv. Klein = Brachiaria brizantha cv. Marandú &gt; Paspalum dilatatum cv. Primo) responded better than temperate (Festuca arundinacea cv. Royal &gt; Agropyron elongatum cv. Hulk), while the temperate Dactylis glomerata cv. Porto did not respond to AMF inoculation. In four of the species, the changes observed in aboveground biomass were explained by the total number of tillers, while, in P. dilatatum, changes were accounted for by the individual weight of mature tillers. On the whole, the screening of cultivated forage grasses revealed that tropical grasses were highly responsive to mycorrhizae, in contrast to a lower effect on the growth of temperate grasses.

Capacidad micotrófica y eficiencia de consorcios con hongos micorrícicos nativos de suelos de la provincia de Buenos Aires con manejo contrastante

We characterized the infective and sporulation capacities of microbial consortia of arbuscular mycorrhizal fungi (AMF) native of Buenos Aires province (Argentina) and determined if some soil characteristics and mycorrhizal parameters could allow to select potentially beneficial inocula. Soil samples were selected from seven locations in Buenos Aires province all under agricultural (A) and pristine (P) conditions. The AMF were multiplied and mycorrhizal root colonization of trap plants was observed at 10 weeks of growth. Spore number in field was low; however, after multiplication spore density accounted for 80-1175 spores per 100g of soil. The principal component analysis showed that the P and Fe soil contents are the main modulators of infectivity and sporulation capacity. The mycorrhizal potential was determined in three locations, being high in Pristine Lobería and Agricultural Trenque Lauquen and low in Junín. Agricultural Lobería (AL) and Pristine Lobería (PL) inocula were selected and their efficiency was evaluated under controlled conditions. Even though shoot dry matter increases after inoculation was not significant (p>0.05) mycorrhizal response was greater than 40% for tomato and 25% for corn, particularly after inoculation with inocula from the agricultural management. These results could be associated to the incipient development of mycorrhizae in both species. Additional research should be conducted to further develop our findings in order to determine the factors involved in the selection of efficient inocula.

New Insights into the Regulation of Aquaporins by the Arbuscular Mycorrhizal Symbiosis in Maize Plants Under Drought Stress and Possible Implications for Plant Performance

The relationship between modulation by arbuscular mycorrhizae (AM) of aquaporin expression in the host plant and changes in root hydraulic conductance, plant water status, and performance under stressful conditions is not well known. This investigation aimed to elucidate how the AM symbiosis modulates the expression of the whole set of aquaporin genes in maize plants under different growing and drought stress conditions, as well as to characterize some of these aquaporins in order to shed further light on the molecules that may be involved in the mycorrhizal responses to drought. The AM symbiosis regulated a wide number of aquaporins in the host plant, comprising members of the different aquaporin subfamilies. The regulation of these genes depends on the watering conditions and the severity of the drought stress imposed. Some of these aquaporins can transport water and also other molecules which are of physiological importance for plant performance. AM plants grew and developed better than non-AM plants under the different conditions assayed. Thus, for the first time, this study relates the well-known better performance of AM plants under drought stress to not only the water movement in their tissues but also the mobilization of N compounds, glycerol, signaling molecules, or metalloids with a role in abiotic stress tolerance. Future studies should elucidate the specific function of each aquaporin isoform regulated by the AM symbiosis in order to shed further light on how the symbiosis alters the plant fitness under stressful conditions.

Mycorrhizal responses in wheat: shading decreases growth but does not lower the contribution of the fungal phosphate uptake pathway

Effects have been investigated of reduced C supply (induced by shade) on arbuscular mycorrhizal (AM) colonisation, mycorrhizal growth responses (MGRs) and on AM-mediated and direct uptake of phosphate (Pi) (using (32)P) in wheat, a plant that does not usually respond positively to AM colonisation. Shading markedly reduced growth and shoot/root dry weight ratios of both AM and non-mycorrhizal wheat, indicating decreased photosynthetic C supply. However, shading had very little effect on percent root length colonised by Rhizophagus irregularis or Gigaspora margarita or on MGRs, which remained slightly positive or zero, regardless of shade; there were no growth depressions under shade. By 6 weeks, when the contributions of the AM pathway were measured with (32)P supplied in small hyphal compartments, R. irregularis had supplied 23 to 28% of shoot P with no significant effect of shading. Data show that reduced C availability did not reduce the contribution of the AM pathway to plant P, so the fungi were not acting physiologically as parasites. These results support our previous hypothesis that lack of positive MGR is not necessarily the outcome of excessive C use by the fungi or failure to deliver P via the AM pathway.

Predicting community and ecosystem outcomes of mycorrhizal responses to global change

Mycorrhizal symbioses link the biosphere with the lithosphere by mediating nutrient cycles and energy flow though terrestrial ecosystems. A more mechanistic understanding of these plant-fungal associations may help ameliorate anthropogenic changes to C and N cycles and biotic communities. We explore three interacting principles: (1) optimal allocation, (2) biotic context and (3) fungal adaptability that may help predict mycorrhizal responses to carbon dioxide enrichment, nitrogen eutrophication, invasive species and land-use changes. Plant-microbial feedbacks and thresholds are discussed in light of these principles with the goal of generating testable hypotheses. Ideas to develop large-scale collaborative research efforts are presented. It is our hope that mycorrhizal symbioses can be effectively integrated into global change models and eventually their ecology will be understood well enough so that they can be managed to help offset some of the detrimental effects of anthropogenic environmental change.