Arbuscular Mycorrhizal Root Colonization Research Articles

Evaluating plant–soil feedback together with competition in a serpentine grassland

Plants can alter biotic and abiotic soil characteristics in ways that feedback to change the performance of that same plant species relative to co-occurring plants. Most evidence for this plant-soil feedback comes from greenhouse studies of potted plants, and consequently, little is known about the importance of feedback in relation to other biological processes known to structure plant communities, such as plant-plant competition. In a field experiment with three C4 grasses, negative feedback was expressed through reduced survival and shoot biomass when seedlings were planted within existing clumps of conspecifics compared with clumps of heterospecifics. However, the combined effects of feedback and competition were species-specific. Only Andropogon gerardii exhibited feedback when competition with the clumps was allowed. For Sorghastrum nutans, strong interspecific competition eliminated the feedback expressed in the absence of competition, and Schizachyrium scoparium showed no feedback at all. That arbuscular mycorrhizal (AM) fungi may play a role in the feedback was indicated by higher AM root colonization with conspecific plant neighbours. We suggest that feedback and competition should not be viewed as entirely separate processes and that their importance in structuring plant communities cannot be judged in isolation from each other.

Mycorrhizal colonization and nitrogen uptake by maize: combined effect of tropical earthworms and velvetbean mulch

Earthworms and mulch can have positive or negative effects on mycorrhizae (fungus-roots) and N uptake by plants. In the present experiment, maize plants were grown under greenhouse conditions with or without tropical earthworms (Balanteodrilus pearsei) and mulch of velvetbean (Mucuna pruriens var. utilis). The formation of vesicles and hyphae of arbuscular-mycorrhizal (AM) fungi in roots and N uptake by maize plants was measured at harvest. The addition of earthworms and velvetbean reduced AM root colonization. Earthworms had no effect on plant root or shoot biomass. In the absence of velvetbean, earthworms reduced AM colonization, but when velvetbean was present, this effect disappeared. The addition of velvetbean mulch, on the other hand, had an effect on plant biomass (above- and belowground) and a positive effect on AM fungal colonization of roots in presence of worms, but a negative effect when worms were absent. When both M. pruriens and B. pearsei were added, shoot and root biomass and N concentrations increased. Vesicle formation was related to velvetbean mulch decomposition as well as the higher N concentration in maize roots. Management of mulch–earthworm interactions may be of value, particularly in low-input and organic agricultural systems, and deserves further investigation.

Arbuscular mycorrhizal fungi and plant symbiosis in a saline-sodic soil

The seasonality of arbuscular mycorrhizal (AM) fungi-plant symbiosis in Lotus glaber Mill. and Stenotaphrum secundatum (Walt.) O.K. and the association with phosphorus (P) plant nutrition were studied in a saline-sodic soil at the four seasons during a year. Plant roots of both species were densely colonized by AM fungi (90 and 73%, respectively in L. glaber and S. secundatum) at high values of soil pH (9.2) and exchangeable sodium percentage (65%). The percentage of colonized root length differed between species and showed seasonality. The morphology of root colonization had a similar pattern in both species. The arbuscular colonization fraction increased at the beginning of the growing season and was positively associated with increased P concentration in both shoot and root tissue. The vesicular colonization fraction was high in summer when plants suffer from stress imposed by high temperatures and drought periods, and negatively associated with P in plant tissue. Spore and hyphal densities in soil were not associated with AM root colonization and did not show seasonality. Our results suggest that AM fungi can survive and colonize L. glaber and S. secundatum roots adapted to extreme saline-sodic soil condition. The symbiosis responds to seasonality and P uptake by the host altering the morphology of root colonization.

Influence of nitrogen source on the viability, functionality and persistence of Glomus etunicatum fungal propagules in an Andisol

This study investigated how two different N sources used as fertilizer (NO 3 − or NH 4 +) interact with an inoculated arbuscular mycorrhizal (AM) fungus ( Glomus etunicatum) in an Andisol from southern Chile. The effect of NO 3 − or NH 4 + on mycorrhizal and non-mycorrhizal wheat plants was measured on key root–soil interface activities: pH, acid phosphatase (P-ase) activity and P availability. Root AM colonization, extraradical mycelium length and spore number were also examined at three stages of AM symbiosis development (120, 150 and 240 days after sowing, DAS). The effect of N-source on AM propagule formation was used as an index of the quality and vigor of AM colonization. Mycorrhizal root length was greater with NO 3 − than with NH 4 + at all times. The NO 3 − source also improved extraradical mycelium density, which reached its maximum at 150 DAS. At each harvest the spore number in the rhizosphere soil was also greater with NO 3 − fertilization. This NO 3 − effect on spore formation ranged from 20% at a 120 DAS to 287% at a 240 DAS increase, compared with NH 4 +. Extraradical mycelium and AM efficiency for P acquisition appeared to be related. The particular fungus/plant metabolism as affected by N sources (NO 3 − or NH 4 +) applied did not result in differential plant growth or in changes in N plant acquisition, but affected AM development and activity. Differences in soil pH, available P or P-ase activity in soil seems not to be responsible for the improved physiological status of mycorrhizal development in NO 3 − fed plants. Mycorrhizal propagule formation in this soil and the high persistence of extraradical mycelium are important factors which may have a strong influence on the next crop, and thus, this aspects should be considered when a cropping system is designed. The influence of N sources on AM performance is of ecological and practical interest in volcanic soils when conventional management is used.

Xyloglucanases in the interaction between saprobe fungi and the arbuscular mycorrhizal fungus Glomus mosseae

We studied the production of xyloglucanase enzymes of pea and lettuce roots in the presence of saprobe and arbuscular mycorrhizal (AM) fungi. The AM fungus Glomus mosseae and the saprobe fungi Fusarium graminearum, Fusarium oxysporum-126, Trichoderma harzianum, Penicillium chrysogenum, Pleurotus ostreatus and Aspergillus niger were used. G. mosseae increased the shoot and root dry weight of pea but not of lettuce. Most of the saprobe fungi increased the level of mycorrhization of pea and lettuce, but only P. chrysogenum and T. harzianum inoculated together with G. mosseae increased the dry weight of pea and lettuce respectively. The AM and saprobe fungi increased the production of xyloglucanases by plant roots. The level of xyloglucanase activities and the number of xyloglucanolytic isozymes in plants inoculated with G. mosseae and most of the saprobe fungi tested were higher than when both microorganisms were inoculated separately. The possible relationship between xylogucanase activities and the ability of AM and saprobe fungi to improve the dry weight and AM root colonization of plants was discussed.

Effects of AM colonization on “wild tobacco” plants grown in zinc-contaminated soil

This greenhouse study aimed to determine the effect of colonization by the arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) on the "wild" tobacco (Nicotiana rustica L. var. Azteca), under soil-zinc (Zn) conditions. Plants of N. rustica were grown in AM or non-AM inoculated substrate and subjected to four soil-[Zn] concentrations (0, 50, 100, and 250 mg Zn kg(-1) dry soil). The AM root colonization increased markedly from 14 to 81% with the increasing soil-[Zn] and the mycorrhizal structures were significantly more abundant at the highest soil-[Zn], suggesting that Zn may be involved directly or indirectly in AM root colonization. In addition, total Zn content or Zn concentrations in shoots and roots were shown to increase as soil-[Zn] increased in both AM and non-AM plants. As for the growth parameters studied, there were no significant differences between treatments despite the increase in Zn content or concentration. The AM roots subjected to the highest soil-[Zn] had a significant reduction by about 50% of total Zn content and Zn concentration compared to non-AM roots. Still, the relative extracted Zn percentage decreased dramatically as soil-[Zn] increased. Soil pH was significantly lower in non-AM than AM treatments at the highest soil-[Zn]. In summary, AM plants (particularly roots) showed lower Zn content and concentration than non-AM plants. In this regard, the AM fungi have a protective role for the host plant, thus playing an important role in soil-contaminant immobilization processes; and, therefore, are of value in phytoremediation, especially when heavy metals approach toxic levels in the soil.

The use of pasture reflectance characteristics and arbuscular mycorrhizal root colonization to predict pasture characteristics of tallgrass prairie grazed by cattle and bison

AbstractAn experiment was conducted to evaluate the potential for using arbuscular mycorrhizal fungal (AMF) root colonization and pasture reflectance characteristics as indicators of changes in tallgrass prairie vegetation resulting from differences in grazing history. The experiment was conducted within the context of a separate long‐term experiment in which eight 4·9‐ha pastures were grazed by either bison or cattle for nine consecutive years. Two separate ungrazed pastures were selected for comparison with grazed areas on the basis of similarity in burning regime, vegetation, soil and topographic characteristics. Four 45 m‐long transects were located in the upland sites within each pasture, and four plots were clipped to ground level along each transect. Reflectance readings were taken with a hand‐held radiometer at each sampling location and a soil core was collected within each plot for analysis of AMF root colonization. Reflectance readings at sixteen different wavelengths were used directly as inputs during multiple regression development or were transformed into each of three vegetation indices (normalized difference vegetation index, soil‐adjusted vegetation index and wide‐dynamic‐range vegetation index) and used in simple linear regressions. Ungrazed pastures were characterized by higher (P < 0·01) grass biomass, total biomass and canopy height than grazed pastures, but had a lower proportional abundance of forbs (P < 0·01) and amounts of forb biomass (P = 0·04). Species of herbivore did not significantly influence above‐ground characteristics that were measured. In general, AMF root colonization was relatively small and was not significantly affected by treatment and, accordingly, the variation was insufficient to test its potential as an indicator of grazing effects on vegetation or its potential relationship with pasture reflectance. Multiple regression equations based on individual wavelength reflectance values explained significantly more of the variation in above‐ground vegetation characteristics than did simple regressions using vegetation indices as predictor variables (r2 values from 0·36 to 0·46 vs. 0·11 to 0·27) and have the potential to predict above‐ground vegetation characteristics in heterogeneous rangelands.

The phosphorus source determines the arbuscular mycorrhizal potential and the native mycorrhizal colonization of tall fescue and wheatgrass

The study assesses the effect of two phosphate (P) sources (soluble superphosphate (SP) and rock phosphate (RP)) on the arbuscular mycorrhizal potential (AMP), the root arbuscular mycorrhizal colonization (AMC) and the growth of tall fescue and wheatgrass of a grassland soil from Argentina. Mycorrhizal potential was assessed with soil samples collected from 2 years for tall fescue and wheatgrass swards before and after field plots were fertilized with 0 and 60 kg P ha −1 as SP or RP. Mycorrhizal potential both at unfertilized and at RP fertilized plots was high (12–14 AM propagules g −1), however fertilization with SP caused a decrease in AMP (0.70–0.95 AM propagules g −1). A range of soil P between 4 and 46 mg P kg −1 and a range of root AMC between 6% and 50% were obtained after fertilization with four rates of SP and RP (0, 15, 30, and 60 kg P ha −1) in plots where tall fescue and wheatgrass were grown during 2 years. Soil P and root mass were higher in the top 10-cm depth than in the 20-cm of the soil profile, but AMC did not change with depth. Shoot dry matter (SDM) production of both grasses did not differ after fertilization with SP or RP, particularly at second year. The AMP positively correlated with the indigenous AMC, and they were not different between tall fescue or wheatgrass. Lineal-plateau relationships between soil P, relative SDM and AMC were established. Highest relative SDM was attained at 6.5 mg P kg −1 in plots fertilized with RP, and at 15.2 mg P kg −1 with SP. Variability in colonization was well accounted by the soil P (at 0–10 cm depth) fertilized with SP ( r 2 = 0.48, P 0.01), but any relationship was found with RP. The AMC decreased with increasing available soil P from plots with SP until 18.3 mg kg −1 (a decrease of 2.2% per mg P kg), after that AMC was stabilized at about 6.9%. Our study clearly showed that fertilization with SP or RP produced similar available soil P content and grasses SDM production. Mycorrhiza root colonization and propagules decreased after fertilization with SP, but fertilization with RP did not decrease mycorrhizal propagules nor colonization. It can be concluded that RP fertilization instead SP could allow obtaining acceptable tall fescue and wheatgrass yield enhancing mycorrhizal potential of soils and indigenous colonization of plants and thus maximizing the use of fertilizer.

Lipid metabolism in arbuscular mycorrhizal roots of Medicago truncatula

The peroxidation of polyunsaturated fatty acids, common to all eukaryotes, is mostly catalyzed by members of the lipoxygenase enzyme family of non-heme iron containing dioxygenases. Lipoxygenase products can be metabolized further in the oxylipin pathway by several groups of CYP74 enzymes. One prominent oxylipin is jasmonic acid (JA), a product of the 13-allene oxide synthase branch of the pathway and known as signaling substance that plays a role in vegetative and propagative plant development as well as in plant responses to wounding and pathogen attack. In barley roots, JA level increases upon colonization by arbuscular mycorrhizal fungi. Apart from this first result regarding JA, no information is available on the relevance of lipidperoxide metabolism in arbuscular mycorrhizal symbiosis. Thus we analyzed fatty acid and lipidperoxide patterns in roots of Medicago truncatula during mycorrhizal colonization. Levels of fungus-specific fatty acids as well as palmitic acid (16:0) and oleic acid (18:1 n − 9) were increased in mycorrhizal roots. Thus the degree of arbuscular mycorrhizal colonization of roots can be estimated via analysis of fungal specific esterified fatty acids. Otherwise, no significant changes were found in the profiles of esterified and free fatty acids. The 9- and 13-LOX products of linoleic and α-linolenic acid were present in all root samples, but did not show significant differences between mycorrhizal and non-mycorrhizal roots, except JA which showed elevated levels in mycorrhizal roots. In both types of roots levels of 13-LOX products were higher than those of 9-LOX products. In addition, three cDNAs encoding CYP74 enzymes, two 9/13-hydroperoxide lyases and a 13-allene oxide synthase, were isolated and characterized. The transcript accumulation of these three genes, however, was not increased in mycorrhizal roots of M. truncatula.

Seasonal variation of arbuscular mycorrhizal fungi in temperate grasslands along a wide hydrologic gradient

We studied seasonal variation in population attributes of arbuscular mycorrhizal (AM) fungi over 2 years in four sites of temperate grasslands of the Argentinean Flooding Pampas. The sites represent a wide range of soil conditions, hydrologic gradients, and floristic composition. Lotus glaber, a perennial herbaceous legume naturalised in the Flooding Pampas, was dominant at the four plant community sites. Its roots were highly colonised by AM fungi. Temporal variations in spore density, spore type, AM root colonisation, floristic composition and soil chemical characteristics occurred in each site and were different among sites. The duration of flooding had no effect on spore density but depressed AM root colonisation. Eleven different types of spores were recognized and four were identified. Two species dominated at the four sites: Glomus fasciculatum and Glomus intraradices. Spore density was highest in summer (dry season) and lowest in winter (wet season) with intermediate values in autumn and spring. Colonisation of L. glaber roots was highest in summer or spring and lowest in winter or autumn. The relative density of G. fasciculatum and G. intraradices versus Glomus sp. and Acaulospora sp. had distinctive seasonal peaks. These seasonal peaks occurred at all four sites, suggesting differences among AM fungus species with respect to the seasonality of sporulation. Spore density and AM root colonisation when measured at any one time were poorly related to each other. However, spore density was significantly correlated with root colonisation 3 months before, suggesting that high colonisation in one season precedes high sporulation in the next season.

Arbuscular mycorrhizal colonization of the dominant plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji

Arbuscular mycorrhizal (AM) colonization was observed on four plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji. The AM colonization of the dominant species, Polygonum cuspidatum, contradicts the conclusion that Polygonaceae are often regarded as being non-mycorrhizal species. The secondary dominant species, Polygonum weyrichii var. alpinum, formed no mycorrhizas. The roots of Cirsium purpuratum, Clematis stans and Campanula punctata ssp. hondoensis, showed a higher percentage of AM colonization than P. cuspidatum. AM colonization and spore density in the rhizosphere soil of P. cuspidatum significantly decreased as elevation increased. AM colonization in roots of Cirsium purpuratum and Clematis stans also tended to decrease with increased altitudes. Cirsium purpuratum and Campanula punctata ssp. hondoensis formed single structural types of Arum- and Paris-type, respectively, whereas P. cuspidatum and Clematis stans formed both Arum- and Paris-type morphologies.

Arbuscular mycorrhizal fungi can decrease the uptake of uranium by subterranean clover grown at high levels of uranium in soil

Subterranean clover inoculated or not with the arbuscular mycorrhizal (AM) fungus Glomus intraradices was grown on soil containing six levels of 238U in the range 0–87 mg kg−1. Increasing U concentration in soil enhanced the U concentration in roots and shoots of both mycorrhizal and nonmycorrhizal plants but had no significant effects on plant dry matter production or root AM colonization. Mycorrhizas increased the shoot dry matter and P concentration in roots and shoots, while in most cases, it decreased the Ca, Mg and K concentrations in plants. The AM fungus influenced U concentration in plants only in the treatment receiving 87 mg U kg−1 soil. In this case, U concentration in shoots of nonmycorrhizal plants was 1.7 times that of shoots of mycorrhizal plants. These results suggested that mycorrhizal fungi can limit U accumulation by plants exposed to high levels of U in soil.

Occurrence and effect of arbuscular mycorrhizal propagules in wheat as affected by the source and amount of phosphorus fertilizer and fungal inoculation

Volcanic ash-derived soils in Chile show very unique behavior and properties as soil system due to their unusual composition characterized by high allophane and stabilized humus content. These soils constitute excellent models to study both natural and man-induced VA mycorrhizal effect over plant nutrition and soil ecology sustainability. This paper studies the effect of Glomus etunicatum inoculation of wheat in a natural volcanic soil fertilized with soluble P or with partially acidulated-rock phosphate (pa-RP) at two rates (17 and 86 kg P ha −1); yield, plant phosphorus acquisition, and mycorrhizal colonized root length, are measured. The influence of these treatments on mycorrhizal mycelium and spore production as well as on soil phosphatase (P-ase) activity was also determined. The inoculation of G. etunicatum, locally isolated, increased significantly the extent of P plant acquisition, spore number, length of extraradical mycelium, and P-ase activity when compared with indigenous arbuscular mycorrhizal (AM) fungi fertilized with pa-RP, at the level of 86 kg ha −1. In concordance with these results, the remaining available P in the experimental soil without inoculation was depressed. A negative impact of soluble P application in G. etunicatum inoculated soil was noted in the P-ase activity, and also in the effectiveness of the applied inoculum in relation to P plant uptake. In soil AM inoculated and fertilized with pa-RP (86 kg ha −1), the enhancement of P-ase activity was related to high mycelium development and spore formation. P plant acquisition in G. etunicatum inoculated plants ranged from 4.96 to 11.57 mg per pot when 86 kg ha −1 of pa-RP is applied compared with the same amount of soluble P. Surprisingly, adding pa-RP does not improve the amount of colonized root length. Fungal root AM colonization and AM propagules (mycelium and spores) were not depressed at higher soluble P supply, but the activity of G. etunicatum measured as P plant uptake was strongly affected. The inoculation with G. etunicatum enhances spore production, particularly at the lower soluble P and at the highest pa-RP levels. A close relationship ( r=0.938) between AM spores and P-ase activity was found. Only in pa-RP treatments the G. etunicatum inoculation reduced the external P wheat requirements. The improvement of P-ase activity by G. etunicatum inoculation, as a biological factor involved in P-cycling in soil, may be an important mechanism related to plant P acquisition. It is concluded that pa-RP is the best source of P, not only by increasing P uptake by interacting with G. etunicatum, but also by enhancing AM propagules (mycelium and spores) remaining in soil.

Bi-directional transfer of phosphorus between red clover and perennial ryegrass via arbuscular mycorrhizal hyphal links

Perennial ryegrass and red clover were grown in low-P soil in separate compartments within rhizoboxes. The compartments were separated by 30-μm pore nylon mesh and a buffer compartment to prevent root-to-root contact but allow hyphal penetration. Both plant species were established as donor and receiver plants. Donors were inoculated with an arbuscular mycorrhizal (AM) fungus, Glomus mosseae, forming hyphal links between donors and receivers. Two rates of P (50 and 100 mg P kg –1) were applied to the soil in the receiver compartments. All other compartments received 50 mg P kg –1. In one experiment 32P tracer was applied to the roots of the donors after 6 weeks' growth to detect P transfer from donors to receivers. In a second experiment, plants harvested 2 weeks later were examined for root AM colonisation, shoot yield and P uptake. Arbuscular mycorrhizal fungal inoculation of donor roots led to increased yield, P concentration and P uptake of receiver shoots. Shoot P concentrations were higher in non-mycorrhizal ryegrass than in non-mycorrhizal clover. The 32P tracer study showed a lower apparent P transfer rate from ryegrass to clover than from clover to ryegrass despite a comparable amount of P transferred. Internal P requirements of the host plants may influence P transfer rates. The AM hyphae formed an important link between the plants but no appreciable net transfer of P was likely to occur.

PLANT GROWTH RESPONSES IN NATURAL ACIDIC SOIL AS AFFECTED BY ARBUSCULAR MYCORRHIZAL INOCULATION AND PHOSPHORUS SOURCES

ABSTRACT The effect of arbuscular-mycorrhizal (AM) fungus Glomus etunicatum inoculation in interaction with two sources of phosphorus (P) [soluble P and partially acidulated phosphate rock (pa-PR)] at three rates (17, 43, and 86 kg P ha−1) was studied in an acidic natural soil using wheat (Triticum aestivum L.) as host plant. Shoot and root dry biomass, AM colonized root length, macro-micronutrients content and soil phosphatase (P-ase) activity were determined after six months of plant growth. The inoculated G. etunicatum fungus, a fungal strain adapted to the prevailing soil conditions, enhanced plant growth (shoot and root biomass) and mineral acquisition of some elements when plants were fertilized with pa-PR but not with soluble P. The nutrient acquisition by AM inoculated plants varied with the source and amount of applied P. When pa-PR was supplied, the inoculated AM fungus enhanced P, potassium (K), aluminum (Al), and manganese (Mn) plant acquisition in comparison with indigenous endophytes alone. Shoot zinc (Zn) and copper (Cu) uptakes were also enhanced by G. etunicatum inoculation only at the intermediate assayed pa-PR level (43 kg ha−1). AM root colonization in the efficient pa-PR treatments, did not relate well to the plant growth and nutrient acquisition in most cases. Nutrients (Ca and Mg) that increased in AM inoculated plants were not those commonly deficient in acidic soils. Nevertheless, some nutrients, which often become limiting under low pH conditions such as P and K were increased by G. etunicatum inoculation plus pa-PR. Changes in rhizospheric soil pH under pa-PR application may be involved in these mycorrhizal effects on nutrient acquisition. The increases in plant biomass as a result of mycorrhizal inoculation do not seem to account for all the changes observed in mineral acquisition. The highest soil P-ase activity was observed at the lowest pa-PR dose showing a negative relationship with P-availability. The inoculation of G. etunicatum was effective in this natural acidic soil in overcoming factors that restrict plant growth and nutrition when pa-PR was applied.

Arbuscular mycorrhizal fungi respond to the substrate pH of their extraradical mycelium by altered growth and root colonization.

• To test the response of arbuscular mycorrhizal (AM) fungi to a difference in soil pH, the extraradical mycelium of Scutellospora calospora or Glomus intraradices, in association with Plantago lanceolata, was exposed to two different pH treatments, while the root substrate pH was left unchanged. • Seedlings of P. lanceolata, colonized by one or other of the fungal symbionts, and nonmycorrhizal controls, were grown in mesh bags placed in pots containing pH-buffered sand (pH around 5 or 6). The systems were harvested at approximately 2-wk intervals between 20 and 80d. • Both fungi formed more extraradical mycelium at the higher pH. Glomus intraradices formed almost no detectable extraradical mycelium at lower pH. The extraradical mycelium of S. calospora had higher acid phosphatase activity than that of G. intraradices. Total AM root colonization decreased for both fungi at the higher pH, and high pH also reduced arbuscule and vesicle formation in G. intraradices. • In conclusion, soil pH influences AM root colonization as well as the growth and phosphatase activities of extraradical mycelium, although the two fungi responded differently.

The early nodulin enod40 is induced by, and also promotes arbuscular mycorrhizal root colonization

Expression of Tenod40, a tobacco (Nicotiana tabacum cv. Xanthi nc) homologue of Msenod40, a gene expressed early during alfalfa (Medicago sativa) nodule development, was significantly and consistently increased (about three-fold) in tobacco roots of plants grown in sand and colonized by the arbuscular mycorrhizal (AM) fungus Glomus intraradices, as compared to non-mycorrhizal controls. In alfalfa (Medicago sativa cv. Gilboa), a similar induction of Msenod40 expression was also detected in mycorrhizal vs. non-mycorrhizal roots. In the same experimental system, the application of the cytokinin 6-benzylaminopurine to non-mycorrhizal roots resulted in a similar increase in Msenod40 expression. A significantly higher incidence of AM fungal colonization (vesicles per cm of root) in M. sativa cv. Gilboa roots was observed following application of luteolin-induced Sinorhizobium meliloti exudates as compared to plants amended with non-induced exudates. Moreover, overexpression of the Mtenod40 gene in Medicago truncatula plants under the control of a 35S constitutive promoter resulted in significantly enhanced colonization as compared to non-transgenic plants. The fact that enod40 gene expression was increased both in a legume and in a non-legume during colonization by the AM fungus, and that colonization levels were higher in enod40 -overexpressing transgenic plants strongly suggests that this gene is involved in the establishment of the fungal partner in its host.

Soil depth distribution of ecto- and arbuscular mycorrhizal fungi associated with Populus tremuloides within a 3-year-old boreal forest clear-cut

Populus tremuloides Michx. is a tree species that have both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) associations. The mechanisms underlying the coexistence of AM and EM are not well understood. In this study, we test the hypothesis that the two types of mycorrhizae are separated along a soil depth gradient. Vertical distribution of EM and AM colonization in roots of P. tremuloides ramets (within a boreal mixedwood 3-year-old clear-cut) was assessed at three different rooting depths (i.e. 0–5, 5–10 and >10 cm) and compared to edaphic properties in the adjacent soil. We predicted that EM colonization would be more abundant in the shallow organic soils and AM colonization would be more abundant in deeper mineral soils. Significantly fewer EM root tips were found in the >10 cm soil depth than in the 0–5 and 5–10 cm depths. Vesicle, AM hyphae and total fungal colonization were significantly lower in the top 5 cm of the soil than in the 5–10 and >10 cm depths. Overall EM root tip mantle formation approximately averaged 86%, while overall AM colonization averaged 6%. A significant negative correlation between EM and AM fungal colonization over all the depths was found (Spearman correlation, r=−0.314, P<0.05). Results of this study support our hypothesis that EM and AM are preferentially partitioned at different soil depths.

Reduced arbuscular mycorrhizal root colonization in Tropaeolum majus and Carica papaya after jasmonic acid application can not be attributed to increased glucosinolate levels

The plant signal compounds jasmonic acid or salicylic acid were applied as abiotic elicitors to leaves of glucosinolate-containing members of the Tropaeolaceae (Tropaeolum majus) and Caricaceae (Carica papaya) and to leaves of the glucosinolate-free plant cucumber. Both compounds are known to induce the accumulation of glucosinolates in Brassica plants. In roots of glucosinolate-containing plants the two compounds enhanced glucosinolate levels or new glucosinolates were accumulated. In all plants treated with jasmonic acid a reduction of root colonization by the arbuscular mycorrhizal fungus Glomus mosseae was observed. No such effect occurred after the salicylic acid treatment. In addition, members of the glucosinolate-containing Tropaeolaceae family were inoculated with the arbuscular mycorrhizal fungus (AMF), and later on the glucosinolate content was determined in roots of mycorrhizal and in roots of non-mycorrhizal plants. Root colonization by the AMF resulted in a large increase of the glucosinolate content, however, the glucosinolate levels in mycorrhizal and in non-mycorrhizal plants showed no effect on root colonization by the AMF. From our results we concluded that the glucosinolate levels can not generally be linked to the non-host status of glucosinolate-containing plants, however, a role of specific glucosinolates in the expression of the non-host status of glucosinolate-containing plants can not be excluded. Moreover, we found that the application of jasmonic acid highly suppresses mycorrhization and this suppression is not a glucosinolate-dependent mechanism.

Temporal and spatial variation of arbuscular mycorrhizas in salt marsh plants of the Tagus estuary (Portugal)

The factors which may influence temporal and spatial variation in plant arbuscular mycorrhizal (AM) colonization and propagule occurrence were evaluated in a Portuguese salt marsh poor in plant diversity. Two distinct sites were studied: a more-flooded (low marsh) and a less-flooded zone (high marsh). AM root colonization, AM fungal spore number and inoculum potential, soil edaphic parameters and tidal flooding time periods were analysed. Levels of AM colonization were considerable in Aster tripolium and Inula crithmoides but very low in Puccinellia maritima and non-existent in Spartina maritima, Halimione portulacoides, Arthrocnemum fruticosum and Arthrocnemum perenne. Fungal diversity was very low, with Glomus geosporum dominant at both marsh zones. Colonization showed no spatial variation within marsh zones but temporal variation was observed in the high marsh, dependent on plant phenological phases. In the low marsh, no significantly seasonal variation was observed. Apparently, plant phenological events were diluted by stressful conditions (e.g. flooding, salinity). Spore density was significantly different between marsh zones and showed temporal variation in both zones. This study showed that distribution of mycorrhizas in salt marsh is more dependent on host plant species than on environmental stresses.