Arbuscular mycorrhizal fungi from three genera induce two-phase plant growth responses on a high P-fixing soil

Plant growth enhancing effects of arbuscular mycorrhizal (AM) fungi are suitably quantified by comparisons of mycorrhizal and non-mycorrhizal plant growth responses to added phosphorus (P). The ratio between the amounts of added P required for the same yield of mycorrhizal and non-mycorrhizal plants is termed the relative effectiveness of the mycorrhiza. Variation in this relative effectiveness was examined for subterranean clover grown on a high P-fixing soil. Plants were either left non-mycorrhizal or inoculated with one of three AM fungal species with well-characterised differences in external hyphal spread. With no P added, plants from all treatments produced <10% of their maximum growth achieved at non-limiting P supply. The growth response of non-mycorrhizal plants was markedly sigmoid. Mycorrhizal growth responses were not sigmoid but their shape was two-phased. The first phase was an asymptotic approach to 25–30% of maximum growth, followed by a second asymptotic rise to maximum growth. Growth effects of Glomus invermaium and Acaulospora laevis were quite similar. Plants in these treatments produced up to four times greater shoot dry biomass than non-mycorrhizal plants. Scutellospora calospora was less effective. The relative effectiveness of AM fungi varied with the level of P application. This is expected to apply to all soils on which a sigmoid response is obtained for growth of non-mycorrhizal plants. In a simple approximation the relative effectiveness was calculated to range from 1.46 to 15.57. Shoot P contents were increased by up to 25 times by A. laevis, significantly more than by the other two fungi. The further mycelial spread of this fungus is thought to have contributed to its relatively greater effect on plant P content.     

Indigenous and introduced arbuscular mycorrhizal fungi contribute to plant growth in two agricultural soils from south-western Australia

Arbuscular mycorrhizal (AM) fungi occur in all agricultural soils but it is not easy to assess the contribution they make to plant growth under field conditions. Several approaches have been used to investigate this, including the comparison of plant growth in the presence or absence of naturally occurring AM fungi following soil fumigation or application of fungicides. However, treatments such as these may change soil characteristics other than factors directly involving AM fungi and lead to difficulties in identifying the reason for changes in plant growth. In a glasshouse experiment, we assessed the contribution of indigenous AM fungi to growth of subterranean clover in undisturbed cores of soil from two agricultural field sites (a cropped agricultural field at South Carrabin and a low input pasture at Westdale). We used the approach of estimating the benefit of AM fungi by comparing the curvature coefficients ( C) of the Mitscherlich equation for subterranean clover grown in untreated field soil, in field soil into which inoculum of Glomus invermaium was added and in soil fumigated with methyl bromide. It was only possible to estimate the benefit of mycorrhizas using this approach for one soil (Westdale) because it was the only soil for which a Mitscherlich response to the application of a range of P levels was obtained. The mycorrhizal benefit ( C of mycorrhizal vs. non-mycorrhizal plants or C of inoculated vs. uninoculated plants) of the indigenous fungi corresponded with a requirement for phosphate by plants that were colonised by AM fungi already present in the soil equivalent to half that required by non-mycorrhizal plants. This benefit was independent of the plant-available P in the soil. There was no additional benefit of inoculation on plant growth other than that due to increased P uptake. Indigenous AM fungi were present in both soils and colonised a high proportion of roots in both soils. There was a higher diversity of morphotypes of mycorrhizal fungi in roots of plants grown in the Westdale soil than in the South Carrabin soil that had a history of high phosphate fertilizer use in the field. Inoculation with G. invermaium did not increase the level of colonisation of roots by mycorrhizal fungi in either soil, but it replaced approximately 20% of the root length colonised by the indigenous fungi in Westdale soil at all levels of applied P. The proportion of colonised root length replaced by G. invermaium in South Carrabin soil varied with the level of application of P to the soil; it was higher at intermediate levels of recently added soil P.     

Improved growth of Cajanus cajan (L.) Millsp. in an unsterile tropical soil by three mycorrhizal fungi

Under glasshouse conditions Cajanus cajan plants grown in a dark red latosol were fertilized with soluble simple superphosphate and hardly soluble rock phosphate and inoculated with three VA mycorrhizal fungi (M₁, Gigaspora margarita; M₂, Scutellospora verrucosa; M₃, Acaulospora rehmii) from the Cerrado ecosystem, Brazil. Only with rock phosphate plant growth was significantly increased by all fungi. Enhanced P uptake corresponded with higher yields and proved to be a characteristic of the VA myccorhizae. A definite relationship between infection intensity and efficiency of VA mycorrhizae was not detected. Spore production was generally more pronounced in the treatment with rock phosphate, especially with M₁ and M₂. Nodulation of Cajanus cajan was greatly improved by all fungi in the treatment with rock phosphate. It is suggested that the increased plant development and nodulation was due to improved uptake of P by mycorrhiza.     

Overlapping expression patterns and differential transcript levels of phosphate transporter genes in arbuscular mycorrhizal,P<Subscript>i</Subscript>-fertilised and phytohormone-treated <Emphasis Type="Italic">Medicago truncatula</Emphasis> roots

A microarray carrying 5,648 probes of Medicago truncatula root-expressed genes was screened in order to identify those that are specifically regulated by the arbuscular mycorrhizal (AM) fungus Gigaspora rosea, by P_i fertilisation or by the phytohormones abscisic acid and jasmonic acid. Amongst the identified genes, 21% showed a common induction and 31% a common repression between roots fertilised with P_i or inoculated with the AM fungus G. rosea, while there was no obvious overlap in the expression patterns between mycorrhizal and phytohormone-treated roots. Expression patterns were further studied by comparing the results with published data obtained from roots colonised by the AM fungi Glomus mosseae and Glomus intraradices, but only very few genes were identified as being commonly regulated by all three AM fungi. Analysis of P_i concentrations in plants colonised by either of the three AM fungi revealed that this could be due to the higher P_i levels in plants inoculated by G. rosea compared with the other two fungi, explaining that numerous genes are commonly regulated by the interaction with G. rosea and by phosphate. Differential gene expression in roots inoculated with the three AM fungi was further studied by expression analyses of six genes from the phosphate transporter gene family in M. truncatula. While MtPT4 was induced by all three fungi, the other five genes showed different degrees of repression mirroring the functional differences in phosphate nutrition by G. rosea, G. mosseae and G. intraradices. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Response of Arbuscular Mycorrhizal Fungi on Wheat (Triticum aestivum L.) Grown Conventionally and on Beds in a Sandy Loam Soil

The present study was undertaken to assess the benefit and compare the functioning of AM fungi on wheat grown conventionally and on beds. Ten treatment combinations were used, treatments 1 and 2: no fertilizers with and without arbuscular mycorrhizal (AM) fungi (In vitro produced Glomus intraradices); 3:100% of recommended NPK: (120 kg ha⁻¹ N; 60 kg ha⁻¹ P; 50 kg ha⁻¹ K), and 4 and 5: 75% of recommended NPK dose with and without AM inoculation in a 5 × 2 split-plot design on wheat using conventional/flat system and elevated/raised bed system. The maximum grain yield (3.84 t ha⁻¹) was obtained in AM fungi inoculated plots of raised bed system applied with 75% NPK and was found higher (although non- significant) than the conventional (3.73 t ha⁻¹) system. The AM inoculation at 75% fertilizer application can save 8.47, 5.38 kg P and 16.95, 10.75 kg N ha⁻¹, respectively, in bed and conventional system. While comparing the yield response with 100% fertilizer application alone, AM inoculation was found to save 20.30, 15.79 kg P and 40.60, 31.59 kg N ha⁻¹, respectively, in beds and conventional system. Mycorrhizal inoculation at 75% NPK application particularly in raised bed system seems to be more efficient in saving fertilizer inputs and utilizing P for producing higher yield and growth unlike non-mycorrhizal plants of 100% P. Besides the yield, mycorrhizal plants grown on beds had higher AM root colonization, soil dehydrogenases activity, and P-uptake. The present study indicates that the inoculation of AM fungi to wheat under raised beds is better response (although non-significantly higher) to conventional system and could be adopted for achieving higher yield of wheat at reduced fertilizer inputs after field validation.     

Host genotype dependency and growth enhancing ability of VA-mycorrhizal fungi for Eleusine coracana (finger millet)

Inoculation of finger millet (Eleusine coracana Gaertn.) plants with one of six different vesicular, arbuscular, mycorrhizal (VAM) fungi increased plant biomass, height, leaf area and absolute growth rate; however, effectiveness of the various VAM fungi varied significantly. Maximum root colonization and mycorrhizal efficacy was observed with plants inoculated with Glomus caledonicum. Among five host genotypes tested for mycorrhizal dependency against G. caledonicum, genotype HR-374 gave the highest plant biomass, mycorrhizal efficacy and root colonization, the inoculation resulting in increased mineral (phosphate, nitrogen, Zn²⁺ and Cu²⁺) content and uptake in shoots.     

Response of Stylosanthes guianensis to endomycorrhizal fungi inoculation as affected by lime and phosphorus applications

The effect of 3 different species of vesicular-arbuscular mycorrhizal fungi on the growth of Stylosanthes guianensis (Aubl.) Sw. cultivated in a sterilized acid and dystrophic soil (Quartzipsament), with 4 levels of lime (0; 0.27; 0.63 and 1.10 meq Ca²⁺/100 g air-dried soil, as Ca(OH)₂) and 2 P levels (0 and 20 mg P/kg soil, as KH₂PO₄) was evaluated under greenhouse conditions. Plants were harvested 40, 60, and 80 days after planting. Stylosanthes guianensis was highly mycotrophic, especially in soil without P fertilization. Mycotrophism was highest in plants inoculated with Acaulospora scrobiculata in soil receiving no P fertilizer and with 0.63 meq Ca²⁺/100 g air-dried soil. Shoot growth increment was as high as 5129% at the third harvest. Inoculation with Glomus macrocarpum presented intermediate results, whereas inoculation with Gigaspora margarita had no significant effect on plant growth. Root per cent colonization and shoot dry weight, as well as root percent colonization and shoot to root ratio were significantly correlated. The occurrence of S. guianensis in very acid and dystropic soils, containing toxic levels of Al³⁺, requires the association with VAM fungi for the plant tolerate such conditions.     

Influence of ectomycorrhizal fungi on the response of Sitka spruce and Japanese larch to forms of phosphorus

Ectomycorrhizal fungi (Paxillus involutus, Suillus grevillei and two unidentified basidiomycetes from excised Sitka spruce mycorrhizas) were isolated from stands of Sitka spruce either in monoculture or in a mixture with Japanese larch in an Irish conifer plantation. The growth of these fungi and their mycorrhizal formation in Sitka spruce and Japanese larch were examined after incubation in modified Melin-Norkrans medium containing either KH₂PO₄, Ca₃(PO₄)₂ or Fe phytate as the phosphorus (P) source. P. involutus and S. grevillei utilized all three P sources. The unidentified basidiomycetes had limited ability to utilize Fe phytate. Basidiomycete 1 showed poor growth on KH₂PO₄ whereas growth of basidiomycete 2 was low on Ca₃(PO₄)₂. Pure culture synthesis studies confirmed that P. involutus and the two basidiomycetes formed mycorrhizas with both tree species but S. grevillei was mycorrhizal only on Japanese larch. P. involutus formed more mycorrhizas in both conifers than the other fungi. Following inoculation with each of the four fungi, shoot and root dry mass of both Sitka spruce and Japanese larch seedlings was enhanced compared with uninoculated/nonmycorrhizal controls. On Fe phytate, Paxillus-inoculated Sitka spruce seedlings had the lowest primary root length and on KH₂PO₄, Suillus-inoculated Japanese larch had the greatest number of short roots. The only differences when Sitka spruce seedlings were grown in either monoculture or in a mixture with Japanese larch mycorrhizal with S. grevillei were primary root length and number of short roots after growth on media containing Fe phytate.     

Field response of wheat to arbuscular mycorrhizal fungi and drought stress

Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.     

Response ofStylosanthes guianensis to endomycorrhizal fungi inoculation as affected by lime and phosphorus applications

The nutrient concentration in the shoots ofStylosanthes guianensis (Aubl.) Sw. cultivated in a sterilized acid and dystrophic soil (Quartzipsament) amended with 4 levels of lime (0; 0.27; 0.63 and 1.10 meq Ca²⁺/100 g soil, as Ca(OH)₂), 2 levels of P (0 and 20 mg P kg^-1 soil, as KH₂PO₄) and not-inoculated or inoculated with 3 vesicular-arbuscular (VA) mycorrhizal fungi was evaluated under greenhouse conditions. The effectiveness of the different fungal species in increasing the nutrient concentration in the shoots varied with the different edaphic conditions. In general, mycorrhiza formation was associated with increases in the concentrations of most of the nutrients analyzed. Under the experimental conditions, the increments in nutrient concentration were higher overall in plants inoculated withAcaulospora scrobiculata. Inoculation ofS. guianensis with VA mycorrhizal fungi was also associated with alterations in the ratios of nutrients in the shoots, which might be important in understanding and explaining the tolerance of mycorrhizal plants to nutritional stresses such as Al and Mn toxicity.     

Increased Growth and Yield of Hydroponically Grown Greenhouse Tomato Plants Inoculated with Arbuscular Mycorrhizal Fungi and Fusarium oxysporum f. sp. radicis-lycopersici

The arbuscular mycorrhizal fungi Glomus monosporum, G. vesiculiferum, G. deserticola, G. intraradices, G. mosseae, and two unidentified species were tested to determine their effect on plant growth and fruit production of tomato (Lycopersicon esculentum Mill.) cv. Trust inoculated with Fusarium oxysporum f. sp. radicis-lycopersici (FORL) under near-commercial greenhouse conditions. Inoculation with G. monosporum and G. mosseae significantly increased fruit yield and fruit number of tomato plants grown hydroponically in sawdust. Plant height and plant dry weight increased significantly when inoculated with G. monosporum and G. mosseae. Further, plants inoculated with G. monosporum and G. mosseae showed significantly lower FORL root infection than the untreated control plants.     

Effect of vesicular-arbuscular mycorrhizal fungi and ofagrobacterium radiobacter on maize growth

An influence of dual inoculation with the rhizosphere bacteriumAgrobacterium radiobacter, and the VAM fungi,Glomus mosseae andGlomus sp., on maize growth and mycorrhizal infection was observed. Separate inoculations of bacteria or fungi showed significant positive effects on the shoot biomass production of pot-cultured plants only at the last of three consecutive harvests. Plant biomass production was enhanced substantially after a dual inoculation with bacteria and fungi. Synergistic interaction of fungal and bacterial inoculation and growth stimulation was evident at all three harvests compared to uninoculated plants and also compared to plants inoculated with fungi or bacteria only. The dual inoculation increased the shoot biomass of plants by approximately 30% as compared with control. No significant differences were found in mycorrhizal infection between plants uninoculated and inoculated with bacteria.Agrobacterium radiobacter seems to be compatible with mycorrhizal symbiosis and can act a synergistic partner of some VAM fungi.     

Benefit and cost analysis and phosphorus efficiency of VA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels

Sorghum [Sorghum bicolor (L.) Moench] was grown in a greenhouse in a low P (3.6 mg kg^-1) soil (Typic Argiudolls) inoculated with the vesicular-arbuscular mycorrhizal fungi (VMAF) Glomus fasciculatum and P added at 0, 12.5, 25.0, and 37.5 mg kg^-1 soil to determine the effects of VAMF-root associations on plant growth, benefit and cost analysis, and P efficiency (dry matter produced/unit P absorbed). Root colonization with VAMF and shoot growth enhancements decreased with increased soil P applications. Mycorrhizal plants were less P efficient than nonmycorrhizal plants. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants were considered the benefit derived by plants from VAMF-root associations. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants with similar P concentrations were considered the costs paid by plants for VAMF-root associations. Values of benefit and cost analysis for VAMF-root associations were highest when soil P was lowest and decreased with increasing P applications. Genotypic differences for calculated costs were pronounced, but not benefits. Benefit and cost analysis.may be helpful to evaluate host plant genotypes and VAMF species to optimize efficiencies of VAMF symbiosis in different soil environments.     

Growth response ofTheobroma cacao L. seeldings to inoculation with vesicular-arbuscular mycorrhizal fungi

Summary The effects of vesicular-arbuscular mycorrhiza (VAM) on the growth and phosphorus uptake of cocoa seedlings (Theobroma cacao L.) grown for 100 days in polythene bags, were studied at five levels of phosphorus fertilization in both steamed and unsterile Bungor Series soil (a fine clayey, kaolinitic isohyperthermic Typic Paleudult). The cocoa seedlings responded well to phosphorus fertilization and mycorrhizal treatments. Plants inoculated with VAM fungi (Gigaspora spp.) gave the most vigorous growth and higher phosphorus in the leaf tissues in unsterile soil compared to plants grown in steamed soil. However, the mycorrhizal effect was significantly more pronounced (P<0.01) in plants grown in steamed than in unsterile soil. High levels of phosphorus application depressed mycorrhizal development. Phosphorus fertilizer applied at the rates of 250 and 500 ug g⁻¹ soil gave maximum root colonization and spore counts in both soil types used.     

Hydroponic sand culture systems for mycorrhizal research

Summary Tomato plants were inoculated withGlomus fasciculatus mycorrhizal fungi while growing in sand through which recycled nutrient solution was automatically passed several times daily. Concentration of P and N in the solution were maintained at relatively low levels. Roots of inoculated plants became highly infected with mycorrhizal fungi, and yield parameters were significantly increased with inoculation over uninoculated control plants.     

AM Fungi and <i>Piriformospora indica</i> Improve Plant Growth of <i>Pinus elliottii</i> Seedlings

Pinus elliottii is an exotic afforestation pine extensively distributed in southern parts of China. In order to understand whether endophytic fungi can affect seedling growth of P. elliottii, Piriformospora indica (Pi), Funnelifcrmis mosseae (Fm), and Diversispora tortuosa (Dt) were inoculated respectively, and the non-inoculated group was set as control. The growth indexes, the contents of soluble sugar and soluble protein, and plant endogenous hormone levels in the leaves of P. elliottii, were analyzed. The results showed that Fm, Dt and Pi colonized the P. elliottii roots to form mycorrhizal structure and chlamydospores arranged in beads respectively. Three fungal inoculants exhibited the stimulated growth responses, whilst Dt illustrated the most positive effect on plant height, single fresh weight, trunk diameter and root system structure, compared with the control. On the other hand, the soluble sugar and soluble protein contents were increased distinctively in mycorrhizal plants. The endogenous IAA, GA3, ZR contents were increased, while the ABA contents were reduced in mycorrhizal plants versus non-mycorrhizal plants. The fungi-induced endogenous hormone changes triggered plant growth improvement of P. elliottii seedlings. This research unraveled the positive effect of AM fungi and P. indica on growth of pine seedlings, while, more application of endophytic fungi to fields needs to be explored.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode,Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.     

Effect of pesticides on ectomycorrhizae ofPinus sylvestris seedlings

Summary Knowledge of the effect of pesticides on the formation of forest tree mycorrhizae is important as pesticides are nowadays used in forestry. The effect of the fungicide Dithane M-45 and the herbicide Gramoxone on the growth ofPinus sylvestris L. seedlings and on the development of their mycorrhizae was studied. Investigations involved seedlings inoculated with pure cultures of mycorrhizal fungi in flasks with perlite under aseptic conditions, in Mitscherlich pots filled with perlite under semi-aseptic conditions, and on peat substrate in outdoor beds. No change in seedling growth and the mycorrhiza formation occurred when water suspension of the fungicide Dithane M-45 was used at the recommended dose. The highest rates of this fungicide had no phytotoxic effect although the growth of treated seedlings was reduced due to complete or partial inhibition of mycorrhizal formation. In contrast, even low doses of Gramoxone reduced the growth of the inoculated and non-inoculated seedlings which were more sensitive than their mycorrhizal fungi. The soil sterilization of outdoor beds with an application of a water suspension of Dithane M-45 at recommended doses reduced mycorrhizal development and seedling growth. Seedlings inoculated simultaneously with pure cultures ofSuillus granulatus showed a slightly better growth than untreated controls.     

Impact of two fluorescent pseudomonads and an arbuscular mycorrhizal fungus on tomato plant growth,root architecture and P acquisition

The ability of fluorescent pseudomonads and arbuscular mycorrhizal fungi (AMF) to promote plant growth is well documented but knowledge of the impact of pseudomonad-mycorrhiza mixed inocula on root architecture is scanty. In the present work, growth and root architecture of tomato plants (Lycopersicon esculentum Mill. cv. Guadalete), inoculated or not with Pseudomonas fluorescens 92rk and P190r and/or the AMF Glomus mosseae BEG12, were evaluated by measuring shoot and root fresh weight and by analysing morphometric parameters of the root system. The influence of the microorganisms on phosphorus (P) acquisition was assayed as total P accumulated in leaves of plants inoculated or not with the three microorganisms. The two bacterial strains and the AMF, alone or in combination, promoted plant growth. P. fluorescens 92rk and G. mosseae BEG12 when co-inoculated had a synergistic effect on root fresh weight. Moreover, co-inoculation of the three microorganisms synergistically increased plant growth compared with singly inoculated plants. Both the fluorescent pseudomonads and the myco-symbiont, depending on the inoculum combination, strongly affected root architecture. P. fluorescens 92rk increased mycorrhizal colonization, suggesting that this strain is a mycorrhization helper bacterium. Finally, the bacterial strains and the AMF, alone or in combination, improved plant mineral nutrition by increasing leaf P content. These results support the potential use of fluorescent pseudomonads and AMF as mixed inoculants for tomato and suggest that improved tomato growth could be related to the increase in P acquisition.