Influence of a plant growth-promoting pseudomonad and vesicular-arbuscular mycorrhizal fungus on alfalfa and birdsfoot trefoil growth and nodulation

Summary In a growth chamber study we examined the influence of a plant growth-promoting rhizobacterium, Pseudomonas putida R-20, and an acid-tolerant vesicular-arbuscular mycorrhizal (VAM) fungus, Glomus intraradices 25, on Medicago sativa L. and Lotus corniculatus L. growth and nodule development. Seedlings were planted in an acidic (pH 5.5), P-deficient soil containing re-established native microflora (minus VAM) and appropriate rhizobia, and inoculated with the rhizobacterium, the VAM fungus, or both. The plants were assayed at three intervals for up to 10–11 weeks. The growth-promoting rhizobacteria alone increased alfalfa shoot mass by 23% compared to all other treatments, but only at 8 weeks of growth, apparently by promoting nodulation and N₂ fixation (acetylene reduction activity). The presence of VAM, either alone or in combination with the rhizobacteria, generally decreased root length but only at 8 weeks also. As a group, the inoculation treatments increased all nodular measurements by 10 weeks of growth. Few treatment effects were found at 7 and 9 weeks for birdsfoot trefoil; neither plant nor nodular measurements differed among treatments. By 11 weeks, shoot mass was increased by the rhizobacteria alone by 36% compared to the control. As a group, the inoculation treatments all showed increased nodular responses by this time. The rhizobacteria stimulated mycorrhizal development on both plant species, but only at the initial samplings. No synergistic effects between the plant growth-promoting rhizobacterium and VAM inoculation were found. Although these results lend credence to the concept of managing microorganisms in the rhizosphere to improve plant growth, they emphasize the necessity for a more thorough understanding of microbial interactions as plants mature.     

Defoliation effects on Plantago lanceolata resource allocation and soil decomposers in relation to AM symbiosis and fertilization

Plants can mediate interactions between aboveground herbivores and belowground decomposers as both groups depend on plant-provided organic carbon. Most vascular plants also form symbiosis with arbuscular mycorrhizal fungi (AMF), which compete for plant carbon too. Our aim was to reveal how defoliation (trimming of plant leaves twice to 6 cm above the soil surface) and mycorrhizal infection (inoculation of the fungus Glomus claroideum BEG31), in nutrient poor and fertilized conditions, affect plant growth and resource allocation. We also tested how these effects can influence the abundance of microbial-feeding animals and nitrogen availability in the soil. We established a 12-wk microcosm study of Plantago lanceolata plants growing in autoclaved soil, into which we constructed a simplified microfood-web including saprotrophic bacteria and fungi and their nematode feeders. We found that fertilization, defoliation and inoculation of the mycorrhizal fungus all decreased P. lanceolata root growth and that fertilization increased leaf production. Plant inflorescence growth was decreased by defoliation and increased by fertilization and AMF inoculation. These results suggest a negative influence of the treatments on P. lanceolata belowground biomass allocation. Of the soil organisms, AMF root colonization decreased with fertilization and increased with defoliation. Fertilization decreased numbers of bacterial-feeding nematodes, probably because fertilized plants produced less root mass. On the other hand, bacterial feeders were more abundant when associated with defoliated than non-defoliated plants despite defoliated plants having less root mass. The AMF inoculation per se increased the abundance of fungal feeders, but the reduced and increased root AM colonization rates of fertilized and defoliated plants, respectively, were not reflected in the numbers of fungal feeders. We found no evidence of plant-mediated effects of the AM fungus on bacterial feeders, and against our prediction, soil inorganic nitrogen concentrations were not positively associated with the concomitant abundances of microbial-feeding animals. Altogether, our results suggest that (1) while defoliation, fertilization and AMF inoculation all affect plant resource allocation, (2) they do not greatly interact with each other. Moreover, it appears that (3) while changes in plant resource allocation due to fertilization and defoliation can influence numbers of bacterial feeders in the soil, (4) these effects may not significantly alter mineral N concentrations in the soil.     

Mycorrhizal colonization with Glomus intraradices and development stage of transformed tomato roots significantly modify the chemotactic response of zoospores of the pathogen Phytophthora nicotianae

The chemotaxic response of zoospores of the plant pathogen, Phytophthora nicotianae, towards exudates from mycorrhizal and non-mycorrhizal transformed tomato roots was studied. A bi-compartmental in vitro system was used to grow Ri T-DNA-transformed tomato roots colonized or non-colonized with the arbuscular mycorrhizal fungus, Glomus intraradices, and to collect root and mycorrhizal exudates. The root and mycorrhizal growth dynamics were first characterized in order to determine two times of exudate sampling. Exudates collected from 16-wk-old mycorrhizal roots were significantly more attractive for P. nicotianae zoospores than exudates from non-inoculated roots. On the contrary, concentrated exudates harvested from 24-wk-old mycorrhizal roots were repulsive to zoospores compared to exudates from non-colonized roots and the water control. In exudates of G. intraradices-inoculated roots, HPLC–MS analyses revealed significantly higher concentrations of proline and isocitrate after 24 wk of growth, while after 16 wk of growth, proline concentration did not differ between exudate types, and the isocitrate concentration was lower in mycorrhizal root exudates. Mycorrhizal inoculation had no effect on the amounts of other amino acids and organic acids and on the sugars quantified within exudates. Our results suggest that modification in exudate composition of mature roots by mycorrhizal colonization may provoke the repulsion of P. nicotianae, and that their capacity to infect host roots may in this manner be reduced.     

Effects of substrate sterilization,fungicide treatment,and mycorrhization helper bacteria on ectomycorrhizal formation of pedunculate oak (Quercus robur) inoculated with Laccaria laccata in two peat bare-root nurseries

Summary Pedunculate oak seedlings (Quercus robur) inoculated with the ectomycorrhizal fungus Laccaria lacata were grown for 1 year on fertilized sphagnum peat in two nurseries. Three factors affecting microbial populations in the substrate were studied, fungicide treatment of the seeds, peat disinfection before sowing (methyl bromide or steam pasteurization), and inoculation with mycorrhization helper bacteria. Treatment of acorns with Iprodione had no depressive effect on mycorrhiza formation. Both disinfection techniques were equivalent, stimulating or depressing mycorrhiza formation depending on the initial microflora in the peat. The introduction of two previously selected mycorrhization helper bacteria (one Pseudomonas fluorescens and one unidentified fluorescent pseudomonad), isolated from L. laccata sporocarps associated with Douglas fir—L. laccata ectomycorrhizas in other nurseries, significantly increased the mycorrhizal rate from 30 to 53% of the short roots. The implications of these results for the controlled mycorrhization of planting stocks and the specificity of mycorrhization helper bacteria are discussed.     

Estimation of mycorrhizal colonization of the roots of oak seedlings inoculated with an ectomycorrhizal fungus,Laccaria amethystea

Six-month-old seedlings of Quercus serrata and Quercus glauca in a nursery were inoculated with the ectomycorrhizal fungus Laccaria amethystea encapsulated in alginate gel and grown in the nursery. The seedlings were collected at 1, 3, and 5 months after the inoculation and examined for colonization of the root system with ectomycorrhizal fungi. The roots within 5 months after the inoculation showed rudimentary ectomycorrhizal colonization. The level of colonization of the root system was estimated based on the intensity of hyphal covering on the root tips by staining with a fluorescent dye and expressed as an index of mycorrhizal colonization (IMC). IMC increased with the time after inoculation and reached values of 4 and 12% in Q. serrata and Q. glauca, respectively at 5 months after the inoculation. The determination of IMC may enable to assess the development of mycorrhizal colonization of the root system that shows rudimentary ectomycorrhizas after the inoculation.     

Changes in rhizosphere microbial activity mediated by native or allochthonous AM fungi in the reafforestation of a Mediterranean degraded environment

This study was carried out in a semiarid degraded area to assess the effectiveness of mycorrhizal inoculation with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L. and Retama sphaerocarpa (L.) Boissier in this area. Associated changes in the soil microbiological properties and aggregate stability related to these AM inocula were also recorded. Eighteen months after planting, G. claroideum had increased available P in the rhizosphere of both shrub species. In general, both inoculation treatments increased water-soluble C and water-soluble and total carbohydrates, G. claroideum being the most effective inoculum, particularly in R. sphaerocarpa. The mixture of native AM fungi was the most effective treatment for increasing the aggregate stability of R. sphaerocarpa soil, while that of O. europaea was increased only by G. claroideum. Increased (dehydrogenase, urease, protease-BAA, acid phosphatase and -glucosidase) enzyme activities, in particular of dehydrogenase and acid phosphatase, were recorded in the rhizosphere of both mycorrhizal shrub species. The mixture of native AM fungi was the most effective treatment for stimulating the growth of O. europaea and R. sphaerocarpa (11.6-fold and 3.3-fold, respectively, greater than control plants). The establishment of mycorrhizal shrub species favoured the reactivation of soil microbial activity, which was linked to an increase in aggregate stability.     

Interactions between mycorrhizal fungi and mycorrhizosphere bacteria during mineral weathering: Budget analysis and bacterial quantification

The impact of ectomycorrhizal fungi or rhizosphere bacteria on tree seedling growth and nutrient uptake is well known. However, few studies have combined those microorganisms in one experiment to clarify their relative contribution and interactions in nutrient acquisition. Here, we monitored the respective contributions of pine roots, two ubiquitous forest ectomycorrhizal fungi Scleroderma citrinum and Laccaria bicolor, and two S. citrinum-mycorrhizosphere bacterial strains of Burkholderia glathei and Collimonas sp., on mineral weathering, nutrient uptake, and plant growth. Pinus sylvestris plants were grown on quartz–biotite substrate and inoculated or not with combinations of mycorrhizal fungi and/or bacterial strains. Magnesium and potassium fluxes were measured and nutrient budgets were calculated. Both ectomycorrhizal fungi significantly increased Mg plant uptake. No significant effects of the two bacterial strains were detected on the K and Mg budgets, but co-inoculating the mycorrhizal fungus S. citrinum and the efficient mineral-weathering B. glathei bacterial strain significantly improved the Mg budget. Similarly, co-inoculating S. citrinum with the Collimonas sp. bacterial strain significantly improved the pine biomass compared to non-inoculated pine plants. In order to better understand this process, we monitored the survival of the inoculated bacterial strains in the quartz–biotite substrate, the pine rhizosphere, and the mycorrhizal niche. The results showed that the two bacterial strains harboured different colonization behaviours both of which depended on the presence of the ectomycorrhizal partner. The populations of the Burkholderia strain were maintained in all these environments with a significantly higher density in the mycorrhizal niche, especially of S. citrinum. In contrast the population of the Collimonas strain reached the detection level except in the treatment inoculated with S. citrinum. These results highlight the need for taking into account the ecology of the microorganisms, and more specifically the fungal–bacterial interactions, when studying mineral weathering and plant nutrition.     

Mycorrhizal community structure,microbial biomass P and phosphatase activities under Salix polaris as influenced by nutrient availability

Low supply of the nutrients nitrogen (N) and phosphorus (P) limit plant growth and spreading, and increase the plant-microbial nutrient competition in subarctic and arctic regions. We investigated the mycorrhizal community structure of a polar shrub willow (Salix polaris) and the microbial turnover in its rhizosphere to explore the adaptation of a mycorrhizal plant in the subarctic tundra. The ectomycorrhizal colonisation ranged from 35 to 64% of the fine root tips and decreased with an increasing soil C/N ratio. In total, 16 ectomycorrhizal morphotypes were found under S. polaris (eight to 13 morphotypes per site, five morphotypes at all four sites). Cenococcum sp. was the most common EM fungus (32% of the ectomycorrhizal fine roots). The abundance of Cenococcum sp. increased with an increasing organic matter content and N/P ratio in the soil. Arbuscular mycorrhizal colonisation of S. polaris was absent or less than 1% of the fine root length. Microbial biomass P accounted for 21–75% of the organic soil P and 6–49% of the total soil P. Microbial biomass P, alkaline and acid phosphatase activities in the rhizosphere increased with increasing soil N concentration. We conclude that a higher N supply decreases the diversity in the mycorrhizal community on polar willows and increases the role of P turnover from the soil microbial biomass for the nutrient supply.     

Effect of arbuscular mycorrhizal fungi on rhizosphere organic acid content and microbial activity of trifoliate orange under different low P conditions

ABSTRACT

Arbuscular mycorrhizal (AM) fungi can improve plant phosphorus (P) uptake; however, information about how AM fungi affect rhizosphere organic acid and microbial activity to alleviate citrus low P stress is limited. Here, a pot experiment was conducted to evaluate the effect of AM fungi (Rhizophagus intraradices, Ri) inoculation on rhizosphere organic acid content, microbial biomass (MB) and enzyme activity of trifoliate orange (Poncirus trifoliata L. Raf.) seedlings grown under three low P conditions. The results showed that mycorrhizal seedlings all recorded higher P concentrations, plant biomass and better root morphology with more lateral and fine roots, but lower root mass ratios, irrespective of P conditions. Mycorrhizal P absorption contribution did not differ significantly among three P conditions. Mycorrhizal seedling rhizosphere soil exhibited lower organic acid content, soil organic P content and ratio of MB-carbon (C)/MB-P, but higher MB and enzyme activity. Additionally, the main organic acids showed a negative relationship with mycorrhizal colonization rate and hyphal length; however, phosphatase and phytase activity had a significantly positive relationship with MB. Therefore, the results suggest that AM fungi inoculation may help citrus to efficiently utilize organic P source by improving microbial activity under low available P conditions.     

Grazing of protozoa on rhizosphere bacteria alters growth and reproduction of Arabidopsis thaliana

Plant roots are densely colonized by bacteria which form the basis of the rhizosphere bacterial food web with protozoa as most effective predators. We established a well defined laboratory system with Arabidopsis thaliana as model plant allowing to investigate in detail the effect of rhizosphere interactions on plant performance. We used this system to analyse separate and combined effects of natural rhizobacteria and the protozoa Acanthamoeba castellanii on plants.Protozoa and bacteria increased plant growth with the effect of protozoa markedly exceeding that of bacteria only. Arabidopsis immediately responded to the presence of protozoa by increasing carbon but not nitrogen uptake. Later protozoa enhanced plant uptake of nitrogen from organic material and prolonged vegetative growth of Arabidopsis resulting in strongly increased seed production. It is concluded that the immediate plant response was based on changes in rhizosphere signalling inducing increased plant carbon fixation rather than on protozoa-mediated increase in nitrogen availability. The subsequently increased plant nitrogen uptake presumably originated from nitrogen fixed in bacterial biomass made available by protozoan grazing, i.e. the microbial loop in soil. The results suggest that Arabidopsis prepared for the upcoming mobilization of nitrogen by increasing carbon fixation and root carbon allocation which paid-off later by increased nutrient capture and strongly increased plant reproduction.     

Effects of low temperature and shade on relationships between nodulation,vesicular-arbuscular mycorrhizal infection,and shoot growth of soybeans

The effects of low temperature and reduced light on a Glycine-Bradyrhizobium-Glomus spp. symbiosis were examined in pot experiments. Soybean plants, Glycine max L. Merr. cv. Tachiyutaka, were grown with N fertilization or inoculation with Bradyrhizobium japonicum plus P fertilization or inoculation with Glomus mosseae in the glasshouse. After the flowering stage, half the pots with soybean plants were subjected to low temperature (15°C 14h/13°C 10 h) with light reduced by shading. At 0, 7, 16, and 28 days after the application of the treatments, the growth, nodulation, vesicular-arbuscular mycorrhizal (VAM) infection and the N and P contents of the soybean plants were measured. In all symbiont-fertilization combinations, the low-temperature treatment reduced the production of dry matter by the soybeans. Nodulation (weight and number) was slightly reduced by this treatment but the proportion of larger nodules was increased. The root length infected by the VAM fungus was little affected by the low-temperature treatment. Both the nodule weight and the infected root length were linearly related to shoot dry weight regardless of treatment and of the symbiont-fertilization combination used. These results suggest that the growth of the symbionts on the root was in balance with the shoot growth of the host, irrespective of climatic conditions, and imply a considerable degree of host control. P inflows to root systems were greatly affected by low-temperature treatment regardless of the symbiont-fertilization combination. This suggests that a simple comparison of P inflows between mycorrhizal and non-mycorrhizal plants may give misleading information on the effects of low temperature or reduced light conditions on P uptake by mycorrhizal plants.     

Quantitative analysis of soluble exudates produced by ectomycorrhizal roots as a response to ambient and elevated CO2

Despite its potential impact on soil carbon flow, few studies have attempted to quantify the effects of elevated carbon dioxide (CO₂) on production of exudates by mycorrhizal plants. In this study we quantified low molecular weight (LMW) organic compounds exuded by non-mycorrhizal (NM) and ectomycorrhizal (ECM) plants in relation to exposure to elevated CO₂. Scots pine seedlings, either colonized by one of eight different ECM fungi or non-mycorrhizal (NM), were exposed to either ambient (350 ppm) or elevated (700 ppm) concentrations of CO₂. Exudation of LMW organic acids (LMWOAs), amino acids, dissolved monosaccharides and total dissolved organic carbon (DOC) was determined and exudation rates were calculated per g root and fungal dry mass. CO₂ had a significant impact on exudation. Under elevated CO₂, exudation of total LMWOAs increased by 120-160%, amino acids by 250%, dissolved monosaccharides by 130-270% and DOC by 180-220% compared to ambient CO₂ treatment. Net CO₂ assimilation rates increased significantly by 41-47% for seedlings exposed to elevated CO₂. Exuded C calculated as a percentage of assimilated CO₂ increased by 41-88% in the elevated CO₂ treatment compared to ambient CO₂ treatment.     

Effects of plant species on microbial biomass phosphorus and phosphatase activity in a range of grassland soils

Soil P transformations are primarily mediated by plant root and soil microbial activity. A short-term (40 weeks) glasshouse experiment with 15 grassland soils collected from around New Zealand was conducted to examine the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil microbial properties and microbiological processes involved in P dynamics. Results showed that the effect of plant species on soil microbial parameters varied greatly with soil type. Concentrations of microbial biomass C and soil respiration were significantly greater in six out of 15 soils under radiata pine compared with ryegrass, while there were no significant effects of plant species on these parameters in the remaining soils. However, microbial biomass P (MBP) was significantly lower in six soils under radiata pine, while there were no significant effects of plant species on MBP in the remaining soils. The latter indicated that P was released from the microbial biomass in response to greater P demand by radiata pine. Levels of water soluble organic C were significantly greater in most soils under radiata pine, compared with ryegrass, which suggested that greater root exudation might have occurred under radiata pine. Activities of acid and alkaline phosphatase and phosphodiesterase were generally lower in most soils under radiata pine, compared with ryegrass. The findings of this study indicate that root exudation plays an important role in increased soil microbial activities, solubility of organic P and mineralization of organic P in soils under radiata pine.     

Infection of valuable broadleaf hardwood trees by Glomus mosseae: growth and mineral nutrition

In the past century, the excessive exploitation of the environment by human beings has resulted in the depletion of valuable broadleaf hardwood trees in Italian forests, creating a need for re-forestation. The aim of this research was to verify whether a vescicular-arbuscular mycorrhizal (VAM) fungus is able to colonise the root of valuable hardwood trees and to evaluate the impact of the VAM fungus on growth and macroelement nutrition of its plant hosts.Four species of valuable broadleaf hardwood trees, Prunus avium L., Fraxinus excelsior L., Acer pseudoplatanus L., and Juglans nigra L., were inoculated with Glomus mosseae, a VAM fungus, and cultivated in a greenhouse. Infection after inoculation and root colonization by the fungus, tree growth, and macro-element nutrition were evaluated two-years after inoculation. G. mosseae formed mycorrhizae on all plants. However, different morphological aspects - predominantly the formation of Arum type arbuscles in P. avium and F. excelsior - were observed. A general improvement of macro-element nutrition from species to species characterised an enhanced growth of mycorrhizal plants. Therefore, it is plausible that the association of VAMs with these broadleaf trees, could overcome the difficulties encountered in the transplanting and the slow growth typical of these tree species.Although numerous articles have reported the beneficial effects of ectomycorrhizal fungi on trees, there is a sparse literature on the association of VAM with tree species. Therefore, this study contributes to the understanding of the role of the symbiosis between valuable broadleaf trees and VAM fungi in macroelement nutrition.     

Interactions between phosphorus availability and an AM fungus (Glomus intraradices) and their effects on soil microbial respiration, biomass and enzyme activities in a calcareous soil

The interactions between soil P availability and mycorrhizal fungi could potentially impact the activity of soil microorganisms and enzymes involved in nutrient turnover and cycling, and subsequent plant growth. However, much remains to be known of the possible interactions among phosphorus availability and mycorrhizal fungi in the rhizosphere of berseem clover (Trifolium alexandrinum L.) grown in calcareous soils deficient in available P. The primary purpose of this study was to look at the interaction between P availability and an arbuscular mycorrhizal (AM) fungus (Glomus intraradices) on the growth of berseem clover and on soil microbial activity associated with plant growth. Berseem clover was grown in P unfertilized soil (−P) and P fertilized soil (+P), inoculated (+M) and non-inoculated (−M) with the mycorrhizal fungus for 70 days under greenhouse conditions. We found an increased biomass production of shoot and root for AM fungus-inoculated berseem relative to uninoculated berseem grown at low P levels. AM fungus inoculation led to an improvement of P and N uptake. Soil respiration (SR) responded positively to P addition, but negatively to AM fungus inoculation, suggesting that P limitation may be responsible for stimulating effects on microbial activity by P fertilization. Results showed decreases in microbial respiration and biomass C in mycorrhizal treatments, implying that reduced availability of C may account for the suppressive effects of AM fungus inoculation on microbial activity. However, both AM fungus inoculation and P fertilization affected neither substrate-induced respiration (SIR) nor microbial metabolic quotients (qCO₂). So, both P and C availability may concurrently limit the microbial activity in these calcareous P-fixing soils. On the contrary, the activities of alkaline phosphatase (ALP) and acid phosphatase (ACP) enzymes responded negatively to P addition, but positively to AM fungus inoculation, indicating that AM fungus may only contribute to plant P nutrition without a significant contribution from the total microbial activity in the rhizosphere. Therefore, the contrasting effects of P and AM fungus on the soil microbial activity and biomass C and enzymes may have a positive or negative feedback to C dynamics and decomposition, and subsequently to nutrient cycling in these calcareous soils. In conclusion, soil microbial activity depended on the addition of P and/or the presence of AM fungus, which could affect either P or C availability.     

Dual inoculation of pretransplant stage Oryza sativa L. plants with indigenous vesicular-arbuscular mycorrhizal fungi and fluorescent Pseudomonas spp.

Summary This study examined the response of rice (Oryza sativa L.) plants at the pretransplant/nursery stage to inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi and fluorescent Pseudomonas spp., singly or in combination. The VAM fungi and fluorescent Pseudomonas spp. were isolated from the rhizosphere of rice plants. In the plants grown in soil inoculated with fluorescent Pseudomonas spp. alone, I found increases in shoot growth, and in root length and fine roots, and decreases in root growth, and P and N concentrations. In contrast, in the plants colonized by VAM fungi alone, the results were the reverse of those of the pseudomonad treatment. Dual inoculation of soil with VAM fungi and fluorescent Pseudomonas spp. yielded plants with the highest biomass and nutrient acquisition. In contrast, the plants of the control treatment had the lowest biomass and nutrient levels. The dual-inoculated plants had intermediate root and specific root lengths. The precentages of mycorrhizal colonization and colonized root lengths were significantly lower in the dual-inoculated treatment than the VAM fungal treatment. Inoculation of plants with fluorescent Pseudomonas spp. suppressed VAM fungal colonization and apparently reduced photosynthate loss to the mycorrhizal associates, which led to greater biomass and nutrient levels in dual-inoculated plants compared with plants inoculated with VAM fungi alone. Dual inoculation of seedlings with fluorescent Pseudomonas spp. and VAM fungi may be preferable to inoculation with VAM alone and may contribute to the successful establishment of these plants in the field.     

Interactions of Glomus clarum with Acetobacter diazotrophicus in infection of sweet potato (Ipomoea batatas), sugarcane (Saccharum spp.), and sweet sorghum (Sorghum vulgare)

Summary Spores of the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus clarum obtained from sweet potatoes grown in soil inoculated with this fungus and with an enrichment culture of Acetobacter diazotrophicus contained A. diazotrophicus and several other bacteria, including a diazotrophic Klebsiella sp. Inoculation of micropropagated sweet potatoes with G. clarum and A. diazotrophicus enhanced spore formation in soil compared to VAM inoculation alone. Plants inoculated with VAM spores containing the bacteria showed additional increases in the number of spores formed within roots. A. diazotrophicus infected aerial plant parts only when inoculated together with VAM or when present within VAM spores. Micropropagated sugarcane seedlings inoculated with the same VAM spores containing the diazotrophs also contained much higher numbers of A. diazotrophicus in aerial parts than seedlings inoculated in vitro with the bacteria alone. When grown in non-sterile soil, the sugarcane seedlings again showed the greatest infection of aerial parts after inoculation with VAM spores containing the diazotrophs. This treatment also increased VAM colonization and the numbers of spores formed within roots. Similar effects were observed in sweet sorghum except that the aerial plant parts were not infected by A. diazotrophicus.     

Azospirillum spp. participation in dry matter partitioning in grasses at the whole plant level

Plant growth-promoting rhizobacteria, particularly those from the genus Azospirillum spp., may affect root functions such as growth and nutrient/water uptake, which in turn may affect shoot growth. Calculations based on data from literature on shoot and root mass of crop grasses (79 plant/bacteria associations were analyzed) revealed that inoculation with Azospirillum spp. increased the shoot-to-root (S/R) ratio in about half of reported cases and decreased the S/R ratio in the other half. In 11 of 35 cases, the S/R ratio increased when the shoot mass increased more than the root mass. In 23 of 35 cases, the root mass did not increase, yet the S/R ratio still increased. Thus, the increase in the S/R ratio indicated that shoot growth responds to inoculation more than root growth. A decrease in the S/R ratio occurred when (a) root growth dominated shoot growth even though both increased (16 of 36 cases), or (b) root growth either increased or remained unchanged, and shoot growth was either unaffected or even decreased (19 of 36 cases). This analysis suggests that: (a) Azospirillum spp. participates in the partitioning of dry matter (both carbon compounds and minerals) at the whole plant level by affecting root functions, and (b) the bacteria affect crop grass through multiple mechanisms operating during plant development.     

Elevated CO2 increases the effect of an arbuscular mycorrhizal fungus and a plant-growth-promoting rhizobacterium on structural stability of a semiarid agricultural soil under drought conditions

In arid and semiarid Mediterranean regions, an increase in the severity of drought events could be caused by rising atmospheric CO₂ concentrations. We studied the effects of the interaction of CO₂, water supply and inoculation with a plant-growth-promoting rhizobacterium (PGPR), Pseudomonas mendocina Palleroni, or inoculation with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith), on aggregate stabilisation of the rhizosphere soil of Lactuca sativa L. cv. Tafalla. The influence of such structural improvements on the growth of lettuce was evaluated. We hypothesised that elevated atmospheric CO₂ concentration would increase the beneficial effects of inoculation with a PGPR or an AM fungus on the aggregate stability of the rhizosphere soil of lettuce plants. Leaf hydration, shoot dry biomass and mycorrhizal colonisation were decreased significantly under water-stress conditions, but this decrease was more pronounced under ambient vs elevated CO₂. The root biomass decreased under elevated CO₂ but only in non-stressed plants. Under elevated CO₂, the microbial biomass C of the rhizosphere of the G. intraradices-colonised plants increased with water stress. Bacterial and mycorrhizal inoculation and CO₂ had no significant effect on the easily-extractable glomalin concentration. Plants grown under elevated CO₂ had a significantly higher percentage of stable aggregates under drought stress than under well-watered conditions, particularly the plants inoculated with either of the assayed microbial inocula (about 20% higher than the control soil). We conclude that the contribution of mycorrhizal fungi and PGPR to soil aggregate stability under elevated atmospheric CO₂ is largely enhanced by soil drying.     

Phosphatase activity and cytokinin content in cowpeas (Vigna unguiculata) inoculated with a vesicular-arbuscular mycorrhizal fungus

We examined the effect of a vesicular-arbuscular mycorrhizal (VAM) fungus Glomus pallidum Hall on the phosphatase activity and cytokinin concentration in cowpea [Vigna unguiculata (L.) Walp] roots at successive stages of plant growth. Both acid and alkaline phosphatase activity were significantly (P=0.05) higher in mycorrhizal than in non-mycorrhizal roots 30 days after inoculation. Similarly, the cytokinin content was significantly increased in mycorrhizal roots compared to non-mycorrhizal roots. Our study suggests that these biochemical changes may improve the growth of mycorrhizal cowpea plants.