Litter-forager termite mounds enhance the ectomycorrhizal symbiosis between Acacia holosericea A. Cunn. Ex G. Don and Scleroderma dictyosporum isolates

The hypothesis of the present study was that the termite mounds of Macrotermes subhyalinus (MS) (a litter-forager termite) were inhabited by a specific microflora that could enhance with the ectomycorrhizal fungal development. We tested the effect of this feeding group mound material on (i) the ectomycorrhization symbiosis between Acacia holosericea (an Australian Acacia introduced in the sahelian areas) and two ectomycorrhizal fungal isolates of Scleroderma dictyosporum (IR408 and IR412) in greenhouse conditions, (ii) the functional diversity of soil microflora and (iii) the diversity of fluorescent pseudomonads. The results showed that the termite mound amendment significantly increased the ectomycorrhizal expansion. MS mound amendment and ectomycorrhizal inoculation induced strong modifications of the soil functional microbial diversity by promoting the multiplication of carboxylic acid catabolizing microorganisms. The phylogenetic analysis showed that fluorescent pseudomonads mostly belong to the Pseudomonads monteillii species. One of these, P. monteillii isolate KR9, increased the ectomycorrhizal development between S. dictyosporum IR412 and A. holosericea. The occurrence of MS termite mounds could be involved in the expansion of ectomycorrhizal symbiosis and could be implicated in nutrient flow and local diversity

Lavendula species as accompanying plants in Cupressus replanting strategies: Effect on plant growth, mycorrhizal soil infectivity and soil microbial catabolic diversity

International audienceThe general objective of this study was to measure the mycorrhizal dependencies of Cupressus and Lavandula species, to assess the contribution of Lavandula species to the mycorrhizal soil infectivity (MSI) and to determine the main effects of the association between a Lavandula species (L. multifida) and Cupressus arizonica on the soil microbial activities, on the growth and the mycorrhizal status of each plant species in experimental conditions. Three species of Lavandula (L. stoechas L., L. dentata L. and L. multifida L.) and three species of Cupressus (C. atlantica, C. sempervirens and C. arizonica) were tested. Plant species were inoculated with an arbuscular mycorrhizal fungus, Glomus intraradices, to determine their mycorrhizal dependencies in controlled conditions. The results showed that L. multifida and L. dentata were representatives of highly mycorrhizal dependent plant species as well C. arizonica and C. sempervirens. The contribution of L. multifida to the MSI was determined by the most probable number (MPN) method. In six different soils, L. multifida enhanced the mycorrhizal soil infectivity, even more if the soil was P deficient. When C. arizonica and L. multifida were grown together, the growth and mycorrhizal colonization of each species were higher than those recorded when each plant species grew alone. The impact of this dual cultivation on the growth and mycorrhizal colonization of C. arizonica was comparable to those recorded with C. arizonica seedlings previously inoculated with G. intraradices. In contrast, the microbial activities of the soil collected from each treatment were different. These results emphasize the role of “resource islands” and “nurse plants” of Lavender plants in the regeneration processes of tree species such as Cupressus spp. In addition, they confirm the role of AM fungi as a major factor contributing to the growth and co-existence of each of the plant species (L. multifida and C. arizonica)

Soil factors influencing the growth response of Acacia holosericea A. Cunn. ex G. Don to ectomycorrhizal inoculation

Australian acacias like Acacia holosericea are excellent candidates for the revegetation of arid zones in Africa. Their high ability to develop multiple symbioses with soil microorganisms is crucial to their rapid development in adverse climatic and edaphic conditions. These symbioses include nitrogen fixation with rhizobia, vesicular arbuscular mycorrhization and ectomycorrhization. We set up an experiment to test the growth response of A. holosericea to ectomycorrhizal inoculation in 13 different soils from Senegal. After autoclaving the soil, the experiment was conducted in a greenhouse for four weeks. Plants were inoculated with Pisolithus albus strain IR100. The following parameters were measured: plant biomass, N, P, K and Ca foliar composition, spontaneous nodulation rate, and ectomycorrhizal colonization. Data were analyzed in light of the physical, chemical and total microbial characteristics of the soil. The results demonstrated a global promoting effect of P. albus inoculation on plant (shoot and root) growth and on foliar P and K, together with a depressive effect on N, while Ca rates were barely affected. Interestingly, spontaneous nodulation with putative water- or airborne rhizobia was stimulated after P. albus inoculation. However, these nodules seemed poorly effective, as they failed to cause any change in plant growth or in foliar N composition within the nodulated and nonnodulated control plants. These results show that plant growth response to ectomycorrhizal inoculation is greatly dependent on soil characteristics, and that root ectomycorrhizal colonization is influenced by biotic factors such as soil microbiota. From a practical point of view, data from the present study demonstrate that it is possible to optimize the effect of fungi on plant growth by screening soils under nursery conditions

Biological control of Striga hermonthica by Cubitermes termite mound powder amendment in sorghum culture

Striga hermonthica (Del.) Benth is an obligate root hemi-parasite of several cereals. Its effect on cereal crops is the main constraint for food production in sub-Saharan Africa. Various control methods have been already proposed, but the infestation by these parasitic plants persists. An appropriated method for Striga management adapted for the African farmer is very much needed. In this study, amendment of soil infested by this phytoparasite with Cubitermes mound powder is proposed as chemical amendment and natural microbial inoculum, to promote plant growth and reduce damage by S. hermonthica on sorghum (Sorghum bicolor L.). The influence of Cubitermes mound powder on the development of several microbial groups (arbuscular mycorrhizal fungi, actinomycetes, saprophytic fungi) was investigated in a pot experiment with sorghum cultured in a sandy soil infested by S. hermonthica. In the amended soil, sorghum growth and mycorrhizal colonization of sorghum plants were significantly greater than in the control treatment. Mycorrhizal colonization was negatively correlated with the number of emerged Striga plants per pot and positively correlated with sorghum growth. The relationship with substrate-induced respiration (SIR) responses showed that amended soil was characterized by its response to hydroxybutyric acid (catabolic marker of mycorrhizal colonization) and non-amended soil by its response to phenylalanine. We noted that the number of emerged Striga plants in amended pots was significantly decreased. Since Cubitermes mound suspensions did not affect Striga seed germination under axenic conditions, it suggests that the amendment with Cubitermes powder reduces S. hermonthica infestation indirectly, i.e. via its effect on the indigenous soil microflora. Overall, it appears that management of Cubitermes mounds is a promising strategy to consider for effective protection of sorghum from Striga infestation

Use of mycorrhizal inoculation to improve reafforestation process with Australian Acacia in Sahelian ecozones

The aim of this study was to determine the effectiveness of the mycorrhizal inoculation on the early development of Australian acacias in a dry tropical environment. Field experiments were carried out with an Australian Acacia species (Acacia holosericea) and four fungal strains were tested: an arbuscular mycorrhizal (AM) fungus (Glomus intraradices) and three ectomycorrhizal isolates belonging to the Pisolithus and Scleroderma genera. Mycorrhizal inoculation has significantly enhanced the growth of A. holosericea seedlings after 4 months of culturing in the greenhouse conditions in a disinfected soil. Mycorrhized and non-mycorrhized (control) plants were then transferred into the field. one of the experiments (Experiment EC5) was used to determine allometric relations based on stem diameter by destructive sampling. Two forms of biomass equations were generated and compared: (i) form I equation incorporated only stem diameter as the independent variable and form 2 equation incorporated stem diameter and height (product of square of stem diameter and height, (DH)-H-2) as the independent variables. As no distinctive advantage of one form over the other was recorded, simplest equation (Eq. (1)) was used for above-ground biomass calculations. Mycorrhizal inoculations with one Pisolithus isolate (P. albus IR100) or G. intraradices have significantly improved the growth of A. holosericea after 18 months as well as after 30 months of plantation. These results show that mycorrhizal inoculation could be of great relevance to improvement of the reafforestation process of degraded soils with leguminous fast growing trees, more particularly in Sahelian ecozones

Fluorescent pseudomonads occuring in Macrotermes subhyalinus mound structures decrease Cd toxicity and improve its accumulation in sorghum plants

Cd-tolerant bacterial strains of fluorescent pseudomonads, mostly belonging to Pseudomonas monteillii, were isolated from termite mound soil (Macrotermes subhyalinus, a litter-forager and fungus-growing termite), in a Sudanese shrubby savanna, Burkina Faso. Such large mounds appeared as sites of great bacterial diversity and could be considered as hot spots of metal-tolerant fluorescent pseudomonads. Microbial isolates were inoculated to Sorghum plants (S. bicolor) in glasshouse experiments with soil amended with CdCl2 (560 mg Cd kg(-1) soil). Microbial functional diversity was assessed at the end of the experiment by measurement of in situ patterns of catabolic potentials. All the bacteria isolates significantly improved the shoot and total biomass of sorghum plants compared to the control. Results concerning root biomass were not significant with some strains. Arbuscular mycorrhiza (AM) was greatly reduced by CdCl2 amendment, and fluorescent pseudomonad inoculation significantly increased AM colonisation in the contaminated soil. The bacterial inoculation significantly improved Cd uptake by sorghum plants. Measurement of catabolic potentials on 16 substrates showed that the microbial communities were different according to the soil amendment. Soils samples inoculated with pseudomonad strains presented a higher use of ketoglutaric and hydroxybutiric acids, as opposed to fumaric acid in soil samples not inoculated. It is suggested that fluorescent pseudomonads could act indirectly in such metabolic processes by involving a lower rate of degradation of citric acid, in line with the effect of small organic acid on phytoextraction of heavy metals from soil. This is a first contribution to bioremediation of metal-contaminated sites with soil-to-plant transfer, using termite built structures. Further data are required on the efficiency of the bacterial strains isolated and on the processes involved. (c) 2006 Elsevier B.V. All rights reserved

Improvement of Cupressus atlantica Gaussen growth by inoculation with native arbuscular mycorrhizal fungi

Aims:The study aimed to determine whether inoculation with native arbuscular mycorrhizal (AM) fungi could improve survival and growth of seedlings in degraded soils of Morocco. Methods and Results:Soil samples were collected from the rhizosphere of Cupressus atlantica trees in the N'Fis valley (Haut Atlas, Morocco). AM spores were extracted from the soil, identified and this mixture of native AM fungi was propagated on maize for 12 weeks on a sterilized soil to enrich the fungal inoculum. Then C. atlantica seedlings were inoculated with and without (control) mycorrhizal maize roots, cultured in glasshouse conditions and further, transplanted into the field. The experiment was a randomized block design with one factor and three replication blocks. The results showed that a high AM fungal diversity was associated with C. atlantica; native AM fungi inoculation was very effective on the growth of C. atlantica seedlings in glasshouse conditions and this plant growth stimulation was maintained for 1 year after outplanting. Conclusions:Inoculation of C. atlantica with AM fungi increased growth and survival in greenhouse and field. Significance and Impact of the Study:The data indicate that use of native species of AM fungi may accelerate reforestation of degraded soils. Further studies have to be performed to determine the persistence of these mycorrhizae for a longer period of plantation and to measure the effects of this microbial inoculation on soil biofunctioning

Arbuscular mycorrhizas and ectomycorrhizas of Uapaca bojeri L. (Euphorbiaceae) : sporophore diversity, patterns of root colonization, and effects on seedling growth and soil microbial catabolic diversity

The main objectives of this study were (1) to describe the diversity of mycorrhizal fungal communities associated with Uapaca bojeri, an endemic Euphorbiaceae of Madagascar, and (2) to determine the potential benefits of inoculation with mycorrhizal fungi [ectomycorrhizal and/or arbuscular mycorrhizal (AM) fungi] on the growth of this tree species and on the functional diversity of soil microflora. Ninety-four sporophores were collected from three survey sites. They were identified as belonging to the ectomycorrhizal genera Afroboletus, Amanita, Boletus, Cantharellus, Lactarius, Leccinum, Rubinoboletus, Scleroderma, Tricholoma, and Xerocomus. Russula was the most frequent ectomycorrhizal genus recorded under U. bojeri. AM structures (vesicles and hyphae) were detected from the roots in all surveyed sites. In addition, this study showed that this tree species is highly dependent on both types of mycorrhiza, and controlled ectomycorrhization of this Uapaca species strongly influences soil microbial catabolic diversity. These results showed that the complex symbiotic status of U. bojeri could be managed to optimize its development in degraded areas. The use of selected mycorrhizal fungi such the Scleroderma Sc1 isolate in nursery conditions could be of great interest as (1) this fungal strain is very competitive against native symbiotic microflora, and (2) the fungal inoculation improves the catabolic potentialities of the soil microflora

Responses of soil microbial catabolic diversity to arbuscular mycorrhizal inoculation and soil disinfection

Although it is usually admitted that arbuscular mycorrhizal (AM) fungi are key components in soil bio-functioning, little is known on the response of microbial functional diversity to AM inoculation. The aims of the present study were to determine the influence of Glomus intraradices inoculum densities on plant growth and soil microflora functional diversity in autoclaved soil or non-disinfected soil. Microbial diversity of soil treatments was assessed by measuring the patterns of in situ catabolic potential of microbial communities. The soil disinfection increased sorghum growth, but lowered catabolic evenness (4.8) compared to that recorded in the non-disinfected soil (6.5). G. intraradices inoculation induced a higher plant growth in the autoclaved soil than in the non-disinfected soil. This AM effect was positively related to inoculum density. Catabolic evenness and richness were positively correlated with the number of inoculated AM propagules in the autoclaved soil, but negatively correlated in the non-disinfected soil. In addition, after soil disinfection and AM inoculation, these microbial functionality indicators had higher values than in the autoclaved or in the non-disinfected soil without AM inoculation. These results are discussed in relation to the ecological influence of AM inoculation, with selected fungal strains and their associated microflora on native soil microbial activity