Summary• The influence of two fluorescent pseudomonads strains (HR13 and HR26) on the ectomycorrhizal symbiosis between Pisolithus alba and Acacia holosericea is reported here.• We measured ectomycorrhizal establishment, fungal growth in the soil (by HPLC) and soil microbial biomass (using the fumigation -extraction method) in treatments with or without pseudomonads.• Bacteria inoculated with the fungal symbiont stimulated ectomycorrhizal formation and shoot or root biomass. Only HR13 significantly increased fungal biomass in the soil. The bacteria stimulated fungal growth and production of phenolic compounds. Sequence analysis of the two fluorescent Pseudomonas revealed 99% homologuey between HR13 and P . monteilii , and 98% between HR26 and P . resinovorans .• It is clear that some bacteria (Mycorrhiza Helper Bacteria) can stimulate the establishment of the ectomycorrhizal symbiosis in tropical conditions.

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-Acacia holosericea plants were inoculated with a strain of Glomus aggregatum IR27 (arbuscular mycorrhizal fungus), Pisolithus tinctorius COI024 (ectomycorrhizal fungus) or with both fungi. Each fungus inoculated alone stimulated plant growth (height and shoot biomass). The response to the dual inoculation was greater than the response to either inoculant one. It may be due to the fact that the co-inoculated plants formed nodules through contaminations. However these nodules are inefficient as the N concentrations were similar in leaves of all inoculated plants with mycorrhizal fungi, alone and together. In thus, P, Ca, K, Mg and Na concentrations were not improved with respect to dual inoculation. The ectomycorrhizal colonization was significantly higher in the dually inoculated treatment than in either of the singly inoculated treatments. acacia / arbuscular mycorrhizas / ectomycorrhizas / dual inoculation Résumé -Influence de la double symbiose endomycorhiziennne et ectomycorhiziennne sur la croissance de Acacia holosericea (A. Cunn. ex G. Don.) en conditions de serre. Des plants de Acacia holosericea ont été inoculés soit avec une souche de Glomus aggregatum IR27 (champignon mycorhizien à arbuscules), soit avec Pisolithus tinctorius COI024 (champignon ectomycorhizien) ou avec les deux symbiotes fongiques. Chaque champignon a stimulé la croissance de la plante hôte (hauteur et biomasse aérienne). La double inoculation a induit une augmentation du développement de la plante supérieure à celle enregistrée lorsque les champignons étaient inoculés séparément. Ceci peut être la conséquence de la formation de nodules dus à des souches de Rhizobia contaminatrices. Toutefois, ces bactéries restent peu efficientes puisque les concentrations en azote dans les feuilles sont similaires dans les traitements avec chaque champignon ou lorsque ces isolats fongiques sont co-inoculés. Les concentrations en P, Ca, K, Mg et Na n'ont pas été modifiées par la co-inoculation. La colonisation racinaire par P. tinctorius COI024 a été significativement améliorée lorsque ce dernier a été inoculé avec le champignon mycorhizien à arbuscules. acacia / mycorhizes à arbuscules / ectomycorhizes / double inoculationAnn. For. Sci. 59 (2002) [93][94][95][96][97][98] 93

Acacia holosericea seedlings were planted in 1-l pots filled with a soil collected from an Australian Acacia plantation in Southern Senegal. After 6 months of culture, mycorrhizosphere soil, roots, galls induced by root-knot nematodes and Rhizobium nodules were sampled from each pot. The diversity of this bacterial group was characterized by siderotyping (pyoverdine IsoElectric Focusing (IEF) analysis) and by restriction fragment length polymorphism (RFLP). The effect of these isolates on the establishment of the ectomycorrhizal symbiosis between an Australian Acacia (A. holosericea) and Pisolithus sp. strain IR100 was studied. In the mycorrhizosphere soil, the population of fluorescent pseudomonads was represented by strains of two different siderovars (groups of bacterial strains presenting an identical pyoverdine-IEF pattern): siderovar 1 (74%) and siderovar 2 (26%). The siderotyping of the isolates around galls of the root-knot nematodes revealed three siderovars (40% from siderovar 1, 40% from siderovar 2 and about 15% from siderovar 3). RFLP of 16S rDNA divided the isolates into four different groups with MspI, two with HhaI and two with HaeIII endonucleases. The establishment of the ectomycorrhizal symbiosis with A. holosericea was promoted by 14 bacterial strains isolated from the mycorrhizosphere soil, three isolates from the roots and four from the galls. Shoot biomass of A. holosericea seedlings was stimulated by eight bacterial isolates from soil, six isolates from galls and seven from roots. These mycorrhiza helper bacteria could have a great ecological importance in tropical areas through the reforestation programs.

Six provenances of Acacia auriculformis have been tested for their ectomycorrhizal dependency with 2 indigenous strains of Pisolithus sp. Moreover, the endomycorrhizal and rhizobial symbiosis, the multiplication of plant parasitic nematodes have been quantified. The results showed significant variations in plant development between provenances. Both the compatibility with and dependency on the ectomycorrhizal strains were different among the provenances. Three main genera of nematodes have been identified, in particular S. cavenessi, which is one of the most pathogenic nematodes of soudano-sahelian crops. Endomycorrhizal structures were sometimes detected on the root systems. These results suggest that provenances of A. auriculiformis that are well adapted to the indigenous environmental conditions (pedoclimatic, soil borne pathogens) must be selected for their mycorrhizal dependency and for their resistance to plant parasitic nematodes. Moreover, the ectomycorrhizal symbiosis has to be considered as an important component of the cultural system.