SummaryArbuscular mycorrhizal (AM) fungi and bacteria can interact synergistically to stimulate plant growth through a range of mechanisms that include improved nutrient acquisition and inhibition of fungal plant pathogens. These interactions may be of crucial importance within sustainable, low-input agricultural cropping systems that rely on biological processes rather than agrochemicals to maintain soil fertility and plant health. Although there are many studies concerning interactions between AM fungi and bacteria, the underlying mechanisms behind these associations are in general not very well understood, and their functional properties still require further experimental confirmation. Future mycorrhizal research should therefore strive towards an improved understanding of the functional mechanisms behind such microbial interactions, so that optimized combinations of microorganisms can be applied as effective inoculants within sustainable crop production systems. In this context, the present article seeks to review and discuss the current knowledge concerning interactions between AM fungi and plant growthpromoting rhizobacteria, the physical interactions between AM fungi and bacteria, enhancement of phosphorus and nitrogen bioavailability through such interactions, and finally the associations between AM fungi and their bacterial endosymbionts. Overall, this review summarizes what is known to date within the present field, and attempts to identify promising lines of future research.

Attachment of certain bacteria to living arbuscular mycorrhizal fungal extraradical hyphae may be an important prerequisite for interactions between these microorganisms, with implications for nutrient supply and plant health. The attachment of five different strains of gfp-tagged soil bacteria (Paenibacillus brasilensis PB177 (pnf8), Bacillus cereus VA1 (pnf8), Pseudomonas fluorescens SBW25 :: gfp/lux, Arthrobacter chlorophenolicus A6G, and Paenibacillus peoriae BD62 (pnf8)) to vital and nonvital extraradical hyphae of the arbuscular mycorrhizal fungi Glomus sp. MUCL 43205 and Glomus intraradices MUCL 43194 was examined. Arthrobacter chlorophenolicus did not attach to hyphae, whereas the other bacterial strains did to a varying degree. Only P. brasilensis showed greater attachment to vital hyphae than nonvital hyphae of both Glomus species tested. Pseudomonas fluorescens showed a higher attachment to vital compared with nonvital Glomus sp. MUCL 43205 hyphae, whereas this relationship was opposite for attachment to G. intraradices. Both B. cereus and P. peoriae showed higher attachment to nonvital hyphae. This study provides novel evidence that under laboratory conditions soil bacteria differ in their ability to colonize vital and nonvital hyphae and that this can also be influenced by the arbuscular mycorrhizal fungal species involved. The significance of bacterial attachment to mycorrhizal fungal extraradical hyphae is discussed.

Arbuscular mycorrhizae are beneficial for crops grown under low-till management systems. Increasingly, it is becoming apparent that bacteria associated with mycorrhizae can enhance the beneficial relationship between mycorrhizae and plants. However, it has been difficult to study these relationships by conventional techniques. In this study actively growing bacteria were identified in soil from an undisturbed fallow field known to contain arbuscular mycorrhizae by using molecular tools to eliminate the need for cultivation. A thymidine analog, bromodeoxyuridine (BrdU), was added to the soil and incubated for 2 days. DNA was extracted, and the newly synthesized DNA was isolated by immunocapture of the BrdU-containing DNA. The active bacteria in the community were identified by 16S rRNA gene PCR amplification and DNA sequence analysis. Based on 16S rRNA gene sequence information, a selective medium was chosen to isolate the corresponding active bacteria. Bacillus cereus strain VA1, one of the bacteria identified by the BrdU method, was isolated from the soil and tagged with green fluorescent protein. By using confocal microscopy, this bacterium was shown to clearly attach to arbuscular mycorrhizal hyphae. This study was the first to use this combination of molecular and traditional approaches to isolate, identify, and visualize a specific bacterium that is active in fallow soil and associates with arbuscular mycorrhizal hyphae.

High numbers of bacteria are associated with arbuscular mycorrhizal (AM) fungi, but their functions and in situ activities are largely unknown and most have never been characterized. The aim of the present study was to study the impact of Glomus mosseae inoculation and plant type on the active bacterial communities in soil by using a molecular approach, bromodeoxyuridine (BrdU) immunocapture in combination with terminal-restriction fragment length polymorphism (T-RFLP). This approach combined with sequence information from clone libraries, enabled the identification of actively growing populations, within the total bacterial community. Distinct differences in active bacterial community compositions were found according to G. mosseae inoculation, treatment with an antifungal compound (Benomyl) and plant type. The putative identities of the dominant bacterial species that were activated as a result of G. mosseae inoculation were found to be mostly uncultured bacteria and Paenibacillus species. These populations may represent novel bacterial groups that are able to influence the AM relationship and its subsequent effect on plant growth.

This study evaluates the potential of Paenibacillus brasilensis strain PB177 to inhibit phytopathogenic fungi commonly causing maize diseases and to colonize maize plants. In vitro assays demonstrated antagonistic activity against the fungal pathogens, Fusarium moniliforme and Diplodia macrospora. The PB177 strain was tagged with the gfp gene, encoding the green fluorescent protein (GFP) and GFP-tagged bacteria were detected attached to maize roots by stereo-and confocal microscopy. The GFP-tagged bacteria were also used to treat maize seeds before challenging the seeds with two phytopathogenic fungi. The results demonstrated that the bacterial cells are mobilized to the maize roots in the presence of the fungal pathogens. The ability of P. brasilensis PB177 to inhibit fungal growth in vitro and its capability of colonization of maize roots in vivo suggest a potential application of this strain as a biological control agent. This is the first report on the successful introduction of the GFP marker gene into a P. brasilensis strain, enabling the direct observation of these promising plant growth promoting bacteria on maize roots in situ.