What are the mechanisms and specificity of mycorrhization helper bacteria?

It has long been known that the presence of free-living microbial communities stimulates mycorrhiza formation, and that this effect can also be induced by specific strains of bacteria isolated from mycorrhizal roots, which have been termed mycorrhization helper bacteria (MHB) (Garbaye, 1994). Although many MHB strains have been isolated and characterized, the mechanisms underlying MHB activity remain to be elucidated. Whereas MHB have been shown to have no particular preference for plant species, they have varying specificities for mycorrhizal fungus strains (Aspray et al., 2006a), although their impacts on growth and colonization of roots by saprotrophic and pathogenic fungi has received little attention. The paper by Lehr et al. (pp. 892–903), in this issue of New Phytologist, provides new evidence for the mechanisms by which one such MHB may operate and also demonstrates that a MHB can promote colonization of roots by the pathogen Heterobasidion abietinum. ‘... , the capacity of bacterial strains to enhance fungus growth in vitro is not a useful indicator of MHB activity, suggesting that effects on the plant are important in determining MHB activity.’ The mycorrhizosphere is recognized as the soil compartment inhabited and influenced by mycorrhizal fungi (Bending et al., 2006). For most plants this region represents the interface at which plants affect, and are affected by, the soil and its inhabitants. The rapid turnover of mycorrhizal fungus hyphae, the presence of mycorrhizal fungus exudates and mycorrhizal fungus-induced changes to plant exudation may all result in changes to the structure and activity of free-living microbial communities relative to the rhizosphere and bulk soil. Although these interactions can potentially impact the mycorrhizal fungus itself, the plant and soil nutrient cycling processes, knowledge of these impacts and their consequences is poorly understood. MHB are the best studied inhabitants of the mycorrhizosphere. Although most studies of MHB have been conducted in ectomycorrhizal systems, MHB have also been shown to occur in arbuscular symbioses (Duponnois & Plenchette, 2003). In ectomycorrhizal systems, MHB have been isolated from the mycorrhizospheres of many tree–fungus symbioses in nursery, plantation and seminatural situations. MHB strains belong to many different bacterial genera, and strains with the potential to act as MHB are readily isolated from the mycorrhizosphere and appear to have a ubiquitous distribution (Poole et al., 2001). Garbaye (1994) proposed five main mechanisms by which MHB could promote mycorrhiza formation, although to date there has been little robust evidence for the relative importance of any of them. Most evidence for MHB mechanisms has been gathered from in vitro studies of the interactions between mycorrhizal fungi and bacteria or their metabolites in the absence of the host plant, although there is evidence that some MHB require close proximity or contact with the plant to exert MHB effects (Aspray et al., 2006b). In their paper, Lehr et al. investigated interactions between Streptomyces sp. AcH505, a MHB of Amanita muscaria and Suillus bovinus growing with Norway spruce (Riedlinger et al., 2006), and strains of the root pathogen, Heterobasidion spp. Previous work in the absence of a host plant had demonstrated that Streptomyces sp. AcH505 produced a metabolite, auxofuran, which specifically stimulated growth of A. muscaria but inhibited growth of pathogenic fungi. In the current study, Lehr et al. (2007) found that Streptomyces sp. AcH505 suppressed the plant defence response. Out of 12 Heterobasidion strains tested, the growth of 11 were sensitive to metabolites produced by the MHB and, in the presence of the MHB, could not colonize Norway spruce roots. However, the growth of one Heterobasidion strain appeared not to be affected by the MHB strain, the presence of which stimulated colonization of the root by the pathogen. This increased pathogenicity presumably occurred because of the associated suppression of the host defence response by the MHB. A number of studies have suggested that plant roots produce metabolites which promote growth of mycorrhizal fungi towards the root. These compounds can act by increasing hyphal extension rates and by promoting hyphal branching, thereby directing growth towards and around the root (Akiyama et al., 2005). Furthermore, colonization of roots by compatible mycorrhizal fungi is also known to be associated with reduced or altered expression of host peroxidase (Mensen et al., 1998; Tarkka et al., 2001), which has been interpreted as indicating suppression of the host defence response by the mycorrhizal fungus and induction of changes in root cell wall structure, which facilitate growth of fungus through host tissue and ectomycorrhiza development. The findings of the current study therefore suggest that Streptomyces sp. AcH505 acts as an MHB by expressing a combination of mechanisms which add to existing signalling mechanisms operating between the mycorrhizal fungus and plant symbionts. The metabolite, auxofuran, appears to stimulate presymbiotic growth of the fungus, adding to the action of root-derived metabolites which direct growth of mycorrhizal hyphae towards the root. In addition, the MHB suppresses the host defence response, thereby adding to existing signalling pathways that occur between the mycorrhizal fungus and its host during mycorrhiza formation. Although the factors responsible for suppression of the host defence response by the MHB could not be determined, involvement of the metabolite auxofuran was ruled out, demonstrating that a combination of factors allow this strain to operate as an MHB. However, final proof of the relative importance of these mechanisms in contributing to MHB activity would need to be conducted in systems containing the ectomycorrhizal fungus. Whether the MHB mechanisms proposed for Streptomyces sp. AcH505 operate in other MHB systems remains to be elucidated. However, the capacity of bacterial strains to enhance fungus growth in vitro is not a useful indicator of MHB activity (Garbaye, 1994), suggesting that effects on the plant are important in determining MHB activity. Furthermore, some MHB appear to have specificity for certain strains of mycorrhizal fungus, suggesting that specific metabolite interactions of the type demonstrated for auxofuran produced by Streptomyces sp. AcH505 may operate more widely. MHB show a range of specificities, with some apparently specific to closely related strains of ectomycorrhizal fungus (Duponnois et al., 1993) and others capable of stimulating mycorrhiza formation by diverse ectomycorrhizal fungi (Aspray et al., 2006a), meanwhile, in one case, an MHB isolated from ectomycorrhizal systems was shown to promote formation of the arbuscular symbiosis (Duponnois & Plenchette, 2003). Inoculation of MHB with low specificity into a nonsterile environment containing complex ectomycorrhizal communities is known to have unpredictable consequences for the establishment of ectomycorrhizal fungus species on roots (Aspray et al., 2006a). In the publication of Lehr et al. (2007), this finding is extended, with a strain identified as an MHB also promoting root colonization by a pathogenic fungus. However, further work would be required to determine the relative extent to which root colonization by Heterobasidion spp. and ectomycorrhizal fungi are promoted by the MHB, and whether stimulation of the pathogen would have any consequences for disease development if ectomycorrhizal fungi were present. Nonetheless, the results have implications for the development of MHB as inocula, and further work is required to determine whether MHB could promote root colonization and disease development by other root pathogenic fungi. Although bacteria with the potential to act as MHB appear to occur everywhere, the activities of most MHB have been demonstrated in laboratory or glasshouse situations, and the extent to which they act in undisturbed mycorrhizospheres at natural cell densities and localization patterns within the soil remains to be demonstrated. However, the study of MHB provides insight into the way in which mixed natural microbial communities stimulate mycorrhiza formation, and MHB themselves have potential for biotechnological exploitation (Brule et al., 2001). Furthermore, bacteria inhabit many niches in close proximity to fungi, and interactions of the type shown by MHB could have a widespread distribution in nature (de Boer et al., 2005). For example, some bacterial strains can enhance the pathogenicity of foliar pathogens (Dewey et al., 1999). Genomics tools provide a promising avenue to unravel interactions within the mycorrhizosphere (Schrey et al., 2005). Identification of the mechanisms by which fungi and bacteria interact, and thereby factors controlling specificity, is clearly an important starting point for understanding not only the role of MHB in natural systems, but for understanding and exploiting the numerous fungus–bacteria interactions which occur in nature.