The release of organic compounds from plant roots into the surrounding soil forms the basis for a versatile community of microorganisms that distinctly influences the productivity of plants (Compant 2005). Fungi and bacteria are a major constituent of this ecosystem, which has been termed the "rhizosphere" (Hiltner 1904, Whipps 2001). They modify growth and distribution of symbiotic and pathogenic organisms (Whipps 2001; Frey-Klett et al. 2005) or improve vitality and resistance of plants against pathogen attack (Wei et al. 1991, van Wees et al. 1997).
Streptomycete bacteria from Araucaria angustifolia stands: characterization and effects on growth of soil fungi in dual culture
The genus Araucaria is an important member of forests of the southern hemisphere. Because of clear cutting only small remnants of Araucaria angustifolia forests still exist in Southern Brasil. Attempts at reforestation have to be supported by exploring site conditions, which are optimal for the growth of this tree. In earlier studies we investigated both light requirement for seedling growth as well as the influence of symbiotic root fungus interactions (mycorrhiza) on nutrient and water acquisition (Breuninger et al. 2000). With regard to the latter, analysis of the root systems of Araucaria trees from forest and grassland (campo) sites revealed mycorrhizal structures (appressoria, penetration and coiled hyphae, vesicles, arbuscules, spores), which are characteristic for the arbuscular mycorrhiza (AM) type. The spores of AM fungi at both sites – forest and campo – were identified. The data indicated that the fungus biodiversity at the forest site was much higher, with 13 species, whereas only 6 species could be identified at the campo site. In addition to the greater biodiversity, the spore number in soil, as well as the percent of mycorrhizal colonization in roots were significantly higher at the forest site than at the campo site. These findings resulted in attempts to increase the spore potential of poor sites by so-called trap cultures with corn.
The rhizosphere, the narrow zone of soil around living roots, is characterized by root exudates, which attract soil microorganisms. These comprise not only fungi but also other microorganisms such as bacteria. Bacteria can affect the establishment of symbiotic root fungus interactions. We have isolated Gram-positive soil bacteria from the rhizosphere of a spruce stand rich with fly agaric (Amanita muscaria) fruiting bodies. Using a co-culture technique in Petri dishes, bacterial isolates were characterized by their effect on the growth of fungal hyphae. A group of bacterial strains was identified which significantly promoted growth of fly agaric hyphae. One of these strains was shown to additionally inhibit growth of pathogenic fungi such as Armillaria obscura (wide host range) and Heterobasidion annosum (causes wood decay in conifers). Taxonomic characterization of the effective bacterial isolates by their morphological appearance, by the analysis of diaminopimelic acid, cell wall sugars, and DNA sequencing (16S rDNA) identified them as actinomycetes, some of which are not yet contained in data banks.
Based on these data, we aim at investigating the role of streptomycetes from the rhizosphere of Araucaria. This will address both organismic and functional diversity, with a focus on bacterial exudates which exert a biocontrol of rhizosphere fungi (symbiotic and pathogenic) and/or affect plant pathogen resistance.
The project is a cooperation between the University of Tübingen and the PUCRS (Roman-Catholic University of Rio Grande do Sul, Brasilien; Prof. Landro Astarita)
Interactions between plants, streptomycetes and other microorganisms
Soil bacteria belonging to the actinomycetes, especially the streptomycetes are commonly found in the rhizosphere of plants. They are regarded as promising biocontrol organisms due to their potential to produce a vast array of secondary substances. They are capable of exhibiting beneficial as well as detrimental effects towards plants, including promotion of symbiosis, improved growth and biotic and abiotic stress resistance but also enhanced disease susceptibility and repressed defence responses.
We determined the specific and selective effects of some streptomycetes towards the development of symbiosis between plants and fungi. The mycorrhiza helper bacterium AcH 505 promotes the growth of mycorrhizal fungi and formation of mycorrhiza between Amanita muscaria and spruce (Picea abies, Schrey et al. 2005, Tarkka et al. 2006, Schrey et al. 2007). AcH 505 produces a fungal-growth promoting secondary metabolite (auxofuran) but also antibiotics that inhibit fungal growth of sensitive species (Riedlinger et al. 2006). The sensitivity of the fungal species largely determines the outcome of the interaction between fungus and AcH 505: species tolerant towards the antibiotic are promoted through auxofuran while sensitive ones are inhibited (Riedlinger et al. 2006). Also, there is specific plant factor involved in the interaction between spruce and AcH 505: the bacterium promotes plant root colonization by strains of a root pathogenic fungus (Heterobasidion sp., Lehr et al. 2007).
In contrast, a further streptomycete, GB 4-2, hinders colonisation of inner root tissues with the pathogenic fungus Heterobasidion sp. by modifying root anatomy and by increasing peroxidase activity in roots and needles. Needle photosynthetic efficiency, which serves as a vitality marker, is increased and needles show increased resistance against the pathogenic fungus Botrytis cinerea (Lehr et al 2008).
Inoculation of roots of the model plant Arabidopsis thaliana with certain streptomycete strains also increases photosynthetic activity and promotes seedling growth. Moreover, leaves show enhanced resistance against infection with the leaf pathogenic fungus Alternaria brassicicola.
The aim of our research is thus to understand how the interaction of plant roots with these beneficial soil bacteria influences the regulation of photosynthesis and triggers defence responses against leaf pathogens.