Friends and foes: streptomycetes as modulators of plant disease and symbiosis

Abstract

The ecological role of soil streptomycetes within the plant root environment is currently gaining increased attention. This review describes our recent advances in elucidating the complex interactions between streptomycetes, plants, pathogenic and symbiotic microorganisms. Streptomycetes play diverse roles in plant-associated microbial communities. Some act as biocontrol agents, inhibiting plant interactions with pathogenic organisms. Owing to the antagonistic properties of streptomycetes, they exert a selective pressure on soil microbes, which may not always be for plant benefit. Others promote the formation of symbioses between plant roots and microbes, and this is in part due to their direct positive influence on the symbiotic partner, expressed as, e.g., promotion of hyphal elongation of symbiotic fungi. Recently, streptomycetes have been identified as modulators of plant defence. By repressing plant responses to pathogens they facilitate root colonisation with pathogenic fungi. In contrast, other strains induce local and systemic resistance against pathogens or enhance plant growth. In conclusion, while streptomycetes have a clear potential of acting as biocontrol agents, care has to be taken to avoid strains that select for virulent pathogens or enhance disease development. We argue towards the use of an integrated screening approach in the search for efficient biocontrol agents, including assays on in vitro antagonism, plant growth, and disease suppression.

Microbiological Parameters of Soil under Sugar Beet as a Response to the Long-Term Application of Different Tillage Systems

“…Thanks to the production of antibiotics, they can protect plants against infestation with pathogens. In addition, they also form symbiotic associations with plants and fungi [23,24]. The variability of actinomycetes in dry soils is small and their greatest numbers were isolated from fertile and moist soils [25].…”

Effect of organic and inorganic fertilizer applications on phosphate solubilizing bacteria in the rhizosphere of maize (Zea mays L.)

“…Diverse PGPR strains have been used successfully for crop inoculations and to enhance plant growth (Kumar et al, 2012); these comprise members of the bacterial genera Azospirillum (Cassán and García, 2008), Bacillus (Jacobsen et al, 2004), Pseudomonas (Loper and Gross, 2007), Rhizobium (Long, 2001), Serratia (De Vleeschauwer and Höfte, 2007), Stenotrophomonas (Ryan et al, 2009) and Streptomyces (Schrey and Tarkka, 2008). Pseudomonas and Bacillus genera are the most commonly investigated PGPR, and often the dominating bacterial groups in the rhizosphere (Morgan et al, 2005).…”

Assessment of the Role of Local Strawberry Rhizosphere—Associated Streptomycetes on the Bacterially—Induced Growth and Botrytis cinerea Infection Resistance of the Fruit

“…Furthermore, more diseased fruit obtained in the streptomycete treated pots might indicate that some actinomycete metabolites perhaps convert the pathogen into more aggressive form such as the monosaccharide, flavin-like compound and valine fragments detected in the metabolites identified in the study. Interestingly, Schrey and Tarkka [35] recently documented the suppressive and promoter activity of the streptomycetes on plant pathogens and recommended caution in their selection and use as biological control agents.…”