Streptomycetes produce various lytic enzymes during their metabolic processes. Therefore, they appear to compete more efficiently against many other microorganisms present in the rhizosoil. The filamentous and sporulating nature of Streptomyces allows them to survive during unfavorable environmental conditions. include many saprophytes, some of them becoming beneficial plant endosymbionts, but also include a few plant pathogens. These microorganisms can increase agricultural production and improve soil fertility and, therefore, have great potential as a supplementary, renewable, and environmentally friendly source of plant nutrients. Mycorrhiza and rhizobia are, therefore, natural miniature fertilizer factories, an economical and safe source of plant nutrients compared to synthetic chemical fertilizers, which substantially contribute to environmental pollution. Mycorrhizal associations (ecto- and endomycorrhizae) are also pivotally important in ensuring plant growth and biomass through increasing nutrient and water uptake and enhancing plant resistance to abiotic and biotic stresses. In such cases, their plant-stimulating activity does not cease, but can continue in the colonized plant tissues. Rhizobacteria are also frequently found endophytically in roots and other plant parts, showing their ability to colonize their hosts. also belongs to the rhizospheric microbial communities, but only very recently their ability to act as plant growth promoters has been emphasized. In most cases, such growth-promoting rhizosphere bacteria belong to the following species: Alcaligenes, Arthrobacter, Azospirillum, Azotobacter, Bacillus, Burkholderia, Enterobacter, Klebsiella, Pseudomonas, and Serratia. For this reason, they might be considered efficient biofertilizers. Rhizosphere bacteria are able to enhance nutrient uptake from the rhizosoil by the plants that they colonize. Rhizobacteria and mycorrhizal fungi are among the microorganisms that have proved to be of highest efficacy in promoting plant growth and, therefore, crop productivity. Most of them are described as producers of metabolites positively interfering with plant life, for example, by enhancing nutrient acquisition or by stimulating plant defense mechanisms towards pathogens. Several different bacterial species have been identified colonizing plant tissues and vessels, from the root system up to the stem, leaves, and other plant organs. Ĭurrently, microbial endophytic communities are the focus of several studies aimed at unraveling and clarifying their role as plant growth promoters and their involvement in plant health. In one such symbiotic interaction, the roots of many plants are infected by specific fungi (mycorrhizal association), rhizobia, and actinobacteria (particularly streptomycetes) that help the plant to acquire nutrients from the soil. The microorganisms grow on plants as a resource of nutrients or habitat niche. Some of these interactions are beneficial, whereas some are detrimental to the plant. Plants are extensively colonized by a range of beneficial microorganisms and acquire a variety of plant–microbe interactions. The present review highlights Streptomyces spp.-mediated functional traits, such as enhancement of plant growth and biocontrol of phytopathogens. Additionally, their ability to promote plant growth has been demonstrated in a number of crops, thus inspiring the wide application of streptomycetes as biofertilizers to increase plant productivity. They are active producers of antibiotics and volatile organic compounds, both in soil and in planta, and this feature is helpful for identifying active antagonists of plant pathogens and can be used in several cropping systems as biocontrol agents. Streptomycetes belong to the rhizosoil microbial communities and are efficient colonizers of plant tissues, from roots to the aerial parts. and their metabolites may have great potential as excellent agents for controlling various fungal and bacterial phytopathogens. In agriculture, plant growth-promoting and biocontrol microorganisms have emerged as safe alternatives to chemical pesticides. There has been many recent studies on the use of microbial antagonists to control diseases incited by soilborne and airborne plant pathogenic bacteria and fungi, in an attempt to replace existing methods of chemical control and avoid extensive use of fungicides, which often lead to resistance in plant pathogens.
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