Exploring plant endophytic Streptomyces for their use in sustainable agriculture

Streptomycetes, together with other bacterial species such as Bacillus spp. and Pseudomonas spp., have been widely studied as valuable sources of strains able to promote plant growth and control plant athogens in agriculture. Streptomyces spp. are fundamental in pharmaceutical industry, however, their practical use in agriculture as plant growth promoting bacteria (PGPB) and biocontrol agents (BCAs) is still in its infancy. Based on the in vitro results, the most promising Streptomyces strains were tested in controlled conditions to evaluate their effect on germination and plant growth of various horticultural crops, including monocots, i.e. maize, wheat, and rice; and dicots, i.e. lettuce, lamb lettuce, rocket, savoy cabbage, soybean, onion, and tomato. The strains showed species-specific promotion of seed germination and plant growth. These effects were dose-dependent, however, higher inoculum dose not always resulted in better performance of the strain. The biocontrol potential of Streptomyces strains was tested on the pathosystem Lactuca sativa S. sclerotiorum. The most active strains were able to reduce the disease incidence by ca. 50%. The disease risk was reduced when the strains were applied to the S. sclerotiorum-infected soil one week prior to lettuce sowing. Our results confirm that Streptomyces have a great potential to act as BCA and PGPB, although, to be able to obtain their optimal performance, the inoculum dose, timing and the application method of individual strains need to be fine-tuned for specific crops.