Genomic and biological characterization of Streptomyces strains isolated from barley

Plant microbiota has received increasing attention in recent years. In particular, the microbiota associated with cereals is being extensively studied to identify bacterial strains that can promote plant health and growth. Barley is the fourth most important cereal worldwide in terms of agricultural production. Intensive barley agriculture requires the use of chemical fertilizers to compensate for nutrient deficiencies in soils and limit pathogen development. The isolation and use of bacteria that can enhance the bioavailability of soil nutrients and inhibit the development of plant pathogens could ultimately limit the use of these chemicals. In this study, we have isolated from a barley microbiota three bacterial strains belonging to the genus Streptomyces. These strains were characterized and named GPA1, GPAT2, and GPN2.

These three closely related isolates were from the same bacterial genus Streptomyces. Based on a phylogenetic analysis, the strains GPAT2 and GPN2 were classified as Streptomyces murinus, while GPA1 was identified as a new species. All strains showed antagonistic activity against two microorganisms that inhibit barley germination: Pseudomonas sp. MRN1 and Fusarium sp. CK. In addition, these strains exhibited different effects on the growth of barley cultivated under hydroponic and axenic conditions. In fact, GPN2 appeared to have no effect whereas the inoculation of barley seedlings with GPAT2 and GPA1 resulted in a reduction and an increase in root length after two weeks of growth, respectively. GPA1 had various Plant Growth-Promoting (PGP) abilities, including phosphate and zinc solubilization and siderophore production. A metabolite profiling of the GPA1 bacterial culture also showed its production and excretion of indole-3-acetic acid (IAA).

In this study, we have characterized three closely related bacteria, which display different effects on barley seedlings growth. These results revealed that the type of interactions of Streptomyces with barley is strain-dependent, suggesting that these interactions may arise from specific molecular mechanisms acquired through coevolutionary processes.