Non-Streptomyces endophytic actinomycetes from Bromus auleticus as a potential biocontrol tool for biotic stress alleviation and plant growth promotion

Plant pathogens can cause significant losses in plant production. To address this issue, it is crucial to develop innovative technologies that promote sustainability in production. One potential solution is the use of plant growth-promoting microorganisms, particularly those with biocontrol properties. In this study, root-endophytic actinomycetes isolated from Bromus auleticus, a South American forage grass, were investigated. Their ability to promote plant growth and their biocontrol activity against fungal phytopathogens were analysed. The actinomycetes were characterised molecularly, morphologically, and physiologically. Their ability to solubilise P, fixate N₂, produce phytohormones, nitrate, siderophores, phenazines and lytic enzymes was assessed. Both in vitro dual-antagonistic and in vivo biocontrol assays were performed. The strains were identified as species of the Micromonospora and Microbispora genus. They were able to produce auxins, solubilise phosphorus and release lytic enzymes, and nitrate. In vitro, both species of Micromonospora aff. halotolerans reduced the growth of fungal phytopathogens by 12%–34%. In particular, the strain Micromonospora aff. halotolerans TWB21 was found to reduce the pre-emergence effects of Fusarium oxysporum growth when the actinomycete was inoculated by seed immersion. Moreover, this strain improved the seed emergence, plant establishment and plant growth parameters under biotic stress. These results remark the importance of endophytic actinomycetes with biocontrol and plant growth-promoting abilities in enhancing their host establishment and biomass production. This reinforces the idea that Micromonospora strains could be applied as biocontrol agents on grasses with agronomic interest.