The influence of Streptomyces violaceoruber and Kocuria rhizophila strains on Origanum vulgare L. growth promotion and bioactive metabolite composition.
Microbial strains belonging to the Actinomycetota phylum are known as plant growth-promoting (PGP) bacteria and are considered promising candidates for the development of biofertilizers due to their metabolic ability to produce diverse bioactive compounds. In this work, the actinomycetotal strains Kocuria rhizophila PA220 and Streptomyces violaceoruber DSM40783 were investigated using a two-step experimental approach: the first step focused on seed germination and provided visual evidence of initial seedling surface attachment via scanning electron microscopy (SEM); the second assessed PGP-associated responses in oregano rooted cuttings and relative differences in metabolite profiles, characterized through gas chromatography-mass spectrometry analysis of flowers volatilome. Seeds and rooted cuttings were inoculated (by priming or direct application, respectively) with the two individual strains or their consortium, alongside a non-inoculated control. For comparison, a commercial biofertilizer was included in the field experiment. Morpho-physiological parameters, including plant elongation and chlorophyll content, were monitored throughout growth. Both strains adhered to seedling surfaces, and S. violaceoruber was associated with higher seed germination rates. Under field conditions, S. violaceoruber was associated with higher chlorophyll content, whereas K. rhizophila was associated with increased shoot elongation at blooming. Increased relative abundances of p-cymene and terpinolene, monoterpenes with relevant bioactive properties, occurred alongside the actinobacterial consortium. Overall, these findings indicate that the tested actinomycetotal strains improved specific plant traits under both controlled and open-field conditions, and moreover align with changes on metabolite profiles. These results support further investigation of Actinomycetota-based biofertilizers as tools for sustainable management of Mediterranean aromatic crops.