Streptomyces calvus Isolate 27 Promotes Plant Growth Through Hormone Accumulation and Bioactive Compounds

Some actinobacterial species have been reported to improve plant growth due to their roles as biostimulants and biological control agents. In this study, the effect of actinobacterial isolate 27, obtained from the rhizospheric soil of melon plants and identified as Streptomyces calvus, was evaluated on the growth of Arabidopsis thaliana and tomato plants. In Arabidopsis, in vitro assays showed that after seven days of interaction, isolate 27 increased fresh weight by 1.4-, 1.5-, and 2.3-fold and lateral root number by 1.7-, 1.3-, and 2.5-fold under physical contact and split-plate systems (MS and ISP2 media), respectively, compared with non-inoculated plants. An increased β-glucuronidase (GUS, encoded by the uidA gene) signal was observed in primary and lateral roots of the Arabidopsis DR5::uidA reporter line during both interaction types, suggesting the activation of auxin-responsive pathways. In addition, isolate 27 rescued the rhd6 (root hair defective 6) mutant phenotype, restoring root hair formation. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that isolate 27 emitted volatile organic compounds (VOCs), including an alcohol and several sesquiterpenes, and that this profile changed during interaction with Arabidopsis plantlets. In soil-based pot assays, inoculation with isolate 27 significantly enhanced the development of Arabidopsis plants after 23 days, both when applied alone and in co-inoculation with Trichoderma atroviride. Furthermore, isolate 27 stimulated tomato plant growth, leading to significant increases in fresh and dry biomass, as well as shoot and root lengths after 28 days. Overall, these results demonstrate that S. calvus isolate 27 promotes plant growth and development through the production of bioactive compounds that modulate plant growth pathways, including hormonal responses, highlighting its potential as a bioinoculant for sustainable and productive agricultural systems.