BackgroundThe required amounts of chemical fertilizers (NPK) are determined by plant yield, and product quality is given less consideration. The use of PGPRs is an environmentally friendly approach that, in addition to increasing yield, also improves fruit quality. This study examined the role of specificStreptomycesstrains in aiding cucumber plants to 1) use fewer NPK fertilizers in the same quantity 2) improve the quality of cucumber fruit, and 3) promote growth and defense system.ResultsIn this study, the effect of 17Streptomycesstrains on the vegetative traits of cucumber seedlings of the Sultan cultivar was evaluated as the first test. Four strains ofStreptomyceswith the highest root and shoot dry weight were selected from the strains. This experiment was performed to determine the interaction effect of selected strains and different amounts of NPK on cucumber yield, quality, physiological and biochemical responses of plants. The first experiment’s results revealed that strains IC6, Y7, SS12, and SS14 increased significantly in all traits compared to the control, while the other strains dramatically improved several characteristics. Analysis of variance (ANOVA) revealed significant differences between the effect of strains, NPK concentrations, and their interactions on plant traits. The treatments containing 75% NPK + SS12, yielded the most fruit (40% more than the inoculated control). Antioxidant enzymes assay showed that SS12 substantially increased the activity of POX, PPO, and the expression of the genes related to these two enzymes. Hormone assay utilizing HPLC analysis revealed that various strains employ a specific mechanism to improve the immune system of plants.ConclusionsTreatment with strain SS12 led to the production of cucumbers with the highest quality by reducing the amount of nitrate, and soluble sugars and increasing the amount of antioxidants and firmness compared to other treatments. A specificStreptomycesstrain could reduce 25% of NPK fertilizer during the vegetative and reproductive growth period. Moreover, this strain protected plants against possible pathogens and adverse environmental factors through the ISR and SAR systems.