LAUSR

Abstract Application of PGPRs that are tolerant to abiotic stresses is a promising approach to reduce the negative effects of drought stress and increase crop production. This study was designed to select and evaluate stress-tolerance PGP Streptomyces strains based on in vitro and in vivo experiments. Our results showed that only a small percentage (14 %) of 107 Streptomyces can produce ACCD. The effect of two selected isolates, IT25 an ACCD producing and C-2012 which did not have this ability but was able to tolerate NaCl 13 %, was evaluated on the growth and yield of tomato plants under normal and drought stress conditions. Under normal conditions, strain IT25 and C-2012 increased tomato yield 35 and 32 % respectively. Besides, decreased in fruit weight (by 60 %) under drought stress was significantly ameliorated in plants treated with Streptomyces strains. Both isolates, increased leaf RWC, proline, MDA, H2O2 and total sugar content and APX activity and decreased CAT and GPX activity under stress conditions. Plants exposed to drought stress exhibited a highly increased expression of ERF1 and WRKY70 in the roots. Both isolates inhibited expression of the genes under stress conditions. Streptomyces strains suppress stress signals, at biochemical and molecular levels to halt the plant's understanding of stress whereas, through increasing leaf RWC, compensated for the decrease in the yield. In conclusion, ACCD producing and salt tolerant Streptomyces both have the potential to prevent yield losses in tomato fruits under drought stress. However, the precise mechanism of their impact on the plant still needs to be addressed in future researches.