Enhancing Tomato Growth and NaCl Stress Using ACC Deaminase-Producing Streptomyces Isolate Alone or In Combination with Azotobacter vinelandii MM1

From the rhizosphere of Tomato plants, grown in saline soil in the western region, of Saudi Arabia, twenty-five actinobacterial strains were isolated on starch nitrate agar medium with 5% NaCl. All the isolates were screened on different concentrations of NaCl up 12%. The isolate SA5 was the most resistant isolate, thus, it was selected for detailed studies. The isolate SA5 showed positive results when screened for indole acetic acid production in a broth medium supplemented with 2 mg/ml L-tryptophan. The ability to reduce endogenous levels of ethylene produced by the plant, through the enzyme ACC-deaminase (1-aminocyclopropane-1-carboxylate) was confirmed in the toluenized cells. The isolates SA5 were identified as Streptomyces sp. SA5. Azotobacter vinelandii can grow in saline and enhance plant growth. Soaking Tomato seeds in Streptomyces (ST) or Azotobacter both culture filtrates (AZ+ST) increased significantly Tomato seed germination, growth and development. Moreover, soil inoculations with the bacterial cells of AZ, ST, or AZ+ST increased the chlorophyll a, b and carotenoid contents of tomato leaves in normal and under the stress of salinity.   There were significant increases in root depth, shoot length and shoot and root dry weights compared to the control under the same level of salinity. The amounts of phosphate, N, Mg, K and proteins present in tomato shoots, grown in normal and saline soil were also increased by soil inoculation. Increasing NaCl concentration increased proline, soluble sugar and esterase contents but soil inoculation decreased the adverse effects of NaCl and decreased them compared to control at the same salinity level. In conclusion, the results of this study indicated that Streptomyces, Azotobacter vinelandii or both could be utilized as biofertilizers in saline soils due to the production of plant growth-promoting agents, siderophore, indole acetic acid, and ACC deaminase, phosphate solubilization enzymes and tolerance to NaCl.