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Soil salinity is one of the worst consequences of global climate change, which is declining the world agriculture production. Pakistan is more vulnerable to climate variability where increasing soil salinity and drought are alarming for agriculture produce. Researchers are devoting attention for the use of microbial strategies to improve plant tolerance against abiotic stresses under extreme environment because of their ecological benefits. Plant growth promoting rhizobacteria (PGPR) induce salinity tolerance in plants with their multiple mechanisms. Despite the fact that bacterial inoculation improves the plant growth but their field application is limited due to inconsistency of results and some environmental constraints. However, efficiency of bacterial inocula for plant growth promotion may be increased under field conditions by employing bacterial consortia instead of using single strain inoculation. Therefore, present study was conducted to evaluate comparative effectiveness of single, dual and triple strain inoculation of salt tolerant PGPR for improving growth and yield of wheat under salt affected conditions. Twenty five rhizobacteria having 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity were isolated from wheat rhizosphere and were tested for their salt tolerance as well as plant growth promotion potential under gnotobiotic conditions. Out of 25 rhizobacterial isolates, 7 isolates were statistically significant for improving growth of wheat under saline conditions. These 7 salt tolerant PGPR were tested for their compatibility of growth and synergism with each other. Out of 7, only 3 isolate found synergistic and showed abilities to coexist. On the basis of rrs (16S rRNA) gene sequencing, these 3 PGPR were identified as Bacillus cereus strain Y5, Bacillus sp. Y14 and Bacillus subtilis strain Y16. Selected PGPR strains showed variable ACC-deaminase activity, P-solubilization, exopolysaccharides production, biofilm formation and auxins biosynthesis, when used alone as well as in their dual and multi-strain combination for different plant growth promoting activities under normal and salt stress conditions. Growth pouch experiment under axenic conditions was conducted to evaluate selected PGPR for inducing salinity tolerance in wheat as single and multi-strain inoculation. In growth pouch assay, multi-strain inoculation increased the total dry weight of wheat up to 28, 54, 59 and 56% at 0, 50, 100 and 150 mM of NaCl salinity levels, respectively, as compared to respective un-inoculated control at same level of salinity. Furthermore, pot and field experiments were conducted in salt affected soils to confirm the consistency of results for plant growth promotion by selected PGPR alone as well as in their possible combinations. In pot experiment, maximum increases in number of tillers were observed with multi-stain inoculation which were up to 54 and 80% more as compared to respective un-inoculated control at 10 and 15 dS m-1 salinity levels, respectively. Multi-strain inoculation also increased the grain yield by 18, 20, 42 and 62 at 1.42 (original), 5, 10 and 15 dS m-1 salinity levels, respectively, as compared to respective un-inoculated control. Further, single and multi-strain inoculations showed positive effect on all growth and physiological attributes of wheat under salt affected field conditions. However, the response of single, dual and triple strain inoculation treatments was highly variable at selected four sites. Maximum increases in grain yield of wheat at field-I, field-II, field-III and field-IV were up to 17, 17, 23 and 26%, respectively, over un-inoculated control by multi-strain inoculation of Y5*Y14*Y16. It might be concluded from this study that multi-strain inoculation with PGPR containing ACC-deaminase could be very promising option for inducing salinity tolerance in wheat, most probably due to better root growth in salinity stress which might be attributed to elimination of root inhibiting factor like production of stress induced ethylene.
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