Bacteria stimulate plant growth via both direct and indirect mechanisms with variable results depending on a number of factors. High salinity is one of the most common environmental stress factor that adversely affect plant productivity by retarding the plant growth and development. To promote plant growth under saline condition, direct use of salt-tolerant bacteria has drawn considerable research interest both in industry and in academics. In the present study, a large number of halotolerant bacteria were isolated, and screened for their tolerance levels of NaCl. In this study, all the isolates at higher NaCl concentrations grew with long stationary phase. This could be due to the synthesis of protective factors and adaptation of current environmental conditions . Phylogenetic analysis of five halotolerant bacterial 16S rDNA gene sequence revealed them to belong to Bacillus and Hallobacillus species. These strains with high salt tolerance were further characterized for the PGP activities including IAA production, P-solubilization, HCN and siderophore production and ACC deaminase activity. Upadhyay et al.  found that only 18% (24 out of 130) of strains isolated from wheat rhizosphere in soils of Varanasi, were found tolerant to 8% of NaCl, while maintaining PGP activities. Siddikee et al. reported that different halotolerant bacteria were able to withstand high salt concentration (1.75 M NaCl) and were able to facilitate plant growth promotion in the presence of growth inhibitory levels of salt. In the present study 32% (27 out of 84) of strains could grow well at 15% NaCl and 25% (21 out of 84) at 20% NaCl. Tolerance of bacterial strains to higher salinity levels in the present study was probably because of the naturalization in the saline habitats. Five high salt tolerating bacteria were studied for their PGP activities. SL3 and J8W had multiple PGP activities (IAA and siderophore positive), however, SL32 and PU62 showed only siderophore and P-solubilization activities respectively.
PGPR that have ACC deaminase activity help plants to withstand stress (biotic and abiotic) by reducing the level of stress ethylene [9, 10]. In the present study none of the five strains studied produced ACC deaminase. Since all the five had mitigating effect of salinity stress on the wheat seedling, there must be other mechanisms by which these halotolerant bacteria are able to stimulate seed germination, root elongation and increase in root biomass. Since IAA secreted by bacteria may promote root growth directly, by stimulating cell elongation or cell division , the observed positive effect of strain Hallobacillus sp. SL3 on wheat seedling could be due to this mechanism. However, strain B. halodenitrificans PU62 which also enhanced these parameters could be due to some other mechanism as it did not show IAA production. It was observed that halotolerant strains, which produce IAA but not ACC deaminase, inhibited root growth rather than promote elongation in the presence of salt , reflecting a higher synthesis rate of ACC under stress. Alternatively, IAA concentration itself may have inhibited root growth. In our study the halotolerant bacteria producing neither IAA nor ACC deaminase enhanced root elongation and root dry weight under salt stress conditions. Also the deleterious effect of salt on seed germination rate was mitigated with inoculation. This could be the result of better water uptake induced by inoculation which is reflected in faster root growth in inoculated seedlings exposed to these stresses. A report showed that the turgor pressure at low water potential (20% PEG 6000) was higher in inoculated seedlings in two wheat cultivars under osmotic stress . Another way of protection is by production of exopolysaccharides which bind with cations, including Na+, and thus decrease the content of Na+ available for plant uptake [8, 25]. Obviously, there could be more than one mechanism that PGPR employ for protection against salt induced stresses.
In this study we have shown that different halotolerant bacteria, isolated from saline habitats are able to withstand high salt concentrations and can facilitate plant growth promotion in the presence of growth inhibitory levels of salt. Given the variation in plant growth promotion, the selection and subsequent commercial use of halotolerant bacteria with multiple PGP activities can be used as bioinoculants for saline environments. This would be an important area for future research.