In vitro investigation of the resistance of bacterial isolates harboring plant growth promoting markers to some abiotic stress conditions

Plant growth-promoting rhizobacteria (PGPR) are defined as a group of microorganisms used as biocontrol agents for plant growth promotion and plant disease management. Endophytes include bacterial or fungal microorganisms that live between/inside plant tissues but do not cause visible disease symptoms in their host. In this study, the tolerance of 5 isolates, which gave remarkable results in terms of PGP (plant growth promoting) markers such as phosphate solubility, indole acetic acid production, siderophore production, nitrogen fixation in different studies carried out in our laboratory, to temperature, pH, salt, drought and mercury, lithium and cobalt heavy metals were investigated under in vitro conditions. In the study, endophyte bacterial isolates with code numbers G96Y1, G105K2, G106K1-SV, V14G3 and V27K3 showed optimum growth at 25oC, 30oC, 35oC, 40oC and 45oC, and V14G3 isolate was the most successful isolates even at 45oC. V27K3 isolate showed optimum growth up to 5% (w/v) salt concentration (including 5%) compared to the control tube. In vivo drought experiments showed that V14G3, G105K2 and G106Y1 isolates were the most successful isolates against drought stress. It was revealed that all three isolates showed significant growth up to 25% (w/v) PEG concentration (up to and including 25%) compared to the control tube. In heavy metal stress experiments, G96Y1 and V14G3 isolates showed growth on NB liquid medium containing 0.05 mM mercury (Hg) and were evaluated as the most resistant isolates against Mercury. When the growth profiles of the isolates at different concentrations of Lithium (Li) metal were examined, they showed a strong growth including NB liquid media containing 15 mM Lithium. When compared with the control tube, it was observed that all isolates were sensitive to Cobalt (Co) metal. They failed to grow at the concentrations tested. In addition, as a result of 16s rRNA sequencing, G96Y1, V14G3, V27K3, G106K1-SV isolates were matched with Acinetobacter calcoaceticus, Bacillus halotolerans, Bacillus zhangzhouensis and Pseudomonas kilonensis strains with 99.80%, 100%, 99.58% and 99.55% similarity rates, respectively.