High salinity abolishes several stages of plant life ranging from the seed germination step to maturity. Many processes are inhibited, such as phytohormone synthesis and regulation, normal root and shoot development, nutrient uptake, photosynthesis, and DNA replication. Plant growth promoting bacteria (PGPB) are naturally colonizing plants and occur in the rhizosphere or non rhizosphere soil and benefi t plant growth by numerous processes. The importance of halotolerant PGPB resides in their ability to adapt to increased salinity by effi cient osmoregulatory mechanism to be able to continue regular cell functions. Thus, halotolerant PGPB are able to provide plants with their activities to challenge osmotic stress by supporting them in the restoration of essential activities, e.g., in their hormonal balance. Halotolerant PGPB stimulate plant growth under high salinity by using similar mechanisms like halosensitive PGPB, such as synthesis of indole acetic acid (IAA), gibberellins (GA), cytokinins (CK), abscisic acid (ABA), solubilization of insoluble phosphate , synthesis and excretion of siderophores , and production of ACCdeaminase to reduce high growth inhibitory levels of ethylene occurring in plants at salt stress conditions. Furthermore, some halotolerant PGPB are even able to colonize plants endophytically , produce various antimicrobial metabolites against pathogenic fungi and bacteria, support plant health by improving systemic resistance and contribute to soil fertility and remediation .
The application of plant growth-promoting bacteria (PGPB) with biocontrol activities as inoculants of crops plants against phytopathogenic fungi and insect pests provides a biological alternative to the use of agrochemicals. Two Pseudomonas sp. strains were isolated from agricultural well water in the area of Bejaia, northeastern Algeria, located rather closely to a lead mine deposit. The isolates S4LiBe and S5LiBe had 16S rRNA gene sequence similarities of 99.4% to 99.7% with Pseudomonas protegens CHAO T and other P. protegens strains. The phenotypic profiles tested with BIOLOG-GN2-microplates were very similar, but showed also some remarkable differences. The isolates S4LiBe and S5LiBe showed plant growthpromoting potential based on the production of the phytohormone indole acetic acid and siderophores and the solubilization of insoluble phosphate. In addition, they produced chitinase and other polymer degrading enzymes. Interestingly, while S4LiBe and S5LiBe were resistant against heavy metals (2.0 mM K2Cr2O7 and 3.0 mM CoSO4, HgSO4, CdSO4 8H2O and PbCl2), the reference strain P. protegens CHAO T was very sensitive to Hg 2+ and Cd 2+ and had lower tolerance towards Co 2+ and Pb 2+ . The isolates S4LiBe and S5LiBe were very active in mycelial growth inhibition assays against Botrytis cinerea, Verticillium dahlia, Fusarium graminearum, Aspergillus niger and A. flavus (growth inhibition between 88% and 48%). Furthermore, S4LiBe and S5LiBe showed effective insecticidal activities, when tested in the Galleria injection assay and they were tested positive for the insect toxin gene fitD alike the reference strain CHA0 T . Finally, inoculation of barley seeds with S5LiBe resulted in significantly stimulated germination rate and growth of seedlings, with increased shoot length, shoot and root fresh weight, shoot and root dry weight as compared to non-inoculated plants.Thus, the heavy metal tolerant isolates S4LiBe and S5LiBe harbor a diverse potential as beneficial bacteria for agricultural application. They may be very useful even in polluted soils for the stimulation of e.g. biomass crops. The demonstration of successful isolation from agricultural well water may open more ready access for a wide variety of this kind of beneficial bacteria for agricultural application.
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