The rhizosphere of Sulla spinosissima growing in abandoned mining soils is a reservoir of heavy metals tolerant plant growth-promoting rhizobacteria

Heavy metals tolerant soil bacteria are known to play essential roles in biogeochemical cycles, biotransformation of metals, bioremediation processes and, plant adaptation. The objective of this study was to isolate and characterize the bacterial strains associated with Sulla spinosissima, a native legume species growing in three abandoned mining sites situated in the Oujda region (East Morocco). Globally, more than 370 bulk and rhizospheric soil bacteria were isolated. Their identification by 16S rDNA sequencing showed that dominant phyla were Firmicutes, Actinobacteria, and Proteobacteria, while at the genus level Bacillus dominated, followed by Stenotrophomonas, Arthrobacter, and Rhodococcus. All the soils contained strains possessing plant growth-promoting traits. The best-performing strains were LMR283 for auxin production (145 mu g mL(-1)), LMR291 for phosphate solubilization (67.6 mg L-1), LMR280 for siderophore production (92.4%) and LMR326 for ACC deaminase activity (105 nmoL alpha-ketobutyrate mg(-1) h(-1)). Among tolerant PGP bacteria, 17 isolates showed antagonistic activity against the pathogen Fusarium oxysporum and 26 produced lytic enzymes. It was relevant that the rhizospheric soils prospected compared to bulk soils contained more interesting isolates for all the studied properties, in particular for soils sampled from Oued El Heimer and Sidi Boubker sites (respectively 79 and 63% of performing strains). Data presented here indicate that Sulla spinosissima growing in heavy metal soils is associated with multifarious active bacterial populations that probably sustain plant tolerance and growth under the prevailing stressful conditions. Superior strains identified are good candidates to be used with selected plants in rehabilitation programs in the contaminated ecosystems.