Characterization of plant growth-promoting bacteria associated with rice cropped in iron-stressed soils

Plant growth-promoting rhizobacteria (PGPR) are able to promote plant growth using a wide variety of mechanisms as well as provide bioprotection against biotic and abiotic stresses. The objectives of this study were to isolate and characterize putative PGPR associated with rice cultivars with a distinct tolerance to iron toxicity grown in two areas: one area with a well-established history of iron toxicity and another without iron toxicity. Bacterial strains were selectively isolated based on their growth in selective media and were identified by partial sequencing of their 16S rRNA genes. Bacterial isolates were evaluated for their ability to produce indolic compounds, siderophores, and ACC deaminase and to solubilize tricalcium phosphates. In vitro biological nitrogen fixation was evaluated for the bacterial isolates used in the inoculation experiments. A total of 329 bacterial strains were isolated. The composition of the bacterial genera and the occurrence of different plant growth-promoting (PGP) traits were significantly affected by the iron conditions and by the cultivar. Strains belonging to the Burkholderia and Enterobacter genera were the most abundant of all the Gram-negative isolates, and those belonging to the Paenibacillus and Bacillus genera were the most abundant of the Gram-positive isolates. A large number of putative PGPR belonging to different bacterial genera presented several PGP traits. Strains belonging to the Burkholderia, Chryseobacterium, and Ochrobactrum genera contributed to plant growth as well as to enhanced nutrient uptake of the rice plants in in vivo experiments. Growth and nutrient uptake of plants inoculated with isolate FeS53 (Paenibacillus sp.) in the presence of an iron excess were similar to those of plants submitted to the control iron condition, indicating that this bacterium can mitigate the effects caused by iron stress.