The Plant Growth-Promoting Potential of Root-Associated Bacteria from Plants Growing in Stressed Environments

Several studies have demonstrated the potential of plant growth-promoting bacteria, including their use as inoculants, in the contexts of both agriculture and enhanced phytoremediation. Despite their promise, there is still a need to characterize and identify plant growth-promoting bacteria amidst isolates cultured to date. Therefore the purpose of this study was to i) screen bacterial endophytes isolated from plants growing in both a chronically nutrient-deficient agricultural soil and a hydrocarbon-contaminated soil for plant growth-promoting potential in vitro and ii) assess the plant growth-promoting capabilities of isolates in vivo. Bacterial isolates belonging to genera common to both environments were screened for plant growth-promoting genotypes and phenotypes including ACC deaminase activity (acds), hydrocarbon degradation (alkB and CYP153), alkaline phosphatase activity (phoD), and nitrogen fixation (nifH). After screening, 28 isolates from 16 genera were subjected to further study in vivo. After using seed germination and root elongation screening in wheat (Trictum aestivum) and sweet clover (Melilotus alba), four isolates were selected for further study—A3 (Pseudomonas sp.) and A9 (Delftia sp.) in sweet clover and A12 (Kluyvera sp.) and B34 (Luteimonas sp.) in wheat—as they promoted early plant growth and development. Plant growth-promoting capabilities were then assessed by inoculating wheat and sweet clover seeds and growing plants in a growth chamber for 60 days in either a marginal agricultural soil or the same soil amended with diesel. Isolates A12 (Kluyvera sp.) and B34 (Luteimonas sp.) increased the acquisition of nitrogen (N) and phosphorous (P) by plants when growing in the marginal agricultural soil. However, plant growth-promoting effects were lost when diesel fuel was added. Further, no effect was observed in sweet clover in either soil condition. Initial screening for plant growth-promoting potential highlighted the importance of including functionality screening as the presence or absence of a plant growth-promoting genotype did not always indicate a positive phenotype. Further, this work exemplified the importance of in vivo screening assays to identify potential plant growth-promoting bacteria. Finally, results showed bacteria with plant growth-promotion potential can be isolated from stressed environments and may promote wheat growth in a marginal agricultural soil