Enhancing pearl millet drought tolerance: a synergistic approach using copper oxide nanoparticles and plant growth-promoting rhizobacteria

Background: Drought poses a substantial risk to plant growth in semiarid and arid regions, where pearl millet is predominantly cultivated. Previous studies showed that plant growth-promoting rhizobacteria (PGPRs), nanoparticles (NPs) and their combinations were used to alleviate the drought stress in plants. In this study, the effects of copper oxide (CuO) NP treatment, PGPR treatment, and their combination on pearl millet seedlings exposed to mild drought stress under greenhouse conditions was investigated. Methods: PGPR from the rhizospheric region of two varieties of pearl millet were isolated, purified, and identified via conventional and molecular methods. The plant growth-promoting abilities of the isolates, such as phosphate solubilization, growth hormone production, and hydrogen cyanide production, were tested and well-performing isolates were identified by 16S rRNA sequencing and BLAST analysis. The identified isolates were selected to investigate the potential advantages conferred on pearl millet exposed to mild drought stress by PGPR, CuO NPs individually or in combination. Results: Of the 11 isolates obtained, two presented promising plant growth-promoting (PGP) traits and were identified as Acinetobacter radioresistens and Stenotrophomonas maltophilia. Treatment with PGPR as well as NPs enhanced morphological characteristics such as shoot length, leaf length and plant height in pearl millet. In addition, treated groups presented increased plant chlorophyll content and antioxidant activity and reduced levels of drought stress markers, such as malondialdehyde, compared with those in control group. The present work proves that the combined action of CuO NPs and PGPR can aid plants in overcoming the harmful effects of drought.