Improvement the nickel tolerance and phytoremediation capacity of safflower plant with bacteria isolated from the hyperaccumulator centaurea ensiformis P.H.davis plant

The increasing levels of nickel (Ni) across the globe are becoming a growing threat to ecosystems, particularly to plants. In agricultural areas, Ni contamination not only reduces crop yield but also poses a risk to food safety by allowing Ni to accumulate in organisms through the food chain. To address this issue, several strategies have been developed to either clean up contaminated soils or cultivate plants with enhanced tolerance. Among these, phytoremediation stands out as a promising approach, and its effectiveness can be significantly improved by integrating supportive agents such as plant growth-promoting bacteria (PGPB). In this study, a total of 13 endophytic bacterial strains identified from the microbiota of Ni-hyperaccumulator Centaurea ensiformis P.H. Davis (Ece sarıbaşı) were characterized for their Ni resistance and PGPB traits. Five selected strains (Peribacillus frigoritolerans, Bacillus sanguinis, Streptomyces pseudovenezuelae, S. canus, and S. griseorubiginosus) were individually inoculated into Carthamus tinctorius L. cv. Yenice (safflower). The objective was to enhance the plant's tolerance to Ni stress and its phytoremediation capacity, while also exploring the underlying physiological and biochemical mechanisms involved. Safflower plants inoculated with different PGPB strains were grown hydroponically under controlled environmental conditions (25°C, 16/8 h light/dark cycle, 250–350 µmol·m⁻²·s⁻¹ light intensity, 45–55% humidity) for 21 days. During the final 7 days, 0.75 mM Ni stress was applied. Ni was found to accumulate more in roots than in shoots, but the distribution varied depending on the bacterial strain used. Based on their contrasting effects, two strains—Streptomyces pseudovenezuelae and S. griseorubiginosus—were selected for further evaluation as part of different phytoremediation strategies. Inoculation with S. pseudovenezuelae increased Ni translocation to shoots by 29.9%, yet had no adverse effects on chlorophyll content, plant height, or biomass. On the contrary, it supported antioxidant defense mechanisms (especially POD and APX), helping the plant tolerate oxidative stress. S. griseorubiginosus, on the other hand, restricted Ni accumulation to the roots by 21.2%, effectively reducing shoot toxicity. This helped preserve membrane integrity, water content, chlorophyll levels, and photosynthetic functionality—ultimately enhancing dry biomass production and overall plant growth performance. In summary, S. griseorubiginosus appears to be a promising and safe biotechnological tool for cultivating safflower in Ni-contaminated agricultural lands where food safety is a concern, particularly through phytostabilization. In contrast, the S. pseudovenezuelae–safflower interaction shows greater potential for phytoextraction-based remediation due to its capacity to promote Ni uptake and translocation. Moreover, future research exploring combined applications of these two bacterial strains could lead to the development of more comprehensive and effective phytoremediation strategies by simultaneously enhancing metal removal efficiency and plant resilience.