Mitigation of Copper Stress in Maize (Zea mays) and Sunflower (Helianthus annuus) Plants by Copper-resistant Pseudomonas Strains

Use of heavy metal (HM) resistant plant growth-promoting rhizobacteria (PGPR) is among the eco-friendly strategies to increase the resistance of crop plants against the HM stress. In this study, we investigated the effects of two copper (Cu)-resistant PGPR strains (Pseudomonas fluorescens P22 and Pseudomonas sp. Z6) on the growth and nutrition of maize (Zea mays) and sunflower (Helianthus annuus) plants grown in a Cu-contaminated soil under glasshouse conditions. Both PGPR strains significantly increased the plant vegetative parameters including shoot biomass, stem height and diameter, and chlorophyll (SPAD values) index in both crops. In both plants, the PGPR inoculations also significantly elevated the uptake of nutrients including potassium, phosphorus, calcium, magnesium (only by P. fluorescens P22), iron, zinc, manganese, and Cu. Magnitude of the nutritional effects varied between the PGPR strains, e.g., in sunflower, inoculation with P. fluorescens P22 and Pseudomonas sp. Z6 led to an increase in uptake of Zn by 42% and 114%, or Mn by 61% and 88%, respectively, in comparison with control plants. Improved performance of the inoculated plants can be attributed to the plant growth-promoting (e.g., production of auxin and siderophore, phosphate solubilization activities, etc.) and stress removal (e.g., production of ACC-deaminase to drop the ethylene level in stressed plants) properties of the PGPR strains, which were uncovered in our in vitro studies prior to the glasshouse experiment. Beside the plant growth-promoting traits of these PGPR strains, their high resistance to Cu toxicity seemed to be of particular importance for plant fitness improvement under Cu toxicity.