The purpose of the current study was to determine the possible improvement in soybean plants’ tolerance against vanadium-induced stress in response to the application of olive solid waste (OSW) and selenium nanoparticles (Se-NPs), by assessing metabolites and plant defense systems. Drawing upon this aim, a pot experiment was performed where the soybean plants were grown with a fertilization treatment (including, control, OSW, Se-NPs, and Se-NPs + OSW) under vanadium stress (including non-stress and 350 mg sodium orthovanadate per kg of soil). Enhancement of hydrogen peroxide (H2O2) and malondialdehyde (MDA) accumulation in vanadium-stressed plants confirmed the oxidative damage in unfertilized plants. Results indicated the positive effects of the combined treatment (Se-NPs + OSW) in improving the plant stress tolerance by causing a balance in the produced ROS and detoxified ROS in the plant. It was mainly stimulated through the improvement of the photosynthetic parameters, anthocyanin metabolism pathway, phenylpropanoid pathway, non-enzymatic antioxidant metabolites (tocopherols, malondialdehyde, polyphenols, and flavonoids), antioxidant enzymes, and biochemical components involved in the ASC/GSH cycle (ascorbate, ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione, glutathione reductase, and glutathione peroxidase), and antioxidant direct scavenging enzymes (peroxidase, catalase, and superoxide dismutase), which finally resulted in higher plant biomass. In conclusion, the simultaneous application of OSW and Se-NPs treatments provided a reliable protection for soybean plants in vanadium-contaminated soils through the activation of antioxidant and non-antioxidant defense mechanisms.

The current research was carried out to evaluate the stress tolerance potential of durum wheat plants, in response to the inoculation of native plant growth-promoting bacteria (PGPB), through assessing PSII photochemistry and photosynthetic traits, as well as grain yield and plant height, and to investigate the possibility of using PGPB as a sustainable alternative or in combination with traditional fertilization plans. A greenhouse experiment included chemical/microbiological fertilization and stress (salinity and drought) treatments. The results indicated that the application of bacterial consortium of four PGPB markedly augmented some biochemical and functional traits in photosystem II, such as effective quantum yield of PSII photochemistry (Y(II)), electron transport rate of PSII (ETR), photosynthesis capacity, transpiration rate and stomatal conductance in unstressed plants, and prevented severe changes in the mentioned traits under drought and salinity conditions. The application of PGPB contributed to enhanced grain yield, too. Furthermore, a better performance of the PGPB inoculation was found in combination with half-dose of the recommended chemical fertilizers. In conclusion, PGPB inoculants maintain or improve the photosynthesis efficiency of durum wheat, grain yield and plant height, particularly under stress conditions, and can help to minimize the consumption of chemical fertilizers.

In this research, the optimum growth conditions for two zinc solubilizing bacteria (ZSB) have been studied for their potential application as bioinoculants to overcome Zn unavailability in soils. For this purpose, a laboratory-scale experiment was carried out to evaluate the zinc solubilizing ability of 80 plant growth promoting bacteria (PGPB) strains isolated from the rhizosphere of barley and tomato plants. To select effective ZSB, isolates were evaluated on Tris-mineral medium supplemented separately with zinc oxide, zinc carbonate, and zinc phosphate at a concentration of 0.1%. Two strains (Agrobacterium tumefaciens and Rhizobium sp.) were selected, based on a clear halo zone around their colonies in the solid medium supplemented with zinc oxide after 10 days of incubation at 29 °C. Results of solubilization at different pH values showed that these strains had solubilization activity in the range of pH 8-10 while no solubilization was observed at pH 6 and 7. The maximum Zn solubilization values were noted at pH 9: 51.4 mg L (Agrobacterium tumefaciens) and 72.1 mg L (Rhizobium sp). According to findings, bacterial growth was affected by different NaCl concentrations under in vitro condition. The salt concentration required for 50% inhibition of absorbance was 2.11 and 2.27% NaCl for Agrobacterium tumefaciens and Rhizobium sp., respectively. The maximum bacterial growth was observed at about 0.8% NaCl concentration.

The current research was performed to investigate the effects of three potassium-solubilizing bacteria (KSB) strains (Pantoea agglomerans, Rahnella aquatilis, and Pseudomonas orientalis) on leaf photosynthetic characteristics in rice (Oryza sativa L. cv. Pajohesh). A pot and a field experiment were conducted in a paddy field. The results indicated that the KSB inoculums significantly enhanced chlorophyll (Chl) a, Chl a+b, SPAD value, and stomatal conductance as compared to the control in both experiments, especially when applied along with half the recommended dose of the potassium chemical fertilizer. KSB inoculations, alone or combined with K fertilizer, significantly increased the value of photochemical quenching, photosynthetic electron transport rate, and the effective quantum efficiency as compared to the control. In conclusion, these native KSB strains could be used as inoculants to reduce consumption of K chemical fertilizer and improve the efficiency of photosynthesis for rice production under the flooding irrigation conditions.

This study aimed to isolate lactic acid bacteria (LAB) from wheat rhizosphere, to characterize their in vitro plant growth promoting activities and to differentiate plant-associated LAB from those associated with foods or human disease through comparative genomic analysis. Lactococcus lactis subsp. lactis and Enterococcus faecium were isolated using de Man-Rogosa-Sharpe (MRS) and Glucose Yeast Peptone (GYP) as enrichment culture media. Comparative genomic analyses showed that plant-associated LAB strains were enriched in genes coding for bacteriocin production when compared to strains from other ecosystems. Isolates of L. lactis and E. faecium did not produce physiologically relevant concentrations of the phyto-hormone indolacetic acid. All isolates solubilized high amount of phosphate and 12 of 16 strains solubilized potassium. E. faecium LB5, L. lactis LB6, LB7, and LB9 inhibited the plant pathogenic Fusarium graminearum to the same extent as two strains of Bacillus sp. However, the antifungal activity of the abovementioned LAB strains depended on the medium of cultivation and a low pH while antifungal activity of Bacillus spp. was independent of the growth medium and likely relates to antifungal lipopeptides. This study showed the potential of rhizospheric LAB for future application as biofertilizers in agriculture.