This research aimed at investigating the isolation and identification of bacterial strains with biological nitrogen-fixing capability and phosphate, potassium, and zinc solubilization activities from a durum wheat field under two different tillage practices including 10 years of conventional tillage (CT) and no-tillage (NT) practices. Attempts were also extended to estimate their relative abundances in the soil as well as to develop accurate mathematical models in determining the effect of different temperatures, NaCl concentrations and pH on the growth, and activity of selected isolates. Twelve effective bacterial strains, including Pseudomonas, Acinetobacter, and Comamonas genera, were identified with a great potential to solubilize the insoluble forms of phosphate (from 11.1 to 115.5 mg l⁻¹ at pH 8), potassium (from 32.2 to 35.6 mg l⁻¹ at pH 7), and zinc (from 1.11 to 389.90 mg l⁻¹ at pH 9) as well as to fix N₂ gas (from 19.9 to 25.2 mg l⁻¹). To our knowledge, this is the first report of the ability of Comamonas testosteroni and Acinetobacter pittii to fix nitrogen and to solubilize insoluble potassium compound, respectively. Three families, Moraxellaceae, Pseudomonadaceae, and Comamonadaceae, showed a higher percentage of abundance in the NT samples as compared to the CT, but only significant difference was observed in the relative abundance of Pseudomonadaceae (P < 0.01). These strains could be definitively recommended as inoculants to promote plant growth in the wide ranges of pH, salinity levels (with maximum growth and complete inhibition of growth from 0.67–0.92% to 3.5–9.3% NaCl, respectively), and temperatures (2.1–45.1 °C).

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.

Through metabolic analysis, the present research seeks to reveal the defense mechanisms activated by a heavy metals-resistant plant, Sesuvium portulacastrum L. In this regard, shifting metabolisms in this plant were investigated in different heavy metals-contaminated experimental sites, which were 50, 100, 500, 1000, and 5000 m away from a man-fabricated sewage dumping lake, with a wide range of pollutant concentrations. Heavy metals contaminations in contaminated soil and their impact on mineral composition and microbial population were also investigated. The significant findings to emerge from this research were the modifications of nitrogen and carbon metabolisms in plant tissues to cope with heavy metal toxicity. Increased plant amylase enzymes activity in contaminated soils increased starch degradation to soluble sugars as a mechanism to mitigate stress impact. Furthermore, increased activity of sucrose phosphate synthase in contaminated plants led to more accumulation of sucrose. Moreover, no change in the content of sucrose hydrolyzing enzymes (vacuolar invertase and cytosolic invertase) in the contaminated sites can suggest the translocation of sucrose from shoot to root under stress. Similarly, although this study demonstrated a high level of malate in plants exposed to stress, caution must be applied in suggesting a strong link between organic acids and the activation of defense mechanisms in plants, since other key organic acids were not affected by stress. Therefore, activation of other defense mechanisms, especially antioxidant defense molecules including alpha and beta tocopherols, showed a greater role in protecting plants from heavy metals stress. Moreover, the increment in the content of some amino acids (e.g., glycine, alanine, glutamate, arginine, and ornithine) in plants under metal toxicity can be attributed to a high level of stress tolerance. Moreover, strategies in the excitation of the synthesis of the unsaturated fatty acids (oleic and palmitoleic) were involved in enhancing stress tolerance, which was unexpectedly associated with an increase in the accumulation of palmitic and stearic (saturated fatty acids). Taken together, it can be concluded that these multiple mechanisms were involved in the response to stress which may be cooperative and complementary with each other in inducing resistance to the plants.