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In the present study, the effect of phytohormone gibberellin on some physiological parameters of Tetraselmis suecica microalgae was investigated. Algae culture was performed under standard conditions with Gillard culture medium, temperature 25 ± 2 C°, light intensity 16: 8 (light/dark), salinity 25 ± 2 mg / l, and 24-hour aeration for ten days. Concentrations of 10, 20, 40, and 60 mg / l were added to the algae culture medium. Growth factors, pigments, photosynthetic factors, and biochemical composition were measured in the log phase. Based on results,  the highest rate of cell growth was observed in the treatment of 60 mg / l, although in other treatments there was a significant difference with the control (P <0.05). The highest amount of protein and carbohydrates was observed in the treatment of 60 mg / l (P <0.05). ). Total lipid content increased from 15.23 in control to 18.53 in 60 mg / l (P <0.05). Antioxidant potential, pigments, photosynthetic factors, and dry weight also showed a significant difference in the concentration of 60 mg / l compared to the control (P <0.05). The highest amount of chlorophyll a and chlorophyll b was observed at concentrations of 60 mg / l (P <0.05). Based on the results, gibberellin has a high potential for cell division, final density, antioxidant potential, cell size, and biochemical composition of Tetraselmis suecica. Also, Tetraselmis suecica can be a good candidate for marine aquatic larvae, drugs, antioxidants, and a rich source of compounds in biofuels.

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Bacteria of the genus Azospirillum are well known as plant growth promoting rhizobacteria. The ability to synthesize phytohormones is considered one of the most important mechanisms to promote plant growth and is widely distributed among plant-associated rhizobacteria.The most important phytohormone produced by Azospirillum is the auxin indole-3-acetic acid, with the L-tryptophan as the precursor. In the present study, we evaluate the capacity of eight Azospirillum strains isolated from rice and wheat, to produce in vitro auxins using plant exudates. Our results show that isolates produced auxins in tryptophan free media, but, generally, the amount produced increased when the tryptophan concentration increased. Some plants root exudates had a similar effect to tryptophan for the auxin production. In this sense, bean, rice and canola root extracts produced, respectively, 93.3%, 96.2%, and 88.31% more auxin than L-tryptophan. Azospirillum sp. isolate A3 had the maximal capacity to produce auxin. Therefore, the effect of cell free supernatant was studied on rice root development. Statistical analysis did not show any significant difference between root number and dry weight of the treated and control seedlings. However, significant differences were observed in root length and wet weight at α=0.01 and α=0.05, respectively.

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The flavonoid and phenolic compounds are among the main pharmaceutical components of medicinal plants. These compounds are considered as effective anti-oxidant sources. Five cyanobacterial extracts were used to stimulate the plant growth and increase production of specific secondary metabolites in Plantago major as a medicinal plant. These cyanobacteria were isolated from the growth bed of the plant in its natural habitats. Nitrate-free BG11 medium was used for preparing axenic monoalgal cultures. Pot experiments were performed by spraying cyanobacterial extracts on the soil of treated plants every 20 days from the time of planting. Growth of plants was evaluated by measuring growth parameters such as plant height, root length, dry and fresh weight of plant, leaf number, leaf area, as well as inflorescence characteristics 60 days after planting. In addition to growth factors, the total amount of phenol and flavonoid of plants was also assessed. Statistical analysis showed that there was a significant difference in the vegetative and reproductive characteristics compared to the control plants. Also, the methanolic extraction of treated and control plants displayed the highest total phenolic and flavonoid content 77.23±3.21 µg of GA mg^-1 and 389.67±34.43 µg of RU mg^-1 in plants treated with Cylindrospermum michailovskoence. Based on the obtained results, cyanobacterial fertilizers are suggested as the biological elicitors to improve the quantity and quality of medicinal plants products. As a result of this study, chemical content of cyanobacterial extracts and the production of plant growth stimulating substances such as phytohormones can be proposed as factors affecting plant growth parameters and metabolites production.