Document Type : Complete scientific research article
Authors
1
soil
2
Dept. of Soil Science College of Soil & Water. University College of Agriculture. Gorgan University of Agricultural Sciences and Natural Resources,Gorgan, Iran
3
Genetics and Agricultural Biotechnology Institute of Tabarestan
4
soil science
5
Max Planck Institute, Magdeburg, Germany
Abstract
Background and objectives: Drought, as an uncontrollable stress factor, impacts nearly all stages of plant growth, either directly or indirectly. The genus Streptomyces is one of the most well-known groups of actinobacteria, with a remarkable ability to thrive in both saline and arid soils, often found in association with plants native to dry and wet environments. Given this, the present study aims to evaluate exopolysaccharide production by actinombacteria isolates under both normal and drought conditions (induced by polyethylene glycol 6000), select the most effective isolate, and assess its impact on growth, physiology, yield indicators, and phosphorus and potassium concentrations in the tomato variety Y (Lycopersicon esculentum) under drought stress.
Materials and methods: In this study, the growth and exopolysaccharide production of five Actinobacteria isolates were evaluated under moisture levels of 0, -0.29, and -0.73 megapascals, at three incubation times (7, 14, and 25 days) under laboratory conditions. The superior isolate was selected for the pot experiment, and its growth-promoting properties, including phosphate solubility and indole acetic acid production, were assessed. The pot experiment was conducted as factorial design arranged in completely randomized design with three replications. The experimental factors included Streptomyces (inoculation Bs-47, no inoculation B0) and moisture levels: 80-90% available water (no stress, D0), 45-60% available water (moderate stress, D1), and 20-30% available water (severe stress, D2). After the plant growth period, growth parameters, physiological indicators, nutrient uptake, and tomato yield were evaluated.
Results: According to the results, isolate 47 recorded the highest production of exopolysaccharides under drought stress levels of -0.73, -0.29, and 0 megapascals, with respective amounts of 4.26, 6.18, and 8.5 grams per liter after 25 days of incubation. The analysis showed that the percentage of total carbohydrates increased with the incubation period and drought stress levels in Streptomyces isolate 47. However, in isolates 16 and 91, total carbohydrates decreased under severe stress at -0.73 megapascals with increasing incubation time. Sequencing results of the selected isolate 47, which exhibited desirable exopolysaccharide production and growth-promoting characteristics, indicated the highest homology with Streptomyces chartreusis, and it was registered in the NCBI database under accession number KJ152149. Infrared spectrum analysis of the exopolysaccharide produced by the superior Streptomyces isolate showed a diverse pattern of absorption peaks ranging from 3450 to 820 cm⁻¹, confirming the polysaccharide nature and strong intermolecular and intramolecular interactions within the exopolysaccharide chains. Drought stress led to a reduction in growth characteristics, physiological traits, and phosphorus and potassium concentrations, while increasing leaf proline levels and antioxidant enzyme activity in the plant. Greenhouse monitoring results showed that under relatively severe stress (45-60% of available water), inoculation with Streptomyces increased aerial biomass (by 35.4%), root biomass (by 30.7%), stem diameter (by 30.6%), chlorophyll index (by 12.3%), relative water content of leaves (by 6.68%), phosphorus concentration (by 19.04%), potassium concentration (by 28.23%), and yield (by 155.3%) compared to the control (without inoculation). Inoculation with Streptomyces under relatively severe and severe stress showed a reduction in catalase enzyme activity by 19.4% and 16.6%, and in superoxide dismutase enzyme activity by 12.5% and 11.6%, compared to the control (non-stress conditions).
Conclusion:The results indicate that inoculation with Streptomyces, possessing desirable traits such as exopolysaccharide production and other growth-promoting characteristics, creates favorable conditions for the plant. By enhancing water retention and improving nutrient uptake in the root environment, it
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