A Newly Isolated JAV4 Strain Enhances Drought Stress Tolerance in Soybean by Modulating Antioxidant Defense and Metabolite Accumulation

다국어 초록 (Multilingual Abstract)

Global warming is a major environmental stressor that limits agricultural productivity, with drought stress being a harmful abiotic stress impacting world agriculture. This leads to imbalances in physiological, oxidative, and metabolic processes, directly impacting photosynthetic levels, nutrient utilization, and crop yield. In crops such as soybean (Glycine max L.), this can reduce their productivity by up to 30% which is aggravated by climate change conditions. This study aimed to isolate and characterize strains of plant PGPR with the ability to tolerate maximum drought stress, evaluating their bio-stimulant effect under PEG-induced stress conditions. Ten bacterial strains were isolated from rhizospheric soil samples in drought environments at Kyungpook National University, of which JAV4 was selected for its tolerance to PEG and high production of exopolysaccharides, indoleacetic acid, siderophores and phosphate solubility, the strain was identified as Bacillus paramobilis, exogenous inoculation was evaluated in pots with eight treatments, this microbe inoculation significantly increased key parameters in stress-induced plants, including the 22.21% increase in relative water content, shoot length 27.80% and root weight 115.45%, in addition to improving spectral indices such as NDVI, CRI1 and MCARI by up to 30%, reflecting greater photosynthetic activity, on the other hand the application of microbe stimulated the activity of CAT and APX and reduced SOD, and ABA contents indicating less oxidation stress, which is why B. paramobilis JAV4 is considered an alternative to conventional fertilizers for its application as a biofertilizer and stimulant, with the potential to enhance the resilience of soybean crops to drought stress conditions. Keywords: Soybean, drought stress, PGPR, ROS, Bacillus paramobilis.

목차 (Table of Contents)

• 1.0 Introduction 1
• 1.1 Context and background 4
• 1.2 Global importance of soybean cultivation 4
• 1.3 Climate change's effects on agriculture 5
• 1.4 The role of rhizobacteria that promote plant growth as a sustainable substitute 5
• 1.5 Problem statement 6
• 1.5.1 Low yields due to drought and the need for biological alternatives 6
• 1.5.2 Presence of voids on the efficiency of Bacillus paramobilis under water stress in soybeans 7
• 1.6 Justification 7
• 1.6.1 Contribution to knowledge about PGPR under conditions of abiotic stress 7
• 1.6.2 Utilization of microorganisms to enhance drought stress tolerance without the application of agrochemicals 8
• 1.6.3 Minimizing synthetic input utilization through the enhancement of agricultural resilience 9
• 1.6.4 Increasing physiological efficiency, yield, and sustainability in soybeans 9
• 1.7 Research Objectives 10
• 1.7.1 Main Objective 10
• 1.7.2 Specific objectives 10
• 1.8 Hypothesis 10
• 2. Literature review 11
• 2.1 Soybean as a strategic crop 11
• 2.1.1 Economic, nutritional, and agro-industrial importance of soybean cultivation 11
• 2.1.2 Production regions, vulnerability to abiotic variables including drought and stress 11
• 2.2 Physiological and chemical impact on plants under drought stress 12
• 2.2.1 Drought impacts morphology, photosynthesis, and root growth. 12
• 2.2.2 ROS production and cell damage 13
• 2.3 Rhizobacteria that promote plant growth and their modes of action 13
• 2.3.1 Effects on plant growth, nutritional absorption, and water retention 14
• 2.3.2 Antioxidant activation and redox regulation by PGPR 14
• 2.4 Genus Bacillus as a multifunctional PGPR 15
• 2.4.1 General characteristics of Bacillus sp. 15
• 2.5 Spectral indices in the evaluation of plant stress 15
• 2.5.1 Defining and applying spectral indices 15
• 3.0 Materials and methods 17
• 3.1. Bacterial Screening 17
• 3.1.1 Strain isolates 17
• 3.1.2 Exopolysaccharide (EPS) production assay 17
• 3.1.3 Indole-3-Acetic Acid (IAA) Assay 17
• 3.1.4 Production of siderophores assay 18
• 3.1.5 Test of phosphate solubilization 19
• 3.1.6 Quantification of secondary metabolites 19
• 3.1.7 Drought stress tolerance assay in vitro 20
• 3.1.8 Molecular identification of the selected strain 20
• 3.1.9 Genomics analysis 21
• 3.2 Experiment pot 21
• 3.2.1 Experimental Design 21
• 3.2.2 Conditions of the experiment 22
• 3.2.3 Treatment application 22
• 3.2.6 Evaluation of plant morphological characteristics and non-invasive physiological reactions 23
• 3.2.6 Measurement of spectral indices assessment 24
• 3.2.7 Antioxidant enzyme activity assays 25
• 3.3 Statistical analysis 27
• 4 Results 28
• 4.1 Bacterial Screening 28
• 4.1.1 Functional characterization of isolates 28
• 4.1.2 Metabolite quantification 29
• 4.1.3 PEG-based drought tolerance 30
• 4.1.4 Molecular identification 31
• 4.1.5 Phylogenetic Tree 31
• 4.2 Pot experiment results 32
• 4.2.1 Percentages in relative water content 32
• 4.2.2 Morphological seedling characteristics 33
• 4.2.3 Measurement of spectral indices 37
• 4.2.4 Quantification of the plants´ antioxidant enzyme activities 40
• 4.2.5 Interaction and Functional Annotation 41
• 2.4.6 ABA modulation under drought 44
• 5. Discussion 46
• 6. Conclusions 48
• 7. Future prospective 49
• 8. References 50