The Role of Arthrobacter pascens 13LEP5 in mitigating Drought and Cold stress in Soybean (Glycine Max (L.) Merr.)

Technical Summary: The Role of Arthrobacter pascens 13LEP5 in Mitigating Drought and Cold Stress in Soybean

Problem Statement
Soybean (Glycine max L. Merr.) production in Europe is increasingly constrained by abiotic stresses, specifically drought and low temperatures. These factors limit yields by disrupting physiological processes such as root hydraulic conductance, leaf expansion, and flowering, while inducing premature senescence. While expanding cultivation areas is one strategy, the authors note that ensuring high-quality, abundant harvests requires advanced agricultural strategies, including the use of biostimulants. Current research on biostimulants for abiotic stress reduction is limited, particularly regarding local European strains and specific combinations of microbial and plant-derived agents. There is a need to evaluate the efficacy of novel bacterial strains, such as Arthrobacter pascens, and their synergistic potential with established symbionts like Bradyrhizobium japonicum and plant-derived protein hydrolysates.

Methodology
The study employed a controlled plant growth chamber experiment to evaluate six treatments on soybean seedlings:

1. Control: Water only.
2. AP: Arthrobacter pascens 13LEP5 (isolated from soybean leaf surfaces in Lithuania).
3. BJ: Bradyrhizobium japonicum (commercial inoculant).
4. PH: Plant-derived protein hydrolysate ("Trainer").
5. BJ+AP: Combination of B. japonicum and A. pascens.
6. BJ+AP+PH: Triple combination of all three agents.

Experimental Design:

• Preparation: Soybean seeds were sterilized and coated with the respective treatments (10 mL solution per 10 g of seeds).
• Growth Stages: Plants were grown through three distinct phases:
  • Optimal Growth (VE–VC): 20/18°C day/night, 80 mL water/day.
  • Stress Induction (VC–V1): A 7-day cycle of alternating temperatures (ranging from 5/4°C to 20/18°C) and reduced irrigation (35 mL/day) to simulate combined cold and drought stress.
  • Recovery (V1–V3): Return to optimal conditions (20/18°C) with increased irrigation (100–150 mL/day) for three weeks.
• Measurements: Data was collected at the VC stage (pre-stress) and V3 stage (post-recovery). Parameters included:
  • Biometric/Structural: 3D leaf area, plant height, convex hull circumference, digital biomass, total voxel volume, and light penetration depth (measured via PlantEye F500 3D multispectral scanner).
  • Physiological/Spectral: NDVI (greenness), NPCI (pigment ratio), PSRI (senescence), Hue Average, and Saturation Average.
• Analysis: Statistical significance was determined using ANOVA and Duncan's Multiple Range Test (DMRT) at p ≥ 0.05.

Key Results

• Strain Identification: Molecular analysis confirmed the isolate as Arthrobacter pascens 13LEP5, showing 99.57% identity with the type strain DSM 20545.
• Biometric Performance:
  • Pre-Stress (VC): The B. japonicum (BJ) single treatment and the triple combination (BJ+AP+PH) generally yielded the highest biometric values (leaf area, height, biomass).
  • Post-Stress Recovery (V3): A shift in efficacy was observed. The single A. pascens (AP) treatment and the triple combination (BJ+AP+PH) demonstrated the most robust recovery.
    • Leaf Area: AP-treated plants achieved the highest 3D leaf area (3905 mm²), significantly outperforming the control (2033 mm²).
    • Height: AP-treated plants were the tallest (68 mm), significantly exceeding the control (47 mm).
    • Biomass & Volume: The BJ+AP combination maintained the highest digital biomass post-stress, while AP and BJ+AP+PH achieved the highest total voxel volumes.
• Structural Complexity: The triple combination (BJ+AP+PH) resulted in the greatest light penetration depth (50 mm) post-recovery, suggesting a modification of canopy architecture that reduced self-shading.
• Physiological and Spectral Indices:
  • Biostimulants significantly improved physiological parameters (NDVI, Hue, Saturation) before stress induction compared to the control.
  • Post-stress, most spectral indices (NDVI, NPCI, PSRI, Hue) converged, showing no significant differences between treatments and the control, indicating that the stress level may not have been severe enough to cause permanent pigment degradation detectable by these indices.
  • Exception: The triple combination (BJ+AP+PH) was the only treatment that showed significantly different results in pigment indices compared to the control. Specifically, it achieved significantly higher saturation values (33.5%) compared to the control (27.4%) and the protein hydrolysate (PH) variant (25.4%), though it did not differ significantly from other biostimulant treatments.

Significance and Claims
The paper claims that biostimulant combinations are generally more effective than single applications for soybean biostimulation. Specifically:

1. Novelty of A. pascens: This study provides the first direct assessment of A. pascens in association with soybean, demonstrating its potential as a biostimulant that significantly enhances growth and stress recovery, likely through auxin (IAA) production and osmolyte accumulation.
2. Synergistic Effects: The triple combination (BJ+AP+PH) showed superior performance in structural development and light penetration, suggesting that protein hydrolysates may act as a carbon/nitrogen source for the bacteria, enhancing their efficacy under post-stress conditions.
3. Structural vs. Pigment Response: The authors conclude that biostimulants exert a more stable, long-term effect on structural plant development (biometrics, architecture) than on pigment accumulation. While spectral indices normalized post-stress, the physical growth parameters remained significantly improved.
4. Practical Application: The study suggests that while single applications (like AP) are effective for recovery, the combination of microbial strains with plant-derived hydrolysates offers the most comprehensive biostimulatory effect.

Limitations and Future Directions
The authors modestly note that the induced stress may not have been severe enough to cause significant chlorophyll degradation, which could explain the lack of differentiation in some spectral indices post-stress. They emphasize that field trials are necessary to determine optimal application timing, dosage, and frequency. Furthermore, the stability and compatibility of the A. pascens and B. japonicum consortium for long-term storage as a commercial product require further evaluation.