Growth and Competitive Infection Behaviors of Bradyrhizobium Japonicum at Different Temperatures

Bradyrhizobium japonicum is a key nitrogen‑fixing symbiont of soybean (Glycine max) and plays a central role in sustainable agriculture by converting atmospheric nitrogen (N₂) into bioavailable forms via root nodulation. Understanding how environmental variables—particularly temperature—affect its growth, infection capacity, and competitive behavior vs other rhizobia (e.g. Bradyrhizobium elkanii) is critical. Temperature determines not just the physiology of free‑living rhizobia in soil/rhizosphere but also the success of nodulation, nodule occupancy, and therefore nitrogen fixation efficiency.

This post synthesizes findings particularly from the study “Growth and Competitive Infection Behaviors of Bradyrhizobium japonicum and Bradyrhizobium elkanii at Different Temperatures” (Hafiz et al. 2021) and related literature. We will cover:

1. The effect of temperature on in vitro growth of B. japonicum and B. elkanii strains

2. Competitive infection / nodulation behavior at different temperature regimes

3. Strain‑specific differences among B. japonicum strains (fast vs slow growers; infection efficiency)

4. Mechanistic and ecological implications: nod gene expression, rhizosphere proliferation, and field nodulation patterns

5. Practical implications for soybean cultivation and inoculation strategies
   

1. Effect of Temperature on In Vitro Growth

Strains Examined

• Several strains of Bradyrhizobium japonicum (Bj11‑1, Bj11‑2 from Fukagawa soil; Bj10J‑2, Bj10J‑4 from Miyazaki soil)

• A reference Bradyrhizobium elkanii strain (BeL7) isolated from same soils. MDPI

Temperature Treatments

• Growth in Yeast Mannitol (YM) liquid medium

• Temperatures tested: 15 °C, 20 °C, 25 °C, 30 °C, 35 °C

• Shaking at ~125 rpm; OD₆₆₀ (optical density at 660 nm) measured daily for 7 days. MDPI

Observations

• B. japonicum strains show optimum growth at lower temperatures (≈15‑20 °C). At these temps, their growth rates are high and stable. At higher temperatures (above ~25‑30 °C), growth declines. Some B. japonicum strains fail to proliferate at 35 °C. MDPI

• In contrast, B. elkanii (BeL7) shows better growth at higher temperatures—optimal at ~25‑35 °C. Its growth is significantly lower at cooler temperatures (~15‑20 °C). MDPI

Quantitative Measures

• Relative growth percentages (OD relative to maximum for each strain) show that B. japonicum retains ~93–100% of maximum performance in the cooler temps (15‑20 °C), but drops as temperature increases. B. elkanii shows low relative growth at low temps, increases at higher temps. MDPI

• Among B. japonicum strains, some variation: e.g., Bj11‑2 vs Bj10J‑4 differ in tolerance thresholds. MDPI
  

2. Competitive Infection / Nodulation Behavior at Different Temperatures

The in vitro growth tells one part of the story. Nodulation (infection of the soybean roots, formation of nodules, and occupancy of those nodules by one strain vs another) is equally important when B. japonicum competes with B. elkanii, or even among B. japonicum strains.

Experimental Design (Competition / Infection)

• Soybean seeds (cv. Orihime, non‑Rj) were surface‑sterilized, then grown in sterilized vermiculite inoculated with mixtures of strains (three per set, from the same soil). Two sets: Fukagawa set, Miyazaki set. MDPI

• Two temperature regimes for plant growth (day/night): 20/18 °C (cool) vs 30/28 °C (warm) in phytotron under 16 h light / 8 h dark cycles. Plants grown for ~3 weeks. MDPI

• After 3 weeks, nodule number and composition analyzed: nodules sampled, DNA extracted, ITS (16S‑23S rRNA internal transcribed spacer) PCR and sequencing to determine which strain occupies each nodule. MDPI
  

Findings

• At low temperature (20/18 °C): B. japonicum strains dominate nodule occupancy. In the Fukagawa set, Bj11‑1 is almost exclusively in nodules; in the Miyazaki set, Bj10J‑2 dominates (with minor presence of another B. japonicum strain). MDPI

• At higher temperature (30/28 °C): B. elkanii (BeL7) dominates nodules in both sets. B. japonicum strains still present but in minor proportions; some do not nodulate effectively at high temps. MDPI

• Soybean growth (shoot/root length & weight) tends to be better at higher temperature (30/28 °C), though nodulation number (i.e. count of nodules) doesn’t always differ significantly between temperature regimes for a given inoculum mix. MDPI
  

3. Strain‑Specific Differences among B. japonicum

Not all B. japonicum strains behave identically. Key differences:

• Fast growers vs slow growers: In the Fukagawa set, Bj11‑1 (characterized as a slow grower) vs Bj11‑2 (fast grower) have similar growth curves at low temps, but when it comes to nodulation competition, Bj11‑1 often dominates over Bj11‑2 under low‑temperature regimes. This suggests that infection capacity (nodulation efficiency) may differ and is not strictly correlated with growth rate. MDPI

• Temperature sensitivity thresholds: Some B. japonicum strains lose capacity to grow or to nodulate when temperature increases beyond ~30‑35 °C, whereas B. elkanii handles high temperatures more robustly. E.g. Bj10J‑2 did not proliferate at 35 °C. MDPI
  

4. Mechanistic and Ecological Implications

Understanding how and why these differences occur helps in predicting field behavior.

Nod Gene Expression

• The nodulation (nod) genes, particularly nodC, have temperature‑dependent expression, at least in B. elkanii vs B. japonicum, which influences infection ability. Higher temperatures seem to up‑regulate nodC in B. elkanii, giving it a competitive edge in infection at warmer temperatures. MDPI

Rhizosphere Proliferation vs Infection

• Two parts to competitive success: proliferation in soil/rhizosphere (how well the bacteria multiply before infection) and infection efficiency (how well they attach, induce nodules, overcome plant defenses, colonize nodules). The studies suggest both are impacted by temperature. E.g. B. japonicum has better proliferation at lower temperatures, whereas at high temperatures, B. elkanii’s proliferation in the rhizosphere plus infection efficiency seem to surpass B. japonicum. MDPI

Latitudinal / Geographic Distribution Patterns

• These temperature‐dependent differences seem to underlie observed latitudinal patterns: in cooler, temperate zones (e.g. northern Japan, northern U.S., higher latitudes), B. japonicum dominates in soybean nodules. In warmer, subtropical/tropical zones (southern Japan, southern U.S., Nepal “south”) B. elkanii tends to dominate. MDPI+2ResearchGate+2
  

5. Practical Implications for Soybean Cultivation & Inoculation Strategies

Given all of the above, what should farmers, breeders, and agronomists consider?

Inoculant Strain Selection

• In cooler climates / growing seasons (or times of the season when soil temperature is low), choose B. japonicum strains that are adapted to low temperature growth and infection (such as Bj11‑1 etc.).

• In warmer soils or seasons, B. elkanii or B. japonicum strains tolerant to higher temperature are preferred.

Timing & Soil Temperature Management

• Seed inoculation and sowing might be timed so that root zone / soil temperatures are favorable to the inoculated rhizobia. For example, avoid sowing too early when soil is too cold (if relying on B. japonicum) or avoid very hot soil if using B. japonicum without heat tolerance.

Mixed Inoculations?

• Potential to use mixed inocula (combining B. japonicum + B. elkanii) to hedge risks, but must monitor for dominance and infection efficiency. Since B. elkanii can dominate at high temps, mixed inocula may result in unpredictable nodule occupancy.

Breeding for Host Compatibility

• Soybean genotypes may differ in their Rj status, response to nod gene signals, or susceptibilities. Some soybean varieties may preferentially interact with certain rhizobia strains. Such host genotype × rhizobia strain × temperature interactions need study.
  

6. Summary & Key Takeaways

Aspect	Temperature Low (~15‑20 °C)	Temperature High (~30‑35 °C)
B. japonicum growth rate	High	Reduced or inhibited
B. elkanii growth rate	Low	High optimal growth
Nodule occupancy / dominance	B. japonicum dominates	B. elkanii dominates
Infection efficiency (nod gene expression)	Favours B. japonicum	Favors B. elkanii
Soybean shoot/root growth	Lower than optimal	Generally better growth metrics
Agricultural implication	Use B. japonicum in cooler zones/seasons	Use B. elkanii or heat‑tolerant strains in warm zones/seasons

Deep Dive: Underlying Biological Mechanisms

To fully understand why B. japonicum and B. elkanii differ in behavior at different temperatures, it is worthwhile to look at:

1. Membrane fluidity and enzyme kinetics: At low temperature, membrane lipids become more rigid, enzyme systems slower. B. japonicum may have adapted membrane lipid composition and enzymes to function better in cooler temps. Conversely, B. elkanii may possess enzyme systems more stable or optimally active at higher temperatures.

2. Signal molecules / flavonoid recognition: Soybean roots secrete flavonoids which induce rhizobial nod gene expression. The capacity of B. japonicum to recognize these signals and induce nod genes may be more efficient at lower temperatures, whereas B. elkanii may need higher temps for that induction. The expression of nodC has been shown to be temperature‑sensitive. MDPI

3. Rhizosphere survival and competition: Soil microbes, competition for nutrients, stress from temperature, moisture etc., affect rhizobial survival. At elevated temperatures, B. japonicum may have poorer survival; B. elkanii more heat tolerant in soil context.

4. Host plant physiology: Soybean root growth, root exudation, nod factor perception, and defense responses are all temperature‐sensitive. Roots at low soil temp have slower growth, possibly affecting infection thread formation. In warm temps, faster root growth may favor rhizobia that can rapidly infect. These host side factors interact with rhizobial strain capabilities.
   

Limitations & Gaps in Current Knowledge

• Mixed colonization detection resolution: The studies noted that sequencing of ITS region sometimes shows only the dominant strain — minor mixed colonization might be below detection thresholds. So, actual mixed infections might be more common than reported. MDPI

• Long‐term field studies: Much of the work is in controlled lab or greenhouse conditions; field soils have complex microbiomes, fluctuating temperatures, moisture, etc., which may modify expected dominance patterns.

• Genomic bases of temperature tolerance: More work needed to identify genes (other than nod genes) in B. japonicum that confer temperature sensitivity or tolerance (e.g. chaperones, membrane components, regulatory elements).

• Interaction with other stresses: Temperature doesn’t act in isolation; moisture, pH, nutrient status, soil texture etc. also matter. Synergistic or antagonistic effects may occur.

Conclusion

Temperature critically influences both the growth and competitive infection behavior of Bradyrhizobium japonicum. At low to moderate temperatures (15‑20 °C), B. japonicum tends to grow well, infect soybean roots efficiently, and outcompete B. elkanii for nodule occupancy. At higher temperatures (≈30‑35 °C), B. elkanii gains the upper hand: its growth, nod gene expression, and infection success surpass B. japonicum. Within B. japonicum there is strain‐level variation in temperature tolerance and infection efficiency. These findings have practical implications for inoculant selection, sowing timing, cultivar choice, and predicting soybean performance under changing climate or in different geographic locations.