Rhizobacteria: The Microscopic “Service Team” Surrounding Plant Roots
1. What are rhizobacteria?
Rhizobacteria refer to a class of beneficial bacteria that colonize the surface of plant roots and the few millimeters of soil immediately surrounding them (known as the rhizosphere). They do not constitute a single species but rather comprise a diverse group of plant growth-promoting bacteria—collectively termed PGPR—including genera such as Pseudomonas, Bacillus, and Azospirillum. These bacteria utilize root exudates (such as sugars and amino acids) as nutrients, and in return, provide a variety of services to the plants.
2. Roles in Agriculture
(1)Direct Growth Promotion: They secrete plant hormones—such as auxins and gibberellins—that stimulate root elongation and increase lateral root formation, thereby fostering more robust seedlings.
(2)Enhancing Nutrient Availability:
- Nitrogen Fixation: For instance, nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, making it available for plant uptake.
- Phosphorus and Potassium Solubilization: They secrete organic acids that dissolve insoluble soil minerals—such as rock phosphate and potassium feldspar—releasing absorbable phosphate and potassium ions.
- Iron Chelation: They produce siderophores (iron-chelating agents) that assist plants in absorbing iron, particularly in calcareous soils where iron deficiency is common.
(3)Biological Disease Control:
- Production of antibiotics (e.g., phenazines) to inhibit pathogenic fungi.
- Secretion of chitinases to degrade the cell walls of pathogenic microbes.
- Competition for colonization sites on the root surface, thereby preventing pathogens from establishing a foothold.
(4)Induction of Systemic Resistance: Through molecules such as bacterial flagellin and lipopolysaccharides, they “awaken” the plant’s immune system, rendering the entire plant resistant to subsequent disease challenges—an effect that can persist for several weeks.
(5)Mitigating Abiotic Stress: For example, they produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, an enzyme that reduces ethylene levels within the plant during periods of drought, waterlogging, or salinity stress, thereby alleviating the damage caused by these adverse conditions.
3. Forms of Application
These bacteria are typically formulated as liquid inoculants or powder preparations, which are applied via seed coating, root dipping, or root drenching. Given their rapid proliferation and swift colonization capabilities, they are ideally suited for concentrated application during the seedling stage or at the time of crop transplanting.
Mycorrhizae: The Subterranean “Trade Network” Between Plants and Fungi
1.What are mycorrhizae?
Mycorrhizae are mutualistic symbiotic associations formed between soil fungi and plant root systems. The most common type is arbuscular mycorrhiza (AM), in which the fungi penetrate the interior of root cells to form tree-like “arbuscule” structures—serving as the interface for the exchange of substances between the two partners. Mycorrhizal fungi cannot survive independently and must rely on plants to provide carbon sources; conversely, plants utilize the fungi’s extensive hyphal networks to acquire water and mineral nutrients.
2.Role in Agriculture
(1)Ultra-long-distance Nutrient Uptake: Fungal hyphae have diameters of only 2–20 micrometers, yet they can extend tens of centimeters beyond the root system, even penetrating through microscopic soil pores. They are particularly efficient at absorbing phosphorus, expanding the effective absorption distance for soil phosphorus by more than a hundredfold. Simultaneously, they facilitate the uptake of trace elements such as zinc and copper.
(2)Significantly Enhanced Drought Tolerance: Hyphae act as microscopic conduits, drawing water from dry soil and transporting it to the root system, thereby enabling plants to maintain higher leaf water potential and photosynthetic rates under conditions of water stress.
(3)Improved Soil Structure: Mycorrhizal fungi secrete a ferric-containing glycoprotein called glomalin. Acting like glue, glomalin binds soil particles together to form water-stable aggregates, thereby increasing soil porosity as well as its water- and nutrient-retention capacities; this effect is particularly pronounced in sandy and degraded soils.
(4)Mitigation of Soil-borne Diseases: The dense network of hyphae forms a physical barrier on the root surface, preventing pathogens (such as Fusarium and Phytophthora) from reaching the root surface. Concurrently, mycorrhizal colonization induces plants to produce defensive substances, such as chitinases and peroxidases.
(5)Alleviation of Heavy Metal Toxicity: Polysaccharides and organic acids present on the surface of hyphae can adsorb and immobilize heavy metal ions—such as cadmium and lead—thereby reducing their translocation to the plant’s above-ground tissues. This characteristic makes them useful for the safe utilization of mildly contaminated agricultural lands.
3.Forms of Application
Products typically take the form of mycorrhiza-inoculated seedling substrates or granular inoculants containing fungal spores; inoculation is most effective during the seedling stage. Because mycorrhizal fungi cannot be cultured in pure artificial media (requiring propagation on living host plants), the production costs and storage requirements for these products tend to be relatively high.
Key Distinctions Between Rhizobacteria and Mycorrhizae
| Dimension | Rhizobacteria | Mycorrhizal Fungi |
| Biological Classification | Prokaryotes (no nucleus; unicellular) | Eukaryotes (possess a nucleus; multicellular hyphae) |
| Spatial Relationship with Roots | Primarily located on the root surface and in the rhizosphere soil; a few enter the root interior (e.g., endophytic nitrogen-fixing bacteria) | Some hyphae invade the interior of root cells to form arbuscules, while extensive hyphae radiate outward |
| Nutrient Exchange Mechanism | Utilize simple organic compounds found in root exudates; do not consume large quantities of photosynthetic products | Require 20–30% of the plant’s photosynthetic products (e.g., glucose) to serve as a carbon source |
| Primary Functional Specialties | Secrete hormones and antibiotics; nitrogen fixation; induce plant resistance | Absorb water, phosphorus, and trace elements; improve soil structure |
| Response to Phosphorus Fertilizers | High-phosphorus soils do not hinder their colonization | High phosphorus levels severely inhibit infection and function (as the plant reduces its carbon investment) |
| Host Range | Corresponding bacteria can be found in the rhizosphere of almost all plants | Approximately 80% of plant species (primarily herbaceous, plus some woody species); however, the Brassicaceae (e.g., rapeseed, cabbage) and Chenopodiaceae (e.g., spinach, beet) families do not form mycorrhizae |
| Difficulty of Artificial Propagation | Easy; can be mass-cultured using fermentation tanks | Difficult; must be propagated on the living root systems of host plants |
| Timing of Application | Suitable during the seedling, transplanting, or active growth stages | Best applied as an inoculum during the seedling stage; direct inoculation in the field often yields inconsistent results |
| Sensitivity to Fungicides | Inhibited by bactericides; however, most fungicides targeting fungi are harmless to them | Strongly inhibited by almost all fungicides (particularly benzimidazoles and triazoles) |
Synergies and Contraindications in Agricultural Applications
Synergistic Effects: “Mycorrhiza-helper bacteria” (such as certain *Bacillus* species) found in the rhizosphere secrete signaling molecules that facilitate the early colonization of host plants by mycorrhizal fungi; concurrently, the hyphal networks formed by these fungi provide migration pathways for the bacteria. Consequently, combined bacterial-fungal inoculants often demonstrate more significant yield-enhancing effects than single-strain inoculants.
Key Contraindications:
- Avoid High-Phosphorus Fertilization: Particularly in fields where mycorrhizal applications are utilized, phosphorus fertilizers should be applied sparingly as a basal dressing; repeated top-dressing with water-soluble phosphorus fertilizers must be strictly avoided.
- Exercise Caution with Fungicides: Fungicides such as Mancozeb, Chlorothalonil, and Carbendazim can severely disrupt and damage the mycorrhizal network.
- Minimize Deep Tillage: Deep plowing severs the hyphal network; therefore, the adoption of no-tillage or deep-loosening tillage practices is recommended.
Summary
- Rhizospheric bacteria act as “multifunctional special forces”: they excel at secreting bioactive substances, preventing disease, fixing nitrogen, and rapidly responding to localized environmental changes.
- Mycorrhizal fungi function as an “underground pipeline network”: they specialize in the long-distance transport of water and immobile nutrients (such as phosphorus and zinc), as well as improving soil structure.
Together, these two groups constitute a plant’s “second genome” and hold immense practical value within the context of sustainable agriculture—specifically in reducing chemical fertilizer use, enhancing stress tolerance, and protecting ecological integrity. In actual agricultural practice, the selection of appropriate microbial inoculants or combinations for application should be guided by specific factors: the type of crop (specifically, its propensity to form mycorrhizae), prevailing soil conditions, and the desired objective (e.g., disease prevention, growth promotion, or soil amendment).
More details about Rhizospheric bacteria and Mycorrhizal fungi,pls contact us.