What is the specific mechanism by which Bacillus subtilis promotes plant growth?

The specific mechanisms by which Bacillus subtilis promotes plant growth can be divided into the following key aspects:

1. Synthesis and regulation of plant hormones

Auxin (IAA) secretion:

      Bacillus subtilis synthesizes indole-3-acetic acid (IAA) through a tryptophan dependent pathway, which directly stimulates plant root growth, increases the number of lateral roots and root hairs, and expands the nutrient absorption area.

Cytokinin and Gibberellin:

      Secreting cytokinins (such as zeaxanthin) promotes cell division, gibberellin (GA) promotes stem elongation, and coordinates overall plant growth and development.

2. Enhancement of nutrient acquisition

Mineral element activation:

      Phosphorus solubilization: Secreting organic acids (such as citric acid and oxalic acid) to dissolve insoluble phosphates (such as calcium phosphate) in the soil, releasing available phosphorus that can be absorbed by plants.

      Potassium solubilization and nitrogen fixation: By secreting extracellular enzymes to decompose potassium containing minerals (such as feldspar), and synergistically fixing nitrogen in the air with nitrogen fixing bacteria, nitrogen sources are replenished.

Siderophores secretion:

      Chelate iron ions in soil to form plant absorbable iron iron carrier complexes, alleviating iron deficiency stress.

3. Enhancement of Adversity Resistance

Accumulation of osmoregulatory substances:

      Inducing plants to synthesize osmoprotectants such as proline and betaine, enhancing cell water retention capacity under stress conditions such as drought and salinity.

Activation of antioxidant system:

      Promote the activity of antioxidant enzymes such as superoxide dismutase (SOD) and peroxidase (POD), eliminate free radicals, and alleviate oxidative damage.

Induced Systemic Resistance (ISR):

      Activating the plant salicylic acid (SA) or jasmonic acid (JA) signaling pathways through lipopeptide compounds such as Surfactin enhances defense responses against pathogens and abiotic stress.

4. Optimization of rhizosphere microecology

Competition and antagonism:

      It preferentially colonizes the rhizosphere, competes with pathogens for space and nutrients, and secretes antibiotics (such as Iturin and Fengycin) to directly inhibit its growth.

Soil structure improvement:

      Decompose organic matter to form humus, increase soil aggregate stability, improve aeration and water retention.

Beneficial microbial synergy:

      Promote the colonization of symbiotic microorganisms such as arbuscular mycorrhizal fungi (AMF) to further enhance nutrient absorption efficiency.

5. Gene expression regulation

Transcriptional level regulation:

      By secreting signaling molecules (such as volatile organic compounds) to affect plant gene expression, upregulate genes related to growth and stress resistance (such as the WRKY transcription factor family).

summarize

      Bacillus subtilis forms a multifunctional network of "growth promotion disease resistance stress resistance" through the synergistic effect of four core mechanisms: hormone regulation, nutritional reinforcement, stress resistance activation, and microecological optimization. This mechanism not only directly promotes plant growth, but also achieves efficient yield increase and ecological protection in sustainable agriculture by improving the rhizosphere environment and activating the plant's own defense system.