Biostimulants are a class of active substances derived from natural materials (such as seaweed extracts, humic acids, protein hydrolysates, and beneficial microorganisms) or synthetic analogs. They do not directly act as plant nutrients but instead systematically enhance plants’ resistance to biotic and abiotic stresses by regulating plant physiological, biochemical, and molecular processes. Their mechanisms of action are complex and multifaceted, centering on synergistically improving cellular defense systems, nutrient absorption efficiency, and endogenous hormone balance, thereby reshaping the plant’s stress resistance network.
Strengthening the Cellular Defense System: Building Multi-Layered Protective Barriers
Plants produce large amounts of reactive oxygen species (ROS) under stress conditions (drought, salinity, extreme temperatures, diseases, etc.), leading to oxidative damage. One of the core functions of biostimulants is to activate or enhance the plant’s endogenous defense system, achieving both “early warning” and “repair” protection.
1. Activating the Antioxidant Defense System
Enhancing antioxidant enzyme activity: Seaweed extracts (especially fucoidan and betaine from brown algae) and humic acids can significantly induce the gene expression and activity of key enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). For example, SOD converts superoxide anions into H₂O₂, while CAT and APX further decompose H₂O₂ into harmless water and oxygen, forming a complete ROS scavenging chain.
Promoting the synthesis of non-enzymatic antioxidants: Biostimulants stimulate plants to synthesize small molecule antioxidants such as glutathione (GSH), ascorbic acid (vitamin C), flavonoids, and carotenoids. These substances can directly quench ROS and participate in the ascorbate-glutathione cycle, maintaining cellular redox homeostasis.
2. Inducing Defense Gene and Stress Protein Expression
Initiating Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR): Certain protein hydrolysates (such as oligopeptides from plants) and microbial metabolites can act as elicitors, mimicking pathogen-associated molecular patterns (PAMPs), activating salicylic acid (SA) and jasmonic acid (JA)/ethylene (ET) signaling pathways, allowing uninfected tissues to enter a defensive state in advance.
Enhancing the accumulation of heat shock proteins (HSPs) and late embryogenesis abundant proteins (LEA): Under high temperature, drought, or cold stress, biostimulants regulate transcription factors (such as HSFs and DREBs) to promote the expression of HSPs (molecular chaperones that prevent protein denaturation) and LEA proteins (which protect membrane structure and protein activity), maintaining cellular structural integrity.
3. Strengthening Physical and Chemical Barriers
Promoting cell wall reinforcement: By inducing the phenylpropanoid metabolic pathway, increasing the deposition of lignin, suberin, and callose, enhancing the mechanical strength of the cell wall, and hindering pathogen invasion and non-stomatal water loss.
Stimulating phytoalexin accumulation: Stimulating plants to synthesize antibacterial substances such as flavonoids and alkaloids, directly inhibiting pathogen growth.
Optimizing Nutrient Absorption and Assimilation: Strengthening the Foundation of Stress Resistance
Under stress conditions, plant metabolism is disrupted, and nutrient absorption is hindered. Biostimulants improve the rhizosphere environment, root system configuration, and nutrient transport efficiency through direct and indirect effects, ensuring that plants still receive sufficient nutritional support for defense and repair under stress.
1. Improving Root System Structure and Function
Promoting root system morphogenesis: Humic acid, seaweed polysaccharides, and some amino acids have auxin-like activity, stimulating the development of lateral roots and root hairs, expanding the absorption surface area. Studies show that the total root length of plants treated with humic acid can increase by 20%-50%.
Enhancing root vitality: Increasing the activity of root dehydrogenase and ATPase, enhancing proton pump function, and providing energy for active nutrient absorption.
2. Activating Soil Nutrients and Promoting Absorption
Chelation and dissolution: Humic acid and fulvic acid have multiple carboxyl and phenolic hydroxyl groups, which can chelate metal ions such as Fe, Zn, Cu, and Mn, preventing precipitation, and dissolving fixed phosphates in the soil, improving the availability of phosphorus and potassium.
Regulating rhizosphere pH: By secreting organic acids or inducing root secretion, locally acidifying the rhizosphere, and activating trace elements in alkaline soils.
Enhancing symbiotic relationships: Some biostimulants can act as “signaling molecules” or carbon sources for mycorrhizal fungi and rhizobia, promoting mycorrhizal colonization and nodulation, and expanding the range of nutrient acquisition (especially phosphorus and nitrogen) through microbial networks.
3. Regulating Nutrient Transport and Assimilation Metabolism
Up-regulating transporter protein genes: Inducing the expression of nitrate transporter proteins (NRT), phosphate transporter proteins (PHT), potassium ion channels (AKT), and trace element transporter proteins. Enhancing the activity of key assimilation enzymes: Increasing the activity of nitrate reductase (NR) and glutamine synthetase (GS) to promote efficient nitrogen assimilation; activating phosphatases to promote organic phosphorus conversion.
4. Accumulating osmoregulatory substances
Under drought or salt stress, biostimulants (such as betaine and proline precursors in seaweed extracts) can promote the accumulation of osmoregulatory substances such as proline, betaine, and soluble sugars in plants, maintaining cell turgor and enzyme activity. This is an extension of nutritional balance under adverse conditions.
Fine-tuning hormone balance: Reshaping the growth-defense trade-off
Plant hormones are the core signaling hub in response to stress. Biostimulants help plants make optimal resource allocation between “growth” and “defense” by influencing hormone synthesis, metabolism, transport, and signal transduction.
1. Directly providing hormones or precursors
Some seaweed extracts contain naturally occurring plant hormones (such as auxin IAA, cytokinins, gibberellins) or their precursor substances, directly supplementing the endogenous hormone pool at very low concentrations.
2. Regulating endogenous hormone synthesis and metabolism
Promoting stress hormone synthesis: Moderately activating key enzymes in abscisic acid (ABA) synthesis (such as NCED) to help plants quickly close stomata, reduce water loss, and induce stress response genes.
Balancing growth and defense hormones: During non-stress periods, biostimulants may slightly increase auxin and cytokinin levels to promote growth; during stress periods, they prioritize strengthening ABA, JA, and SA pathways, inhibiting excessive growth and shifting resources towards defense.
Regulating hormone degradation: Affecting the activity of oxidases (such as the ABA degradation enzyme CYP707A) to prolong or shorten the action window of specific hormones.
3. Enhancing hormone signal sensitivity and interaction
Regulating receptors and signaling components: Affecting the expression abundance or activity of hormone receptors (such as the ABA receptor PYR/PYL), amplifying or weakening signal output.
Coordinating hormone signal interactions: JA and SA, auxin and ABA signaling pathways often exhibit antagonistic or synergistic interactions. Biostimulants optimize signal network integration by regulating nodes such as the MAPK cascade and transcription factors (e.g., MYC2, NPR1), thus avoiding growth inhibition caused by excessive defense responses.
Integrated Synergy: Building a Systemic Stress Resistance Network
The three major mechanisms of action of biostimulants are not isolated but form a highly synergistic network through signal transduction and metabolic reprogramming:
1.Early Warning and Signal Amplification: Biostimulants act as “triggering factors,” entering cells through membrane receptors or osmosis, triggering second messengers (such as Ca²⁺, ROS acting as signaling molecules), activating the MAPK cascade and hormone network, and amplifying stress resistance signals.
2.Metabolic Redirection: Under the guidance of hormones and defense signals, carbon and nitrogen metabolism shifts from growth to the synthesis of defense compounds (such as antioxidants, phytoalexins) and osmoregulatory substances.
3.Optimized Resource Allocation: Improved nutrient absorption provides substrates and energy for defense processes; enhanced cellular defense protects membrane structure and function, which in turn ensures the efficiency of nutrient absorption and transport; precise hormone balance directs the synergistic operation of the entire system.
Actual Stress Resistance Performance:
- Drought Resistance: Achieved through ABA-mediated stomatal regulation + osmotic adjustment + root system optimization + antioxidant protection, enabling efficient water use and conservation.
- Salt Tolerance: Regulates the SOS pathway to reduce Na⁺ accumulation and promote K⁺ absorption; enhances compatible solute synthesis; protects photosynthetic organs.
- Extreme Temperature Resistance: Induces HSPs and membrane lipid remodeling, maintaining membrane fluidity and enzyme activity.
- Disease Resistance: Enhanced physical barriers + phytoalexin accumulation + systemic resistance induction, reducing reliance on chemical pesticides.
Conclusion
Biostimulants represent an important tool in the shift of modern agriculture from “external input” to “activating endogenous potential.” Their essence is to enhance the “physiological resilience” of plants by regulating their epigenetics, transcriptome, proteome, and metabolome. By simultaneously acting on the three pillars of cellular defense, nutrient absorption, and hormone balance, biostimulants help plants achieve the following in adverse conditions:
- Faster perception and response speed
- Stronger damage repair capabilities
- More optimal resource allocation strategies
- More persistent systemic memory
This multi-target, systemic mode of action gives them a greater advantage than single-function products in coping with the complex combination of stresses brought about by climate change. Future research will further focus on the precise target sites of different biostimulant components, synergistic effects of formulations, and precise application strategies based on crop-environment interactions, thereby maximizing their value in sustainable agriculture.