Plant immune inducers are substances that enhance a plant’s disease resistance by activating its own immune system. Chitooligosaccharides (COS) are a prominent example of this class, functioning through multiple mechanisms to induce resistance. The following sections outline the concept of plant immune inducers and the specific functions of chitooligosaccharides.
What Are Plant Immune Inducers?
Plant immune inducers—also known as plant resistance inducers, polysaccharide-based pesticides, or “plant vaccines”—are substances capable of activating a plant’s immune system and enhancing its stress tolerance. These agents do not directly kill crop pests or pathogens; instead, they function by triggering or activating the plant to produce resistance-related substances, thereby enabling the plant to resist pathogens or inhibit the growth of harmful microorganisms.
The core mechanism of action for plant immune inducers involves regulating endogenous signaling pathways to help plants withstand biotic and abiotic stresses—such as pathogens, pests, drought, and soil salinity—while simultaneously promoting growth and improving crop yield and quality. Unlike traditional chemical pesticides that target pathogens directly, plant immune inducers target the plant itself, achieving pest and disease control by bolstering the plant’s natural defenses against harmful external organisms.
There is a wide variety of plant immune inducers, including chitosan, Chitosan Oligosaccharide, seaweed polysaccharides, β-glucans, pectic polysaccharides, xylan, mannan, arabinogalactan, carrageenan, and alginates. As of the end of 2024, eight main types of plant resistance-inducing pesticides had been registered in my country—including amino-oligosaccharin, S-abscisic acid, lentinan, chitosan, oligosaccharides, glucan-oligosaccharides, chitooligosaccharide acetate, and amide-oligosaccharide acetate—comprising a total of 270 registered products.
The Role of Chitosan Oligosaccharide as Plant Immune Inducers
Chitosan Oligosaccharide are oligosaccharides produced through the enzymatic hydrolysis of deacetylated chitin; the raw material is derived from the shells of marine shrimp and crabs, and the product is manufactured using biological enzymatic hydrolysis technology. Its primary function is to activate the crop’s own immune system, enabling the plant to actively resist diseases and environmental stresses while simultaneously promoting growth and increasing both quality and yield; hence, it is known as a “plant vaccine.” Its mechanism of action involves a complete signaling cascade, ranging from recognition at the cell surface to intracellular signal transduction and the subsequent synthesis of defense compounds.
Chitosan Oligosaccharide are oligosaccharides produced through the enzymatic hydrolysis of deacetylated chitin; the raw material is derived from the shells of marine shrimp and crabs, and the product is manufactured using biological enzymatic hydrolysis technology. Its primary function is to activate the crop’s own immune system, enabling the plant to actively resist diseases and environmental stresses while simultaneously promoting growth and increasing both quality and yield; hence, it is known as a “plant vaccine.” Its mechanism of action involves a complete signaling cascade, ranging from recognition at the cell surface to intracellular signal transduction and the subsequent synthesis of defense compounds.
1. Specific binding to plant cells
Chitosan Oligosaccharide (COS) bind to plant cells via specific sites located on the cell wall and plasma membrane. Research utilizing laser confocal microscopy has demonstrated that COS molecules bind specifically to plant cells, marking the initial step in activating the plant immune system.
2. Activation of the salicylic acid signaling pathway
The salicylic acid (SA) signaling pathway is central to plant defense against biotrophic pathogens. Studies indicate that COS can induce plant resistance through SA-mediated signaling pathways. Research on Arabidopsis thaliana has revealed that the SA pathway plays a critical role in COS-induced resistance against Tobacco Mosaic Virus (TMV). Experiments show that COS induces resistance in both wild-type and jasmonic acid (JA) pathway-deficient *Arabidopsis* plants but fails to induce resistance in SA pathway-deficient plants, confirming the central role of the SA pathway in COS-induced resistance.
Research on chili peppers further confirms that COS treatment significantly upregulates the expression of SA pathway marker genes, such as NPR1, PR1, PR5, and PR10. The upregulation of these genes indicates the effective activation of systemic acquired resistance (SAR) in the plants.
3. Activation of the jasmonic acid signaling pathway
In addition to the SA pathway, COS can also activate the jasmonic acid (JA) signaling pathway. Studies have found that when COS induces resistance in Arabidopsis against Pseudomonas syringae pv. tomato, both the SA and JA pathways are activated. Notably, COS can still induce resistance in the JA-deficient mutant jar1, suggesting that the two pathways play redundant roles in COS-induced resistance.Chitosan Oligosaccharide treatment increases the expression level of lipoxygenase, a key enzyme in the jasmonic acid biosynthesis pathway.
4. Synergistic activation of calcium signaling pathways
Calcium ions act as second messengers, playing a crucial role in plant immune signal transduction. Research has shown that chitooligosaccharides induce resistance against Cucumber Mosaic Virus (CMV) through the synergistic interaction between the salicylic acid pathway and the calcium signaling pathway. A combination of proteomic analysis and experimental data indicates that this synergistic mechanism is a vital pathway by which Chitosan Oligosaccharide induce disease resistance in plants.
5. Induction of reactive oxygen species (ROS) burst and MAPK cascades
Upon binding to plant cells, Chitosan Oligosaccharide rapidly trigger the production of signaling molecules such as NO and H2O2, leading to a reactive oxygen species (ROS) burst. ROS not only possess direct antimicrobial activity but also function as signaling molecules involved in activating subsequent defense responses.
Chitosan Oligosaccharide also activate mitogen-activated protein kinase (MAPK) cascades. Using mRNA differential display analysis, 96 differentially expressed bands were identified following Chitosan Oligosaccharide treatment of tobacco plants; sequencing and homology comparison of one of these bands revealed it to be a MAPK gene closely associated with Chitosan Oligosaccharide-induced disease resistance.
6. Enhancement of defense enzyme activity
Chitosan Oligosaccharide treatment significantly boosts the activity of various defense enzymes. Studies on chili peppers have shown that such treatment enhances the activities of enzymes including catalase, peroxidase, superoxide dismutase, and phenylalanine ammonia-lyase. These enzymes play critical roles in scavenging ROS, reinforcing cell walls, and synthesizing defense-related compounds.
7. Significant disease control efficacy
Chitosan Oligosaccharide demonstrate remarkable disease control efficacy across various crops. In chili peppers, a 100 mg/L Chitosan Oligosaccharide treatment achieved a 56.65% control efficacy against Cucumber Mosaic Virus, outperforming Ningnanmycin (34.34%)(1) at the same concentration. In practical applications, Chitosan Oligosaccharide can increase yields by 10%–15% in rice and wheat and by over 25% in vegetables, while also reducing chemical pesticide usage by more than 30%(2). To date, Chitosan Oligosaccharide products have been deployed across more than 270 cities and counties nationwide, covering 31 major crop varieties with a cumulative application area exceeding 300 million mu-times.
Conclusion
Plant immune inducers represent an eco-friendly and sustainable plant protection strategy that enhances disease resistance and crop yields by activating the plant’s own immune system. As a key example of this class of agents, Chitosan Oligosaccharide effectively boost plant resistance against viral, bacterial, and fungal diseases through multiple mechanisms—including specific binding to plant cells, activation of salicylic acid and jasmonic acid signaling pathways, coordination of calcium signaling, induction of reactive oxygen species (ROS) bursts and MAPK cascades, and enhancement of defense enzyme activity. Consequently, they hold broad prospects for application in the field of green agricultural pest and disease control.