Document Type : Review Paper
Authors
1
Faculty of Plant Protection, Agricultural Research Center of Khorasan Razavi Province, Mashhad, Iran
2
Department of Plant Pathology, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran
Abstract
Introduction[1]
Chitosan is a natural polymer industrially derived from the deacetylation of chitin. Chitosan is recognized as a safe and biocompatible biological compound that acts by stimulating plant growth, inducing the plant’s general defense response, and increasing plant resistance to pathogens, including bacteria, fungi, nematodes and especially viruses. This compound has demonstrated significant efficacy as an elicitor of plant resistance to viral infections. Unlike other plant pathogens, effective treatments for viral infections are currently lacking. Two main strategies in managing plant viral diseases are the use of resistant cultivars and the induction of plant defense mechanisms using elicitors.
Elicitors are physical or chemical stimuli derived from biotic or abiotic sources that can induce physiological, morphological responses and the accumulation of phytoalexins in plant cells. Chitin is a linear polysaccharide that is known as the most abundant natural polymer after cellulose. Chitosan is obtained by removing the amide group from the chitin chain. The extraction of chitosan from natural sources containing chitin depends on various steps including deproteinization, deacetylation and demineralization. The chemical structure of chitosan depends on the molecular weight, the deacetylation degree (DD) and its pattern. Unlike chitin, chitosan is a positively charged polymer and is very strongly cationic and readily soluble in acidic conditions. The higher degree of deacetylation of chitosan, the more positive charges is formed on its surface.
Chitosan contributes to the reduction of plant diseases through two primary mechanisms. The first is direct antimicrobial action against pathogens, such as damage to the plasma membrane, interaction with DNA and RNA (electrostatic interactions), and deposition on the microbial surface and the second is induction of plant defense responses resulting from downstream signaling, transcription factor activation gene transcription and ultimately cellular activation. Chitosan has the ability to inhibit viral infections both locally and systemically. The effectiveness of chitosan depends on the dose and the method of its application and the molecular structure of chitosan especially the molecular weight and degree of acetylation. However, the antiviral activity of chitosan decreases with increasing molecular weight. The possible reason for this may be the ability of small chitosan molecules to better penetrate into plant tissue. Chitosan also prevents the replication of viruses and viroids and enhances the host’s hypersensitivity response during infection. Increasing plant cell resistance as a result of using chitosan in plants such as potatoes, tomatoes, and cucumbers has been proven by various researchers. Regarding the antiviral of chitosan, it should be noted that it is also able to suppress viral infections in many different plant families which affects the occurrence of a hypersensitivity response (HR) to viral infections and prevents the systemic spread of viruses in the plant. Some reports indicate that it stimulates the plant’s defense response and improves its general defense mechanisms. These results have been proven in mung bean, pea and peanut plants by foliar spraying against their viruses. Studies show that several pathways are activated after chitosan application, most of which are related to the activation of the plant’s innate immunity for example, accumulation of salicylic acid (SA) and activation of systemic acquired resistance (SAR) in the plant. SA acts as a signaling molecule that activated multiple pathways and initiates various changes in plant cell physiology and gene expression. Salicylic acid has a negative effect on viral replication. The translocation of the virus within the host plant plays a very important role in the replication of the virus. This translocation may be local intercellular or systemic transmission in the host. Since chitosan also induces the formation of calloses, the cell-to-cell movement of the virus is also reduced by the deposition of calloses around the plasmodesmata and the reduction of their permeability. One of the nanoparticles that has received great attention today is nanoparticles composed of chitosan. Due to the large positive charge on the surface of this molecule, it can target the virus coat protein and also increase growth parameters in the plant.
Conclusion:
Chitosan and its derivatives are regarded as powerful tools for activating various plant defense pathways. Based on the results of research and extension experiments conducted on the farms, this component showed high efficiency as an inducer of plant resistance against viral infections. In addition to reducing infection severity, it contributed to improved crop yield. Therefore, the use of this polymer and nanoparticle products based on it is a suitable alternative to traditional chemical pesticides, which is an important step towards reducing the use of chemical pesticides and establishing sustainable agriculture.
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