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Chitosan

 

Overview

 

Chitosan is derived from chitin, which is found in the exoskeletons of shellfish such as shrimp, lobster or crabs and the cell walls of some fungi. [1] Chitosan is considered environmental friendly for agricultural uses as it is easily degradable and non-toxic to humans. Chitosan shares a number of biochemical similarities with the cellulose found in plant cell walls. In common with cellulose, it is a long-chained linear, neutrally charged polymeric polysaccharide. Additionally, like cellulose, chitosan is used to construct mechanical and physical barriers that provide structural stability in plants.

 

Chitosan has been shown to induce/elicit natural defence responses in plants and it has been used as a natural compound to control pre and postharvest pathogenic diseases. It has been shown that fungal, bacterial and viral plant pathogens may be controlled by chitosan application. Although the research is somewhat variable, chitosan application has also been shown to increase yields, independent of pathogenic disease control, in several agricultural crops. Further, research has shown that chitosan increases the flavonoids and terpenoids in herbaceous resin producing plants.

 

Chitosan is found in many commercial agricultural products including General Hydroponics Chi Foliar Spray and several Grotek products. It is also likely an undisclosed component in many other additives that are sold through the hydroponics industry today.

 

Fungicidal Activity of Chitosan

 

Chitosan has been shown in a number of studies to be a potent elicitor of plant defense mechanisms which, in turn, have allowed plants to resist or tolerate a range of diseases.

 

Research has shown that chitosan is not only effective in halting the growth of plant pathogens, but it also induces morphological changes, structural alterations and molecular disorganization of the pathogen cells. It has been shown that various plant cells can respond to fragments of chitosan and initiate defense reactions, which lead to cause an accumulation of antibiotic phytoalexins to prevent infection of fungal diseases, stimulation of plant growth, induction of the root system and strengthening stem of plants. Further, chitosan induces SAR (systemic acquired resistance) in plants. Therefore, the antimicrobial action of chitosan comprises of more than one mode of action.

 

To date, there is strong evidence indicating that after chitosan application, plants can acquire enhanced tolerance to a wide variety of pathogenic microorganisms.

 

What this means, is that chitosan is shown to be an effective agent for preventing/controlling fungal, bacterial and viral pathogens in crops. Therefore, its addition to hydroponic solutions, or when applied via foliar, chitosan protects plants from disease.

 

The degree of chitosan’s anti-microbial activity varies amongst bacteria and fungal species. Some research suggests that chitosan promotes beneficial bacteria (e.g. Bacillus subtilus) and fungi (e.g. Trichoderma harzianum) while being antagonistic towards pathogenic microorganisms. Therefore, chitosan is shown to suppress pathogenic microorganisms while having no impact on, or even stimulating some beneficial bacteria and fungi numbers.[2] For example, research has demonstrated that the combination of chitosan and Bacillus subtilis AF 1 (bacteria), when used as seed treatments, showed better control of A. niger (causing crown rot of groundnut) and Fusarium udum (causing wilt of pigeon pea) than AF 1 culture alone.[3] Additionally, studies with tomato crops have shown that the combination of chitosan with Bacillus pumilus increased the host defence reaction of the treated roots.[4] For cucumber plants grown in the presence of nutrient solutions amended with chitosan, and inoculated by P. aphanidermatum, the host reactions were similar to those observed on chitosan-treated tomato roots.[5] In other research, Bacillus subtilis strain GB03, B. amyloliquefaciens strain IN937a, and B. subtilis strain IN937b were trialed together with chitosan. Results showed that the combination the bacilli strains with chitosan resulted in significant growth promotion that was correlated with induced pathogen resistance in tomato, bell pepper, cucumber and tobacco.[6]

 

Another study demonstrated that a combination T. harzianum (fungi) and chitosan activated host defense genes leading to physical and biochemical changes in plant cells involved directly or indirectly in disease suppression. The authors of this study concluded that inducing plant defense mechanisms by applying T. harzianum and chitosan, “particularly in combination”, could provide protection of tomato plants against Fusarium crown and root rot, caused by Fusarium oxysporum.[7]

 

Thus, chitosan can be used in conjunction with some beneficial bacteria and fungi to further increase disease resistance in plants. This makes chitosan ideal for use with some species of beneficial bacteria and fungi (‘bennies’) in hydroponics.

 

Growth Promoting Activity of Chitosan

 

Pathogens such as Pythium and Fusarium can greatly reduce yields. Hence, by ensuring that pathogens are unable to infect crops the application of chitosan, through its disease preventive mechanisms, is shown to increase yields. However, improvements in plant growth have been reported after the application of chitosan treatment to a range of crops, which are thought to be independent of the effects on disease control.

 

For example, Pornpeanpakdee et al. and Nahar et al. found that the growth of orchids (Dendrobium and Cymbidium respectively) was enhanced when chitosan was supplied to micropropagated plants growing under aseptic (sterile) conditions. This is supported by further studies showing enhanced growth in sterile conditions such as tissue-cultured grapes and the growth of the medicinal herb Aztec Sweet Herb (Phyla dulcis). Research has also shown, chitosan promotes shoot and root growth in Daikon radish (Raphanus sativus L.), and hastens flowering time and increases flower number in passionfruit (Passiflora edulis Sims). Chitosan promotes growth of cabbage (Brassica oleracea L. var. capitata L.), chitin oligosaccharide increases the chitinase activity of rice, and chitosan promotes vegetative growth, cut flower weight, flower numbers and hastened flowering time in lisianthus (Eustoma grandiflorum).[8],[9]

 

These findings, however, should be balanced with information where other authors have found there were no growth promoting benefits displayed where chitosan was used in similar crop trials.[10]

 

Therefore, while it is undoubtable that chitosan is a fantastic component for reducing the incidence of disease in crops, some skepticism should be applied to any claims that chitosan increases yields beyond its effects on disease control (at least for now).

 

Antiviral Activity of Chitosan

 

Chitosan has been shown to control viral diseases in plants.[11] While the antiviral mechanisms of chitosan are still not completely understood, it has been shown that chitosan can inhibit the replication of viruses through interfering with a few steps in virus life cycle or through improving the host antiviral immune responses to accelerate the process of viral clearance.[12]

 

The antiviral effect of chitosan is dependent on the concentration. For example, Prospieszny et al. (1991) sprayed bean plants with chitosan concentrations ranging 0.00001- 0.1% 15 min before inoculation with alfalfa mosaic virus and found increasing inhibition of the virus with increasing concentration where complete inhibition at 0.01% was obtained.[13] Another study showed the increase in chitosan, chitosan acetate, and chitosan hydrochloride concentration from 0.00005 to 0.01% to cause an increase in infection inhibition from less than 50% to 100% against bacteriophage.[14]

 

Flavonoid and Essential Oil Benefits

 

Chitosan acts as a pathogen preventive through, among other things, eliciting plant defense mechanisms against disease. Although elicitors were first used to increase plant resistance to pathogens, it was found that the mechanism involved increased polyphenol levels. Therefore, although elicitors do not kill pathogens directly, they trigger plant defense mechanisms which results in an increase of polyphenolic compounds. Polyphenolic compounds are important in plants for several reasons. Firstly, they protect plants from biotic and abiotic stress factors. Indeed, some of these phenolic compounds are only induced when stress factors are present. Secondly, most of these metabolites are responsible for the organoleptic and qualitative properties of produce originating from such plants. For example, anthocyanins, constitute a pigment group responsible for the color of a great variety of fruits, flowers and leaves, and flavan-3-ols are polyphenols involved in the bitterness and astringency of tea, grapes and wine. Therefore, the application of chitosan not only allows us to control plant disease but to also increase the phenolic content of plants. For example, it has been shown that chitosan increased the total polyphenol content of grapes and strawberries. [15] Additionally, one study has shown that MeJA (Methyl jasmonate) and chitosan applications (together and separately) increased flavonoid and terpenoid content is several species of resinous plants.[16]

 

Based on this information, while more crop specific research is needed, it is probable that chitosan increases the quality of herbaceous essential oil producing crops.

References

 

[1] Muzzarelli R. A. (2010) Chitin nanostructures in living organisms. In Chitin Formation and Diagenesis

[2] Sharp R. G. (2013) A Review of the Applications of Chitin and Its Derivatives in Agriculture to Modify Plant-Microbial Interactions and Improve Crop Yields: Agronomy 2013, 3, 757-793; doi:10.3390/agronomy3040757

[3] K Manjula, A R Podile (2001) Chitin-supplemented formulations improve biocontrol and plant growth promoting efficiency of Bacillus subtilis AF 1: Canadian Journal of Microbiology, 2001, 47(7): 618-625, 10.1139/w01-057

[4] Benhamou, N., Kloepper, J.W., Tuzun, S., 1998. Induction of resistance against Fusarium wilt of tomato by combination of chitosan with an endophytic bacterial strain: ultrastructure and cytochemistry of the host response. Planta 204, 153–168.

[5] El Ghaouth, A., Arul, J., Wilson, C., Benhamou, N., 1994 Ultrastructural and cytochemical aspects of the effect of chitosan on decay of bell pepper fruit. Physiol. Mol. Plant Pathol. 44, 417–432.

[6] Kloepper J.W. et al. Application for Rhizobacteria in Transplant Production and Yield Enhancement: Proc. XXVI IHC – Transplant Production and Stand Establishment Eds. S. Nicola, J. Nowak and C.S. Vavrina Acta Hort. 631, ISHS 2004

[7] El-Mohamedy, R.S.R., Abdel-Kareem, F., Jabnoun-Khiareddine, H., and Daami- Remadi, M. 2014. Chitosan and Trichoderma harzianum as fungicide alternatives for controlling Fusarium crown and root rot of tomato. Tunisian Journal of Plant Protection 9: 31-43.

[8] Sharp R. G. (2013) A Review of the Applications of Chitin and Its Derivatives in Agriculture to Modify Plant-Microbial Interactions and Improve Crop Yields: Agronomy 2013, 3, 757-793; doi:10.3390/agronomy3040757

[9] K. Ohta, A. Taniguchi, N. Konishi, and T. Hosoki (1999) Chitosan Treatment Affects Plant Growth and Flower Quality in Eustoma grandiflorum

[10] Chibu, H.; Shibayama, H.; Arima, S.; Effects of chitosan application on the shoot growth of rice and soybean. Jpn. J. Crop Sci. 2002, 71, 206–211 and . Khan, W.; Prithiviraj, B.; Smith, D.L. Effect of foliar application of chitin and chitosan oligosaccharides on photosynthesis of maize and soybean. Photosynth. Res. 2002, 40, 621–624.

[11] Kulikov, S.N.; Chirkov, S.N.; Il’ina, A.V.; Lopatin, S.A.; Varlamov, V.P. Effect of the molecular weight of chitosan on its antiviral activity in plants. Prikl Biokhim Mikrobiol. 2006, 42, 224–228.

[12] Wei Wang, Shi-Xin Wang and Hua-Shi Guan (2012) The Antiviral Activities and Mechanisms of Marine Polysaccharides: An Overview: Mar. Drugs 2012, 10, 2795-2816; doi:10.3390/md10122795

[13] Pospieszny, H., Chirkov, S., Atabekov, J., 1991. Induction of antiviral resistance in plants by chitosan. Plant Science 79, 63 – 68.

[14] Kochkina, Z.M., Chirkov, S.N., 2000. Effect of chitosan derivatives on the reproduction of coliphages T2 and T7. Microbiology 69, 208-211.

[15] Yolanda Ruiz-García and Encarna Gómez-Plaza (2013) Elicitors: A Tool for Improving Fruit Phenolic Content: Agriculture 2013, 3, 33-52; doi:10.3390/agriculture3010033

[16] MATHEW R. and SANKAR D. P (2014) Comparison of Major Secondary Metabolites Quantified in Elicited Cell Cultures, Non-Elicited Cell Cultures, Callus Cultures and Field Grown Plants of Ocimum