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The chitosan (CHT) biopolymer is a de-acetylated chitin derivative derived from the outer shell of shrimp, shellfish, lobster, or crabs, as well as the cell wall of fungi. Because of its biodegradability, environmental non-toxicity, and biocompatibility, it is an ideal resource for sustainable agriculture. The CHT emerged as a promising agent used as a plant growth promoter. It induces plant growth by influencing plant physiological processes like nutrient uptake, cell division, cell elongation, enzymatic activation, and synthesis of protein that can eventually lead to increased yield. It also alters plant defense responses by triggering multiple useful metabolic pathways. Depending on the structures, chitosan is useful for industrial and agricultural applications.
Chitosan (CHT) is a poly (1,4)-2-amino-2-deoxy-β-D glucose, a de-acetylation derivative of chitin, found in arthropod exoskeletons, which includes crustaceans like lobsters, shrimps and crabs, insects, mollusk radulae, beaks of cephalopod and fish, and lissamphibian scales . The discovery of chitosan (pronounced as Kite-O-San) dates back to 1811 when a French Professor Henri Braconnot of Natural History first found “chitin” from which it is derived. He found a mushroom extract that would not dissolve in sulphuric acid, and he called it ‘fungine’ . In 1823, it was named ‘chitin’ after another scientist Auguste Odier extracted it from cuticles of beetle and called it ‘chiton’. Chitin was the first man-identified polysaccharide, about 30 years prior to cellulose. The concept was further recognized when the existence of nitrogen in the chitin was demonstrated by Lassaigne in 1843. Professor C. Rouget undertook the alkaline treatment of chitin in 1859, resulting in an acid dissoluble substance, unlike chitin itself. Hoppe-Seiler gave the name “chitosan” to de-acetylated chitin . Although chitin has long been an unused natural component, interest in this biopolymer and its derivatives like CHT has grown significantly in recent years due to its diversified biological properties.
The biopolymer CHT is safe, cheap and its chemical structure can easily be converted to develop relevant polymers for specified applications. These features make CHT a molecule of great significance in a wide range of potential users, from health care and biotechnological industries to farmers . It is biodegradable, environment friendly for agriculture, and not toxic to humans or other organisms . It has shown efficacy in increasing crop growth, yield, and quality. The CHT has been documented as an elicitor of plants’ natural defense response and has been utilized as a natural product to combat pathogenic diseases before and after harvest . It functions as an antifungal , antibacterial , antiviral , and bionematicidal agent . Chitosan has been widely utilized as a coating agent of different nuts, cereals, fruits, and vegetables to protect from post-harvest losses, and increase the duration of storage and preservation . A wide range of studies showed that foliar application of CHT improves plant growth, yield and induces synthesis of secondary metabolites like polyphenolics, flavonoids, lignin, and phytoalexins in plants . It influences seed plasma membrane permeability, enhances sugar and proline concentration, boosts peroxidase (POD), phenylalanine ammonia-lyase (PAL), tyrosine ammonialyase (TAL), and catalase (CAT) activities .
Chitosan functions as a plant growth promoter in various crops such as beans, potato, radish, gerbera, soybean, cabbage, and other crops. As a result of plant growth promotion, it also enhances yield. Chitosan has a major influence on the growth rates of shoots, roots, flowering, and the number of flowers. As chitosan molecules are extremely hydrophilic, they reduce stress damage in plant cells by decreasing water content and accelerating several biological macromolecules’ activities. Three trials were conducted on orchids to determine the effect of CHT on organogenesis; the results showed that CHT could produce positive results at a very low concentration . The results also suggested that CHT was working as a consequence of other metabolic processes rather than merely enhancing nitrogen nutritional quality or as a source of energy for the production of carbohydrates. Both Pornpeanpakdee et al.  and Nahar et al.  found that orchid growth (Dendrobium and Cymbidium) was stimulated by the supply of CHT to micro-propagated plants that grow under sterile conditions. This is corroborated by other findings showing increased growth in aseptic conditions like tissue cultured grapes  and the growth of Phyla dulcis in liquid bioreactors .
Significant growth improvements have been found by several studies in daikon radishes , cabbage , soybean sprouts , sweet basil , and also in ornamental crops, including Gerbera  and Dendrobium orchids  by various modes of application such as in vitro, in vivo, soil application, pot application, and biofertilization. To increase maize yield, a mixture of CHT and plant-growth-promoting rhizobacteria can be utilized as biofertilizers . It is utilized in potted freesia cultivation as a biostimulator . Vasudevan et al.  reported that the use of CHT formulation could accelerate the length of root and shoot and yield of rice grain. It also promotes the growth of plants such as pepper, cucumber, and tomato raised in the nursery. Therefore, we have enlisted some important agricultural crops that showed improved plant growth and development due to the application of CHT (Table 1).
Table 1. Effects of chitosan (CHT) on plant growth and development.
|Plant Species||CHT Effects||Mode of Application||References|
|Rice (Oryza sativa L.)||Increased plant growth,
higher photosynthesis rate
|Soybean (Glycine max)||Increased plant growth||Soil application|||
|Rape (Brassica rapa L.)||Increased plant growth and content of leaf chlorophyll||Hydroponic pot application|||
|Maize (Zea mays L.)||Increased plant growth and grain weight||Biofertilization|||
|Improved seed germination||In vivo|||
|Improved seed germination and vigor index||In vivo|||
|Potato (Solanum tuberosum L.)||Increased tuber size||In vivo|||
|Increased plant growth and yield||In vitro and in vivo|||
|Tomato (Solanum lycopersicum)||Improved fruit quality and productivity||In vivo|||
|Increased seed germination and vigor index||In vivo|||
|Daikon radishes (Raphanus sativus)||Increased plant growth||In vivo|||
|Cabbage (Brassica oleracea)||Increased plant growth||In vivo|||
|Soybean sprouts (Glycine max)||Increased plant growth||In vivo|||
|Okra (Hibiscus esculentus L.)||Increased plant growth, and yield||In vivo|||
|Eggplant (Solanum melongena)||Increased plant growth, and yield||In vivo|||
|Bean (Phaseolus vulgaris)||Increased leaf area, and carotenoids and chlorophylls levels||In vitro|||
|Chili (Capsicum frutescence L.)||Increased plant growth, yield, and thousand seed weight||In vivo|||
|Increased leaf area, canopy diameter, and plant height||In vivo|||
|Bell pepper (Capsicum annuum)||Increased fruit weight, diameter, and yield||In vivo|||
|Turmeric (Curcuma longa)||Increased plant growth, and yield||In vivo|||
|Ajowan (Carum copticum)||Increased seed germination, vigor index, dry weight, and radical length||In vivo|||
|Artichoke (Cynara scolymus)||Improved seed germination and plant growth||In vivo|||
|Cucumber (Cucumis sativus)||Increased plant growth and improved quality||In vivo|||
|Chickpea (Cicer arietinum)||Increased plant growth||In vivo|||
|Increased seed germination and vigor index||In vivo|||
|Coffee (Coffea arabica)||Increased plant height and leaf area||In vivo|||
|Strawberry (Fragaria × annanasa)||Increased fruit yield and total antioxidant activities||In vivo|||
|Increased fruit yield||In vivo|||
|Watermelon (Citrullus lanatus)||Increased plant growth||In vivo|||
|Mango (Mangifera indica)||Increased plant growth, fruit size and weight||In vivo|||
|Grapevine (Vitis vinifera L.)||Increased plant growth||In vivo|||
|Basil (Ocimum ciliatum and
|Increased plant growth and
|Phyla dulcis||Increased plant growth||In vitro|||
|Freesia (Freesia corymbosa)||Increased plant growth||In vivo|||
|Gerbera jamesonii||Increased plant growth||In vivo|||
|Dendrobium aggregatum||Increased plant growth||In vitro|||
|Cymbidium insigne||Increased plant growth||In vitro|||
|Kemiri sunan (Reutealis trisperma)||Increased plant growth||In vivo|||
|Scots pine (Pinus sylvestris L.)||Increased plant growth||In vivo|||
Chitosan, a chitin derivative, is the second most widely distributed abundant natural polymer. Over the last decades, the number of uses of CHT and its derivatives has significantly increased. The availability of information on biocompatible and biological characteristics of CHT makes it a potential bioactive substance for agriculture. CHT is a versatile nontoxic compound with multiple modes of action to positively impact plant health. Its application can mitigate the broad use of chemical pesticides, at least in part. To date, there is ample evidence to suggest that plants may achieve improved growth and development after the application of CHT, suggesting that the utilization of natural elicitors like CHT may be an essential component of sustainable agriculture.
While a lot of work has been done, several issues still remain unclear pertaining to the mechanisms of inducing plant immunity, accelerating plant growth, and development. In that regard, research and development should pay attention to discovering new derivatives of CHT, as their effective chemical alteration might significantly improve its chemical and physical characteristics, and enhance its field applicability by ensuring low mammalian toxicity. CHT and its derivatives apparently rely on their molecular weight for the majority of physiological activity and functionality. In addition, further study is needed to confirm whether biopolymers like CHT have the ability to influence physiological processes.
Therefore, future studies should also concentrate on understanding the details at the molecular levels, which can offer an insight into the unknown biochemical mechanisms of CHT. Combined proteome and transcriptome study of known proteins and genes would enhance our knowledge of the complex CHT-mediated signal pathway and allow for improving biotechnological approaches in plant growth promotion. A better understanding of CHT’s mode of action in plants and pathogens would improve the possibility of its effective application. Furthermore, the collaboration and participation of research organizations, government regulatory authorities, and industries will be the primary key to the success of CHT use by unraveling its innate immunity-induced activities, growth enhancement in plants, and biotechnological prospects for sustainable agriculture.