1. Introduction

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, mollusc radulae, beaks of cephalopod and fish, and lissamphibian scales [1]. 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’ [2,3]. 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 [4]. 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 [5,6]. It is biodegradable, environment friendly for agriculture, and not toxic to humans or other organisms [7]. It has shown efficacy in reducing disease incidence and 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 [8]. It functions as an antifungal [9], antibacterial [10], antiviral [11], and bionematicidal agent [12]. 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 [13,14]. 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 [15,16]. 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 [17].

2. Effect of Chitosan Biopolymer on Plant Growth

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 [42,43,44]. 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. [43] and Nahar et al. [44] found that orchid growth (Dendrobium and Cymbidium) was stimulated by the supply of CHT to micropropagated plants that grow under sterile conditions. This is corroborated by other findings showing increased growth in aseptic conditions like tissue cultured grapes [45] and the growth of Phyla dulcis in liquid bioreactors [46].

Significant growth improvements have been found by several studies in daikon radishes [47], cabbage [48], soybean sprouts [49], sweet basil [50], and also in ornamental crops, including Gerbera [51] and Dendrobium orchids [42] 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 [52]. It is utilized in potted freesia cultivation as a biostimulator [53]. Vasudevan et al. [54] 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.

3. Concluding Remarks and Future Perspectives

Chitosan, a chitin derivative, is the second most widely distributed abundant natural polymer. Over the last decade, 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 tolerance to a broad range of pathogenic micro-organisms, and promote 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 pathogens’ growth inhibition by CHT, 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 boost its antimicrobial efficacy, 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 or metabolism in microbes. Future studies may aim at explaining the real target molecule on the cell membrane, or even other intracellular targets in case of an antimicrobial mechanism of action. Moreover, further investigations are also required for pathogen resistance mechanisms against this polymer.

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. It may provide significant benefits if gene mutant strains of microbes can be developed to study the antimicrobial 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 infection control and 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 antimicrobial characteristics, innate immunity-induced activities, growth enhancement in plants and biotechnological prospects for sustainable agriculture.