A review of chitosan and its usage in agriculture

In many plants, the use of biostimulants is one of the methods to reduce the harmful effects of non-biological stresses and increase their performance and quality. Several substances with elicitor properties, including chitosan, have been identified to stimulate responses to stress and defense mechanisms (Kowalski et al., 2006). Chitosan, as a carbon source, may stimulate the growth of beneficial microbes in the soil, increase the conversion of organic matter to minerals, and help the root system of plants in absorbing more nutrients from the soil, thus promoting plant growth (Cho et al., 2008). Nowadays, the use of chitosan as a non-toxic, biodegradable, and environmentally friendly substance to reduce and improve the effects of various stresses, such as drought and salinity, has gained attention. Lianju and colleagues (2011) reported that pre-treatment of wheat seeds with chitosan reduces the effects of salinity stress on the plant by increasing stem length, root length, stem dry weight, root dry weight, and chlorophyll content. They also found that chitosan leads to an increase in proline content and a decrease in malondialdehyde in the plant. It has also been reported that priming rice seeds with chitosan increases growth, proline content, and total carbohydrates under salinity stress (Ruan and Xue, 2002). Malondialdehyde is produced as the end product of the oxidation of non-saturated fatty acids in the cell membrane. Oxidative stress increases the malondialdehyde content (Weber et al., 2004). Chitosan may reduce malondialdehyde levels by chelating metal ions or binding with lipids, thereby reducing lipid oxidation (Xue et al., 2002). Treating chickpea seeds with chitosan at different stress levels increased proline and total carbohydrate content (Mahdavi and Safari, 2015). The accumulation of proline during salinity stress may be due to its stimulation of proline synthesis from glutamic acid, reduction of its translocation through phloem, and prevention of its oxidation during stress (Lutts et al., 1996). Under saline stress conditions, proline acts as an osmolyte, reduces cell osmotic potential, and absorbs toxic ions (Woodward and Bennett, 2005). The accumulation of carbohydrates under stress helps protect plant cells from stress by regulating osmotic pressure, maintaining turgor pressure, and stabilizing membranes and proteins (Bohnert et al., 1995). Chitosan may contribute to an increase in soluble proline and total carbohydrates in chickpea leaves, and consequently, help regulate cell osmotic pressure to reduce the damaging effects of salinity stress on this plant (Mahdavi and Safari, 2015). Rice treatment with chitosan before mild water stress has reduced the damage caused by drought stress in this plant. This effect has been attributed to the production of secondary metabolites by rice, leading to the closure of stomata and reduced transpiration (Boonlertnirun et al., 2007). Seed coating with chitosan may enhance seed germination and increase drought tolerance in rice seedlings. High concentrations of chitosan may prevent water absorption by seeds due to the adhesive coating it forms on the outer part of the seeds (Ruan and Xue, 2002). Under mild water stress conditions, the protein concentration decreases, while pre-treatment of seeds with low concentrations of chitosan (0.5-4.0%) increases it. Under severe stress conditions, the significant decrease in protein content in seedlings can be attributed to both protein degradation and reduced synthesis (Jiang and Ren, 2004). When seedlings are exposed to high stress and low concentrations of chitosan, the protein concentration increases. Under water stress conditions, active oxygen accumulates in plants, and a defense system is established in plants to accelerate protective enzyme activity, effectively protecting them from oxidative damage and thus maintaining normal plant function (Wang et al., 2002). When plants are exposed to mild water stress, all defense systems are activated to counteract the damage caused by active oxygen. In many plants, it has been demonstrated that drought stress affects the activities of catalase, peroxidase, and superoxide dismutase enzymes (Jiang and Ren, 2004). In marigold seedlings, catalase and peroxidase enzymes play a key role in protecting the seedlings from mild water stress. Pre-treatment of seeds with chitosan at concentrations of 0.5-4.0% resulted in increased catalase enzyme activity (Mahdavi et al., 2013). Chitosan can quench free radicals OH and O2 and has a protective effect on DNA. The mechanism of chitosan’s scavenging of free radicals may be related to its unique structure, which is composed of numerous accessible amino and hydroxyl groups that react with free radicals (Xie et al., 2001). Chitosan is a versatile biopolymer, and there is evidence that chitosan possesses antifungal properties. The antimicrobial property of chitosan is attributed to its positively charged amino groups, which interact with the negatively charged cell membranes of microorganisms, resulting in the inhibition of mRNA and protein synthesis, interaction with DNA and RNA, and abnormalities in mycelium in fungi (Long et al., 2014). Chitosan can completely inhibit the germination of fungal pathogens such as P. expansum and B. cinerea and cause damage to the plasma membrane in both pathogens (Liu et al., 2007).