In an increasing population, the control of fungal plant disease has become vital to meet food supply needs. Phytopathogenic fungi are the most common plant pathogens and cause many severe diseases. Over time, the widespread use of synthetic fungicides to combat fungal crop diseases has increased resistance to fungal pathogens (Bagheri et al., 2019). Most vegetable diseases are due to fungal pathogens (Koike et al., 2007). The role of chitosan as a biocontrol agent against fungal pathogens has been studied extensively (Abbey et al., 2019; dos Santos Gomes et al., 2021; Riseh et al., 2023). Chitosan inhibits mycelial growth, sporulation, spore viability, and germination of the fungal pathogens, probably due to chitosan’s ability to bind DNA to inhibit RNA synthesis in the target organism (Roller and Covill, 1999). Chitosan imposes fatalities against other fungi and oomycetes, such as Phytophthora cinnamomi, Phytophthora palmivora, Gremmeniella abietina, Cryphonectria parasitica, and Heterobasidion annosum (Kuyyogsuy et al., 2018; Silva-Castro et al., 2018; Matei et al., 2020). Chitosan protected the S. tuberosum plant against late blight disease by eliciting the induced systemic resistance, pattern recognition receptors, and several other defence-related genes, hormones, and enzymes (Zheng et al., 2021b). Furthermore, it can restrict mycelial growth and spore germination by regulating several genes of metabolism, cell membrane structure and function, and ribosome biogenesis (Huang et al., 2021). Chitosan and its derivatives attained a growth inhibition of up to 100% against Phytophthora cambivora and significantly reduced the mycelial growth in Phytophthora plurivora and Phytophthora × alni (Silva-Castro et al., 2018). It could result from enhanced activities of catalase (CAT), peroxidase (POD), polyphenol oxidase, and phenylalanine ammonia lyase (PAL) and the expression of HbPR1, HbGLU, HbASI, and HbCAT genes with chitosan application (Kuyyogsuy et al., 2018). The same study found a positive correlation between chitosan and callose and lignin depositions in Hevea brasiliensis leaves, which could provide additional fungal resistance. Furthermore, Guo et al. (2013) reported that 0.5 mg/mL of chitosan reduced growth in Fusarium oxysporum (15%) and Alternaria solani (57%). Similar effects were observed by Younes et al. (2014), where chitosan effectively suppressed the growth of Aspergillus niger, F. oxysporum, and A. solani.

The antibacterial activity of chitosan encompasses both discouraging bacterial growth (bacteriostatic) and destroying them (bactericidal). Although the mode of chitosan activity against bacteria is still under discussion, the most accepted model is the electrostatic interaction model governing chitosan binding to the bacterial membrane. The functional NH₂ groups of GlcNAc subunits give chitosan a polycationic nature. The positive moiety binds electrostatically with the negatively charged components of bacterial cell membrane including extracellular polymeric substances and proteins (Khan et al., 2020). A few studies noted that chitosan can also bind to DNA and cause extensive nucleic acid degradation (Dananjaya et al., 2016; Wang et al., 2022). Chitosan binding induces permeabilisation of the bacterial cell surface and facilitates intracellular leakage. It degrades bacterial biofilm and causes cell death. However, it all depends on how well chitosan can perforate the plasma membrane, which directly depends on its MW and the bacterial type (Verlee et al., 2017). Several other factors can regulate chitosan’s antibacterial potential, such as the ratio of its monomeric units (GlcNAc and GLcN), the DDA, the acetylation pattern, solubility, and environmental effects (Li and Zhuang, 2020). Nonetheless, no substantial relation was found between the source of chitosan to its antibacterial activity. It is observed in several gram (+) and gram (−) bacteria that the minimum inhibitory concentration (MIC) of chitosan (similar MW) decreases with increasing DDA (Verlee et al., 2017). Thus, a higher DDA would mean more effective chitosan against bacteria. Antibacterial activity of chitosan has been noted against several important gram (+) bacteria (such as Staphylococcus aureus, Bacillus cereus, Enterococcus faecalis, Micrococcus luteus, and Bacillus subtilis) and gram (−) bacteria (including Escherichia coli, Vibrio cholera, Shigella dysenteriae, Bacteroides fragilis, and Pseudomonas aeruginosa) (Benhabiles et al., 2012; Younes et al., 2014; Goy et al., 2016; Yadav et al., 2020).

Plant viruses negatively impact plants and cause a wide range of symptoms, including discolouration, distortion of plant parts, and loss of vigour, affecting yield. Chitosan application has been found significantly effective against several viruses, including potato virus X, tobacco mosaic and necrosis viruses, alfalfa mosaic virus, peanut stunt virus, cucumber mosaic virus, potato virus X (PVX), and beans mild mosaic virus (Phaseolus vulgaris) (Pospieszny et al., 1991; Pospieszny, 1997; Chirkov et al., 2001; Faoro et al., 2001). Since chitosan’s antiviral activity depends on its MW, low MW chitosan shows higher antiviral activities than high MW. Chitosan application inhibits systemic propagation of viroid (Pospieszny et al., 1991) and induces plant resistance by callose formation and enhanced ribonuclease content. Treated leaves show less accumulation of viruses than the control (Chirkov et al., 2001).

The underground plant parasitic nematodes attack various crops’ root systems, causing root galls, stunted growth, and increased susceptibility to pathogen attack and abiotic stress (Yan and Xie, 2015). Accumulating body of evidence has shown that exogenous application of chitosan triggers plant defence-related pathways in various crops, such as potato, tomato (Vasyukova et al., 2001; Fan et al., 2020; Boamah et al., 2023), barley, banana (Maciá-Vicente et al., 2009; Suarez-Fernandez et al., 2021), and soybean (Mwaheb et al., 2017). Consistently, chitosan also promotes host resistance to Meloidogyne spp. root-knot nematodes (RKNs). Similarly, water-soluble chitosan induces resistance against root-knot nematodes (Escudero et al., 2017; Chakraborty et al., 2020).

Insect herbivores are among the most notorious crop pests that cause significant direct and indirect crop loss. The exogenous application of chitosan has been found effective against multiple pests, including chewing and sucking insects. The insecticidal effect of chitosan has been tested against various insect herbivores: lepidopterans (Helicoverpa armigera, Plutella xylostella, Spodoptera littoralis, and Tuta absoluta), hemipterans (Aphis gossypii, Rhopalosiphum padi, Sitobian aveane, Metapophium dirhodum, Myzus persicae, and Hyalopetrus prun), coleopterans (Callosobruchus maculatus), and hymenopterans (Solenopsis invicta) (Rabea et al., 2005; Sahab et al., 2015; Sabbour and Solieman, 2016; Zhang et al., 2003; Zhu et al., 2021). Chitosan-mediated changes have been shown to reduce egg deposition (Sahab et al., 2015), insect performance (Zhang et al., 2003), population densities (Sabbour and Solieman, 2016), and feeding behaviour. It also causes morphological changes in the midgut of insect herbivores and reduces digestive enzymes’ activity, increasing pest mortality (Rabea et al., 2005).

It is suggested that chitosan induces plant defence through an intricate correspondence with several phytohormones such as JA, abscisic acid (ABA), indole acetic acid (IAA), SA, and gibberellic acid (GA) (Jogaiah et al., 2020; Ji et al., 2022). These phytohormones are critical to inducing defence against several plant pathogens and pests.

Chitosan promotes JA concentration in plant tissues under attack by activating the octadecanoic pathway (Rakwal et al., 2002; Iriti et al., 2009). The pathway is responsible for JA biosynthesis by oxidising linolenic acid and increasing phytoalexin content. Chitosan-induced JA signalling promoted the accumulation of several other secondary metabolites such as glucosinolates and anthocyanins glucosides (Iula et al., 2022). These compounds regulate stress-induced oxidative damage given their ROS scavenging nature. Furthermore, JA can trigger signal transduction and activates defence-related genes against pathogen invasion. Peian et al. (2021) reported that chitosan activated multiple JA-biosynthesis-related genes such as VvLOX, VvAOC, VvAOS, and VvCOI1 to induce production of methyl jasmonate, a vital defence hormone, in Vitis vinifera L. under fungal stress.

ABA-mediated signal transduction is critical for plants to respond to biotic and abiotic stresses (Mauch-Mani and Mauch, 2005; Lee and Luan, 2012). Chitosan is reported to stimulate ABA biosynthesis to protect the plant against P. palmivora in H. brasiliensis (Kuyyogsuy et al., 2018). In Fragaria × ananassa ‘Fugilia’, TOPLESS-related 3 (TPR3) and HISTONE DEACETYLASE 19 (HDA19) are overexpressed during pathogen attack. These proteins interact with the cell wall and reduce its stability. Nonetheless, both TPR3 and HDA19 were restricted by chitosan integration into such plants while restoring cell wall stability and fruit quality (Peian et al., 2021). Chitosan and ABA treatments induced the expression of HbPR1, HbGLU, HbASI, and HbCAST and the deposition of lignin and callose (Kuyyogsuy et al., 2018). ABA also regulates callose deposition intensity and speed (Flors et al., 2005). Furthermore, the same study (Kuyyogsuy et al., 2018) links chitosan with upregulated NCED activity, a crucial enzyme for ABA biosynthesis. Thus, chitosan and ABA seem to work together to defend plants against stressful scenarios. However, their precise crosstalk is still unclear. Chitosan can activate the pathogen-associated molecular pattern (PAMP) and promote H⁺ and Ca²⁺ entry in the cytosol; this activates mitogen-activated protein kinases (MAPKs) and the production of JA, ABA, and phytoalexins (El Hadrami et al., 2010; Iula et al., 2022).

In Helianthus annuus L., chitosan magnified IAA and phenol content during stress (Li et al., 2019; Bakhoum et al., 2020). Similarly, chitosan treatment upregulated IAA and SA content in Arabidopsis thaliana by altering their gene expression patterns in the roots. It resulted in restricted expression of WOX5 in the apical root meristem and arrested root development (Lopez-Moya et al., 2017). Chitosan-induced IAA amassing could be triggered by upregulated genes of the tryptophan-dependent biosynthesis pathway (ami1, aao1, and yuc2) and reduced expression of IAA translocation gene (pin1) (Lopez-Moya et al., 2019).

Similar implications of chitosan treatment were observed in GA biosynthesis. Seed priming with chitosan improved germination rate, lipase activity, and seedling growth through increased GA levels in peanut plants (Zhou et al., 2002). Chitosan was also linked with the enhanced impact of GA on plant physiology in another study with P. vulgaris L. (Pereira et al., 2017). Nonetheless, the understanding of chitosan–GA crosstalk is still preliminary and needs more attention.

Chitosan triggers signal transduction for phytoalexins production, secondary metabolites, and enzymatic and non-enzymatic antioxidants in defence responses to (a)biotic stresses (Pongprayoon et al., 2022). Chitosan triggered signalling pathways in strawberry fruits during oxidative stresses. It induces chloroplast-related genes. Peroxiredoxin-ROS scavenger genes have related to the cellular levels of ROS in the signalling networks of the chloroplast (Awad et al., 2015). Chitosan application in arbuscular mycorrhizal (AM) tomato (Solanum lycopersicum L.) promoted the growth of plants. Among the possibilities to evaluate the influence of chitosan on tomato growth and flowering may be its anti-transpirant properties to activate ROS scavenging to increase stomatal conductance and xylem vessel growth. In addition, applying chitosan to leaves increased plants’ photosynthesis rate and consequently improved plant growth and development (El Amerany et al., 2020). Plants treated with chitosan developed an increased capacity to produce enzymatic antioxidants such as CAT, POD, superoxide dismutase (SOD), and glutathione reductase (GR) to mitigate the effects of oxidative stress in salinity stress (Alkahtani et al., 2020). Chitosan’s involvement with vide supra specialised molecules has been reported in several plant species under various stresses. Chitosan stimulates the activity of several defence-related enzymes such as POD in Prunus persica L. fruits or Phoenix dactylifera L. roots (Abdellatef et al., 2022). Chitosan treatment can amplify PAL activity as well in many crops like Triticum aestivum, V. vinifera, and O. sativa, resulting in increased levels of phenolic and flavonoid compounds through phenylpropanoid pathway (Li et al., 2013). In V. vinifera, chitosan elevated PAL activity and enhanced the antioxidant defence mechanisms against Botrytis cinerea by upregulated CAT, POD, and SOD activities (Peian et al., 2021).

The foliar application of chitosan encouraged O₂ ⁻ scavenging and restricted H₂O₂ generation and lipid peroxidation to manage stress in sweet peppers (Alkahtani et al., 2020). In white clover, chitosan in dehydration-responsive element-binding protein (DREB) responsive pathway upregulated DREB2, DREB4, and DREB5 genes (Ling et al., 2022). It activated Y2K and Y2SK genes, which encode dehydrins (DHNs) that produce water-stress tolerance. Notably, these genes are critical to stress tolerance and antioxidant defence.

Chitosan activates signalling pathways in cells by binding to specific cellular receptors, activating important secondary messengers such as Ca²⁺, nitric oxide (NO), ROS, and transcription factors (TFs) (vide supra section 4.2 for ROS). These molecules play a critical role in triggering several biochemical responses. It is worth noting that chitosan, along with Ca²⁺ treatment, increases Ca²⁺ influx into the cytosol. This elevated cytosolic Ca²⁺ level is associated with enhanced callose formation. Notably, chitosan without the Ca²⁺ application did not form callose, suggesting that chitosan-induced callose synthesis depends on the presence of Ca²⁺ (Koühle et al., 1985).

Similarly, NO plays a critical role in diverse vital physiological phenomena and provides defence against stress scenarios (Corpas et al., 2001; Kolbert et al., 2019). Chitosan promotes the generation of NO and phosphatidic acid. Nonetheless, it inhibits the phospholipase-mediated signalling pathway in the presence of an NO scavenger. Thus, it seems plausible that chitosan elicits defence responses in a NO-dependent pathway (Tocci et al., 2011). Moreover, chitosan can regulate photosynthesis and stomatal movement (Mukarram et al., 2023) in NO-dependent signalling, considering NO has critical roles in stomatal movement in stressed plants (Garcıía-Mata and Lamattina, 2001; Neill et al., 2008).

Many chitosan-modulated genes, including defence-related genes and TFs related to signalling pathways, are involved in biotic stress responses. Povero et al. (2011) demonstrated that several WRKY genes of A. thaliana respond to chitosan treatment. WRKY gene family is attributed to defending against pathogens or pathogen-mimicking stimuli. Among these chitosan-elicited WRKY TFs, At5g13080 (WRKY75), At3g01970 (WRKY45), At2g46400 (WRKY46), and At4g31800 (WRKY18) are specifically involved in pathogen responses. Chitosan also overexpressed other MYB TFs such as MYB31 (At1g74650) and MYB15 (At3g23250). The modulation of these TFs by chitosan highlights its impact on regulating biotic stress responses and emphasises its potential as a valuable tool in enhancing plant defence mechanisms.