Chitosan-induced biotic stress tolerance and crosstalk with phytohormones, antioxidants, and other signalling molecules

Abstract

Several polysaccharides augment plant growth and productivity and galvanise defence against pathogens. Such elicitors have ecological superiority over traditional growth regulators, considering their amplified biocompatibility, biodegradability, bioactivity, non-toxicity, ubiquity, and inexpensiveness. Chitosan is a chitin-derived polysaccharide that has recently been spotlighted among plant scientists. Chitosan supports plant growth and development and protects against microbial entities such as fungi, bacteria, viruses, nematodes, and insects. In this review, we discuss the current knowledge of chitosan's antimicrobial and insecticidal potential with recent updates. These effects are further explored with the possibilities of chitosan's active correspondence with phytohormones such as jasmonic acid (JA), salicylic acid (SA), indole acetic acid (IAA), abscisic acid (ABA), and gibberellic acid (GA). The stress-induced redox shift in cellular organelles could be substantiated by the intricate participation of chitosan with reactive oxygen species (ROS) and antioxidant metabolism, including hydrogen peroxide (H₂O₂), superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). Furthermore, we propose how chitosan could be intertwined with cellular signalling through Ca²⁺, ROS, nitric oxide (NO), transcription factors (TFs), and defensive gene activation.

Keywords: antimicrobial; antioxidants; biopolymer; chitooligosaccharides; chitosan; insecticidal; oxidative stress; phytohormones.

Figure 1

Chemical structure of chitin, chitosan, and its derivatives.

Figure 2

Proposed modus operandi for chitosan-induced biotic stress tolerance in higher plants. These effects can be identified as indirect (A, B) and direct (C). Indirect effects include chitosan action on soil properties that improve water retention. Improved soil–water content promotes soil microbiota. Such microbiota include several chitinolytic organisms that can dissolve insect exoskeletons by chitinase enzymes (see panel A). Other beneficial soil organisms can promote source-sink potential. A higher nutrient status combined with increased soil–water retention promotes plant innate defence, e.g., antioxidant capacity (see panel B). In contrast, the direct effects of chitosan include cytotoxic action on pathogen/pest cells. Chitosan can cause hydrolysis of the cell wall components. This, along with metal chelation and cation efflux from the cytoplasm, disrupts ROS-antioxidant metabolism in pathogen/pest cells. This could result in cell degradation or death of plant invaders. It is suggested that these direct actions can rely on chitosan’s intricate crosstalk with certain phytohormones such as JA, SA, ET, ABA, and GA. A few studies suggested a genomic approach for such adjustments such as activation of defensive genes and TFs (see panel C). Apart from this, other signalling pathways could be facilitating chitosan-induced biotic tolerance in plants. Nevertheless, their understanding is still in preliminary phase and demands more investigation. PSII, photosystem II; CA, carbonic anhydrase; NR, nitrate reductase; ROS, reactive oxygen species; SAR, systemic induced resistance; PAMP, pathogen-associated molecular pattern; NPR1, natriuretic peptide receptor A/guanylate cyclase A; SA, salicylic acid; JA, jasmonic acid; ET, ethylene; GA, gibberellic acid; ABA, abscisic acid; TFs, transcription factors.