Foliar application of chitosan enhances growth and modulates expression of defense genes in chilli pepper (Capsicum annuum L.)

Phytophthora capsici is one of the primary pathogens causing a global problem of severe losses in chilli production. The use of conventional fertilisers and fungicides to improve chilli production had been shown to elevate environmental and health issues. Hence, the foliar application of chitosan, natural deacylated chitin, to enhance growth and resistance in chilli pepper plants was investigated. The chilli plants were grown for 14 days before receiving chitosan application and 33 days before Phytophthora infection, physiological parameters were recorded during the growth period, and expression of resistance related genes was quantified at 72 hours after infection. Our results showed that physiological parameters, such as increment of height and leaves number, and chlorophyll content indicated an improved growth process in chitosan treated plants compared to the control. Plant resistance to Phytophthora infection was also investigated following chitosan application to highly (CM334), moderately (LABA), weakly (LADO) resistant and susceptible (15080) cultivars. The disease incidence and severity indices were reduced in chitosantreated plants, except in highly and moderately resistant cultivars. Further, expression was also quantified for defence-related genes, including 9-lipoxygenase (CaLOX), Ca2+-bound calmodulin 1 (CaCaM1), receptor-like cytoplasmic protein kinase (CaPIK), Pto-interacting1 (CaPTI1) and resistance gene analogue 2 (CaRGA2). The results suggest that CaLOX, CaPTI1 and CaRGA2 genes were involved in defence mechanism to Phytophthora, with increased expression during infection. However, expression levels were reduced when Phytophthora infection was coupled with foliar chitosan application, indicating that chitosan may play a direct role in decreasing the pathogenicity of Phytophthora. In conclusion, this study suggests the promising role of chitosan as an alternative to conventional fertiliser and fungicide in chilli pepper plant.