Eco-friendly chitosan polymer mitigates disease severity and mediates plant resistance against Beet curly top Iran virus in tomato

A tomato-infecting virus known as Beet curly top Iran virus (BCTIV) cause a significant disease for tomato plants and several other plant species around the world. Chitosan polysaccharide is a natural biopolymer that has been utilised as an exo-elicitor to enhance plant defence mechanisms against a variety of plant diseases. This study investigates the efficacy of chitosan in combating BCTIV disease on tomato plants and modulating the host–virus interaction under greenhouse conditions. Twenty-four hours before the virus inoculation, tomato plants were sprayed with a protective chitosan solution at different concentrations (0.5, 1, 1.5, and 2 mg/mL). Tomato plants were inoculated with a BCTIV infectious clone using an Agrobacterium-inoculation method. The findings clearly demonstrated a reduction in the severity of the disease in chitosan-treated plants as compared to Mock-plants, with the percentage decreasing from 61.53% to 75.28% in 1.5 mg/mL treated plants and from 9.01% to 28.43% in 0.5 mg/mL treated plants. In addition, the utilisation of chitosan has the potential to deactivate the accumulation of BCTIV within the host tissues. The virus accumulation was greatly alleviated in 1, 1.5, and 2 mg/mL-treated plants by 71.29%, 90.11%, and 93.14%, respectively, and over the mock plants. Furthermore, it was found that chitosan applied at all tested concentrations increased the relative expression and mRNA accumulation of genes related to resistance, including the pathogenesis-related protein gene PR-1, the HSP90 gene, and the AGO2a antiviral gene. These genes reached their maximum by 22.9-, 12.93-, and 4.44-fold increases, respectively, over the untreated control. According to gas chromatography–mass spectroscop (GC-MS) fractionation profile, chitosan increased 28 bioactive metabolic components, such as n-hexadecanoic acid, heptanone, 1,2-dimethylbenzene, dicarboxylic acid, and cis-11-octadecenoic acid methyl-ester, to improve metabolic pathways. Results reported here revealed that foliar application of chitosan decreases the rate of the disease severity and virus accumulation in BCTIV-infected tomato plants. This effect is associated with increased gene expression and defence-related factors, enhancing tomato resistance to BCTIV infection. Consequently, chitosan treatments could be part of an integrated approach for reducing the severity of BCTIV disease in tomato and other host plants.