The role of hydrogen peroxide in chitosan-induced resistance to osmotic stress in rice (Oryza sativa L.)

Chitosan is a biopolymer with multiple agricultural applications. The objective of this research was to identify the mechanism required for the chitosan response. Chitosan clearly induced resistance to osmotic stress (a surrogate for drought stress) in the 'Leung Pratew 123' ('LPT123') rice (Oryza sativa L. 'Leung Pratew123') by enhancing plant growth and maintenance of the photosynthetic pigments during osmotic stress, but not in the derived mutated line, LPT123-TC171. Hydrogen peroxide (H2O2) was increased after osmotic stress in both lines, but higher levels were found in the LPT123 cultivar. Chitosan application did not affect the H2O2 or glutathione content under the osmotic stress condition in the LPT123 cultivar, but decreased H2O2 accumulation in the LPT123-TC171 line. The 20-fold lower glutathione level in the LPT123 cultivar suggested a low glutathione-ascorbate cycle activity that would lead to the higher H2O2 levels. Whereas, the chitosan-mediated reduction in glutathione levels in the LPT123-TC171 line during osmotic stress suggested a higher glutathione-ascorbate cycle activity leading to low H2O2 levels. Additionally, a higher peroxidase and catalase activity following chitosan treatment of the LPT123-TC171 line supports the lower observed H2O2 level. The lipid peroxidation after osmotic stress was decreased by chitosan treatment in LPT123, but not in LPT123-TC171. The exogenous H2O2 application with chitosan treatment in LPT123-TC171 could enhance plant growth during osmotic stress. It is concluded that the limited H2O2 level, the signal molecule for chitosan responses in the LPT123-TC171 line, resulted in no beneficial effects of chitosan application for osmotic stress. Therefore, H2O2 is proposed to be one of the key components for plant growth stimulation during osmotic (drought) stress by chitosan.