Oxidative modification of the cell wall in plants exposed to the atmospheric pollutant ozone

The goal of this research was to define the role of the cell wall in plant exposure to the atmospheric pollutant ozone (O₃) and to determine if oxidative breakdown products of the cell wall could act as elicitors of plant response to O₃. To determine if cell wall components are sensitive to oxidation, cell walls were isolated from tomato leaves ( Lycopersicon esculentum cv. Roma) and exposed to O₃ or pure oxygen. Ozone reduced the quantity of wall-bound caffeic acid and vanillin in cell wall extracts thought to have limited enzymatic activity. To determine if this oxidative modification also occurred in vivo, tomato plants were exposed to O₃ or filtered air, and the soluble and insoluble (wall-bound) phenolic compounds were analyzed. Exposure to O ₃ decreased the quantity of wall-bound vanillin within 2 to 3h of exposure, with a return to initial levels at 5h of exposure. Vanillin quantity in the control samples was unchanged throughout the experiment. A vanillin glycone compound was detected in the buffer soluble fraction of O₃-treated samples at 3h of exposure. Control and O₃-treated samples harvested at 5h of exposure did not contain this compound. An unsuccessful attempt was made to synthesize the vanillin glycone molecule to test its ability to elicit a plant response to O₃. As vanillin is toxic to the cell, protocatechuic aldehyde, an oxidative breakdown product of caffeic acid which is structurally similar to but less toxic than vanillin, was tested for its ability to elicit an O₃ response. Protocatechuic aldehyde was introduced into leaves by syringe infiltration, spray application and petiole immersion, and changes in the transcript levels of β-1,3-glucanase and glutathione peroxidase genes were measured. Protocatechuic aldehyde elicited an increase in transcript levels of β-1,3-glucanase and glutathione peroxidase genes 24h after treatment. The infiltration and spray application methods were shown to be the most effective, although some variability in the expression levels was likely due to the difficulty of controlling the quantity of the compound actually entering the leaf. A model was created to illustrate the relationship between the O₃-induced modification of cell wall phenolics and the hypothesized signaling events.