The reactions of plant hormones with reactive oxygen species: chemical insights at a molecular level

The reactions of two plant hormones, namely jasmonic acid (JA) and methyl jasmonate (MJ), with different reactive oxygen species (ROS) were investigated using the density functional theory. Different reaction sites and mechanisms were explored, as well as solvents of different polarity, and pH in aqueous solution. The thermochemical viability and kinetics of the investigated reaction pathways were found to be strongly influenced by the reacting ROS. All the investigated pathways were found to be exergonic, both in aqueous and lipid solution and for both JA and MJ, when the reactions involve ^•OH and ^•OCH₃. On the contrary, for the reactions with peroxy radicals (^•OOH and ^•OOCH₂CHCH₂) only a few hydrogen transfer pathways were found to be thermochemically viable. The reactions involving ^•OH were found to be diffusion-controlled, with both JA and MJ, regardless of the polarity of the solvent. This led to the hypothesis that the direct ^•OH scavenging activity of JA and MJ might play a role in the beneficial effects of the jasmonate family regarding the antioxidant defense of plants against metal-induced oxidative stress. The deprotonated fraction of JA is, to some extent, more reactive than the neutral fraction toward ROS. This, together with the acid-base equilibria inherent to some ROS, make the pH an influential environmental factor on the overall reactivity of JA toward ROS.