Could an increase of hydrogen peroxide efflux in soybean (Glycine max) and Arabidopsis thaliana under elevated CO2 explain an observed induction of foliar salicylic acid?

Background/Question/Methods

           Elevated atmospheric CO₂ downregulates chemical defenses of soybean (Glycine max) against insect herbivores, but a mechanism governing this response has not been established. Elevated CO₂ stimulates the rates of photosynthesis and electron transport chain increasing the production of hydrogen peroxide. We hypothesize that foliar salicylic acid might rise as a consequence of increased hydrogen peroxide accumulation in soybean chloroplasts under elevated CO₂, providing a link between photosynthesis and plant defense. To test whether elevated CO₂ could increase photochemical production of hydrogen peroxide, soybean were acclimated to shade for 3 weeks and then exposed to full afternoon sunlight through sudden removal of shade. Non-photochemical quenching (NPQ) was continuously measured in soybean leaf tissue following light exposure. To test the connection between antioxidant scavenging of hydrogen peroxide and salicylic acid, wild-type and vitamin C deficient mutants of Arabidopsis were grown under ambient and elevated CO₂. Rosettes from individual plants were analyzed for salicylic acid content.

Results/Conclusions

            When exposed to instant high light, soybean grown under elevated CO₂ exhibit a marked decline in NPQ, in contrast to plants grown under ambient CO₂ (MANOVA, p = .014, N = 4 with 2 biological replicates). This result indicates that plants under elevated CO₂ absorb light energy that would be instead dissipated as heat under ambient CO₂; this implies that photochemical production of hydrogen peroxide would increase under elevated CO₂.  In Arabidopsis, salicylic acid levels in the vitamin C deficient mutants were significantly increased relative to their wild-type counterparts regardless of CO₂ level (paired t-test, p = .038, N = 4 with 2 technical replications). These results provide support for a link between antioxidant capacity to quench hydrogen peroxide and salicylic acid in the chloroplast.  

            Our results add to a growing body of evidence that rising CO₂ indirectly induces salicylic acid levels in plant tissue by increasing foliar content of hydrogen peroxide. This causal relationship bears important implications to plant disease resistance and plant-insect interactions under global change. The induction of salicylic acid by elevated CO₂ is thought to antagonize biosynthesis of jasmonic acid. This upset in cross-talk between hormonal defense signals would explain the decline of jasmonic-acid based defenses in soybean under elevated CO₂.