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?
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.
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.