Elevated CO₂ Reduces the Resistance and Tolerance of Tomato Plants to Helicoverpa armigera by Suppressing the JA Signaling Pathway

Both resistance and tolerance, which are two strategies that plants use to limit biotic stress, are affected by the abiotic environment including atmospheric CO₂ levels. We tested the hypothesis that elevated CO₂ would reduce resistance (i.e., the ability to prevent damage) but enhance tolerance (i.e., the ability to regrow and compensate for damage after the damage has occurred) of tomato plants to the cotton bollworm, Helicoverpa armigera. The results showed that elevated CO₂ reduced resistance by decreasing the jasmonic acid (JA) level and activities of lipoxygenase, proteinase inhibitors, and polyphenol oxidase in wild-type (WT) plants infested with H. armigera. Consequently, the activities of total protease, trypsin-like enzymes, and weak and active alkaline trypsin-like enzymes increased in the midgut of H. armigera when fed on WT plants grown under elevated CO₂. Unexpectedly, the tolerance of the WT to H. armigera (in terms of photosynthetic rate, activity of sucrose phosphate synthases, flower number, and plant biomass and height) was also reduced by elevated CO₂. Under ambient CO₂, the expression of resistance and tolerance to H. armigera was much greater in wild type than in spr2 (a JA-deficient genotype) plants, but elevated CO₂ reduced these differences of the resistance and tolerance between WT and spr2 plants. The results suggest that the JA signaling pathway contributes to both plant resistance and tolerance to herbivorous insects and that by suppressing the JA signaling pathway, elevated CO₂ will simultaneously reduce the resistance and tolerance of tomato plants.