Elevated CO<sub>2</sub> Reduces the Resistance and Tolerance of Tomato Plants to <em>Helicoverpa armigera</em> by Suppressing the JA Signaling Pathway

<div><p>Both resistance and tolerance, which are two strategies that plants use to limit biotic stress, are affected by the abiotic environment including atmospheric CO<sub>2</sub> levels. We tested the hypothesis that elevated CO<sub>2</sub> 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, <em>Helicoverpa armigera</em>. The results showed that elevated CO<sub>2</sub> 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 <em>H. armigera</em>. Consequently, the activities of total protease, trypsin-like enzymes, and weak and active alkaline trypsin-like enzymes increased in the midgut of <em>H. armigera</em> when fed on WT plants grown under elevated CO<sub>2</sub>. Unexpectedly, the tolerance of the WT to <em>H. armigera</em> (in terms of photosynthetic rate, activity of sucrose phosphate synthases, flower number, and plant biomass and height) was also reduced by elevated CO<sub>2</sub>. Under ambient CO<sub>2</sub>, the expression of resistance and tolerance to <em>H. armigera</em> was much greater in wild type than in <em>spr2</em> (a JA-deficient genotype) plants, but elevated CO<sub>2</sub> reduced these differences of the resistance and tolerance between WT and <em>spr2</em> 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 CO<sub>2</sub> will simultaneously reduce the resistance and tolerance of tomato plants.</p> </div>