Elevated CO 2 decreases the response of the ethylene signaling pathway in M edicago truncatula and increases the abundance of the pea aphid

Summary The performance of herbivorous insects is greatly affected by plant nutritional quality and resistance, which are likely to be altered by rising concentrations of atmospheric CO 2 . We previously reported that elevated CO 2 enhanced biological nitrogen (N) fixation of M edicago truncatula , which could result in an increased supply of amino acids to the pea aphid ( A cyrthosiphon pisum ). The current study examined the N nutritional quality and aphid resistance of sickle , an ethylene‐insensitive mutant of M . truncatula with supernodulation, and its wild‐type control A17 under elevated CO 2 in open‐top field chambers. Regardless of CO 2 concentration, growth and amino acid content were greater and aphid resistance was lower in sickle than in A17. Elevated CO 2 up‐regulated N assimilation and transamination‐related enzymes activities and increased phloem amino acids in both genotypes. Furthermore, elevated CO 2 down‐regulated expression of 1‐amino‐cyclopropane‐carboxylic acid ( ACC ), sickle gene ( SKL ) and ethylene response transcription factors ( ERF ) genes in the ethylene signaling pathway of A17 when infested by aphids and decreased resistance against aphids in terms of lower activities of superoxide dismutase ( SOD ), peroxidase ( POD ), and polyphenol oxidase ( PPO ). Our results suggest that elevated CO 2 suppresses the ethylene signaling pathway in M . truncatula , which results in an increase in plant nutritional quality for aphids and a decrease in plant resistance against aphids.