Elevated CO₂ modifies the performance of herbivorous insects mainly by altering host plant nutritional quality and resistance. We previously reported that elevated CO₂ enhanced biological nitrogen fixation (BNF) of Medicago truncatula , which resulted in an increased supply of amino acids to the pea aphids ( Global Change Biology 2013, 19: 3210-3223 ). Current study examined the N nutritional quality and aphid resistance of sickle, an ethylene-insensitive mutant of M. truncatula with super-nodulation, and its wild-type control A17 under elevated CO₂ in open-top field chambers.

Our results showed that under ambient CO₂, the ethylene-insensitive mutant sickle, which produced more nodules and exhibited a stronger BNF than the wild-type A17, grew better and was less resistant than the wild type A17 and consequently supported higher numbers of pea aphids than A17. T he increased BNF in both genotypes under elevated CO₂ provided sufficient N so that the plants could produce greater biomass and more pods than under ambient CO ₂ . Furthermore, elevated CO ₂ tends to suppress the ethylene signaling pathway in wild-type A17 plants so that increased nodulation and BNF satisfy the increased N requirement for growth under elevated CO ₂ . By decreasing the ethylene signaling pathway, however, e levated CO₂ reduced plant resistance against the pea aphids.

In summary, impairment of ethylene signaling pathway by elevated CO ₂ has two important effects in M. truncatula : it increased the nodulation and BNF and thereby increased the phloem amino acids supporting aphid reproduction; On the other hand, elevated CO₂ decreased the ethylene-dependent resistance of wild-type M. truncatula against the pea aphids. The two effects of the ethylene signaling pathway would synergistically increase the fitness of pea aphids under elevated CO₂. This research has been published in New Phytologist (Huijuan Guo, Yucheng Sun^*, Yuefei Li, Xianghui Liu, Wenhao Zhang and Feng Ge^*. Elevated CO₂ decreases the response of the ethylene signaling pathway in Medicago truncatula and increases the abundance of the pea aphid. http://onlinelibrary.wiley.com/doi/10.1111/nph.12484/abstract) . This project was supported by the “National Basic Research Program of China” (973 Program) (No. 2012CB114103), and the National Nature Science Fund of China (No. 31000854, No. 31170390, No. 31221091 ).