Global atmospheric CO₂ concentration has been increasing at an accelerating rate, and is anticipated to reach at least 750 ppm by the year 2100 (IPCC, 2007). Rising atmospheric CO₂ levels can dilute the nitrogen (N) resource in plant tissue, which is disadvantageous to many herbivorous insects. Aphid, a phloem sucker insect, appear to be an exception that warrants the further study. The current study used an N-fixing-deficient mutant (dnf1) of Medicago truncatula and its wild-type control to determine how elevated CO₂ (750 ppm vs. 390 ppm) affects the nutritional interactions among host plant, pea aphid and its endosymbiont Buchnera.

Our results showed that elevated CO₂ has the potential to increase pea aphid density on legume plants via bottom-up effects on nutritional quality, and a functional BNF system is crucial for obtaining this positive response. It is further suggested that pea aphids are able to manipulate amino acid metabolism both in host plants and in their bacteriocytes under elevated CO₂ when enhanced BNF is present. This study has generated several significant findings: First, our results support the view that amino acids imbalances increased under elevated CO₂ through the legume/aphid interaction, which also involved the aphid endosymbionts. Second, BNF provides increased N nutrition, especially via nonessential amino acids, to pea aphids, which enhances their population growth under elevated CO₂. Finally and perhaps most importantly, the results suggest that legumes may suffer greater damage from aphids if atmospheric CO₂ levels continue to increase.

 
This research work has been published in Global Change Biology (Huijuan Guo, Yucheng Sun*, Yuefei Li, Bin Tong, Marvin Harris, Keyan Zhu-Salzman and Feng Ge*. 2013. Pea aphid promotes amino acid metabolism both in Medicago truncatula and bacteriocytes to favor aphid population growth under elevated CO₂). 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).