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Title: Increased protein carbonylation in leaves of Arabidopsis and soybean in response to elevated [CO2]

item Booker, Fitzgerald
item Fiscus, Edwin
item Ort, Donald
item Huber, Steven

Submitted to: Photosynthesis Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2008
Publication Date: 8/1/2008
Citation: Qiu, Q., Huber, J., Booker, F.L., Jain, V.L., Leakey, A., Fiscus, E.L., Yau, P., Ort, D.R., Huber, S.C. 2008. Increased protein carbonylation in leaves of Arabidopsis and soybean in response to elevated [CO2]. Photosynthesis Research. Vol 97, pp. 155-166.

Interpretive Summary: The worldwide burning of fossil fuels to produce energy is raising the concentration of carbon dioxide in the air, which has the potential to affect global climate patterns. Rising levels of carbon dioxide, however, are beneficial to most plants. Net photosynthesis, plant growth, and sometimes yield, are stimulated by elevated carbon dioxide levels. Plant water status and drought tolerance can also be improved by elevated carbon dioxide. It was unexpected then that we would discover evidence of oxidative stress in soybean and a model plant system (Arabidopsis) that were treated with elevated carbon dioxide concentrations. We detected increased levels of carbonyl groups in leaf proteins from treated plants. Protein carbonylation is known to result from oxidative stressors such as ionizing radiation and ozone, but not from elevated carbon dioxide. Increased protein carbonylation was found in leaf tissues from Arabidopsis treated in growth chambers with high carbon dioxide levels and in leaf tissues from soybean exposed to either ozone or elevated carbon dioxide in open-top field chambers. In contrast, protein carbonyl levels were lower in soybean leaf tissue samples from a free-air carbon dioxide enrichment treatment compared with controls. Separation of proteins by electrophoresis showed that carbonyl concentrations in Rubisco, the main protein component in leaves and heart of the photosynthesis process, were higher in plants treated with elevated carbon dioxide as well as ozone compared with control plants. Further investigations using two-dimensional electrophoresis and mass spectrometry suggested that elevated carbon dioxide stimulated expression of other enzymes associated with oxidative stress responses. The consequences of increased protein carbonylation remain unclear, although it is generally assumed that oxidized proteins lose enzymatic activity and may be preferentially degraded. Increased oxidative stress with elevated carbon dioxide may be related to higher leaf temperatures, chloroplast injury, accelerated developmental processes, or other as yet unidentified processes. Future studies will be directed toward understanding the impact of protein carbonylation on leaf photosynthetic activity, and the possible significance of elevated carbon dioxide-induced oxidative stress to photosynthetic adaptation and the response of plant growth to elevated carbon dioxide.

Technical Abstract: While exposure of C3 plants to elevated carbon dioxide concentrations would be expected to reduce production of reactive oxygen species (ROS) in leaves because of reduced photorespiratory metabolism, results obtained in the present study suggest that exposure of plants to elevated carbon dioxide concentrations can result in increased oxidative stress. First, in Arabidopsis and soybean, leaf protein carbonylation, a marker of oxidative stress, was often increased when plants were exposed to elevated carbon dioxide concentrations. In soybean, increased carbonyl content was often associated with loss of leaf chlorophyll and reduced enhancement of leaf photosynthetic rate (Pn) by elevated carbon dioxide concentrations. Second, two-dimensional (2-DE) difference gel electrophoresis (DIGE) analysis of proteins extracted from leaves of soybean plants grown at elevated carbon dioxide concentrations or ozone revealed that both treatments altered the abundance of a similar subset of proteins, consistent with the idea that both conditions may involve an oxidative stress. The 2-DE analysis of leaf proteins was facilitated by a novel and simple procedure to remove ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from soluble soybean leaf extracts. Collectively, these findings add a new dimension to our understanding of global change biology and raise the possibility that oxidative signals can be an unexpected component of plant response to elevated carbon dioxide concentrations.