Proteomic response of rice seedling leaves to elevated CO2 levels

Previous investigations of plant responses to higher CO2 levels were mostly based on physiological measurements and biochemical assays. In this study, a proteomic approach was employed to investigate plant response to higher CO2 levels using rice as a model. Ten-day-old seedlings were progressively exposed to 760 ppm, 1140 ppm, and 1520 ppm CO2 concentrations for 24 h each. The net photosynthesis rate (P-n), stomatal conductance (G(s)), transpiration rate (E), and intercellular to ambient CO2 concentration ratio (C-i/C-a) were measured. P-n, G(s), and E showed a maximum increase at 1140 ppm CO2, but further exposure to 1520 ppm for 24 h resulted in down regulation of these. Proteins extracted from leaves were subjected to 2-DE analysis, and 57 spots showing differential expression patterns, as detected by profile analysis, were identified by MALDI-TOF/TOF-MS. Most of the proteins belonged to photosynthesis, carbon metabolism, and energy pathways. Several molecular chaperones and ascorbate peroxidase were also found to respond to higher CO2 levels. Concomitant with the down regulation of P-n and G(s), the levels of enzymes of the regeneration phase of the Calvin cycle were decreased. Correlations between the protein profiles and the photosynthetic measurements at the three CO2 levels were explored.