| As the atmospheric CO2 concentration continues to rise, its impact on plant growth and development, and the potential for plant-based remediation strategies, has generated significant research activity. The molecular mechanisms, however, by which plants modulate their response to CO2, and the role of various effectors of this response, such as metabolites or plant hormones, are not well understood. In many studies, growth of Arabidopsis on high concentrations of exogenous carbohydrates (such as glucose or sucrose) has been used to mimic high CO2 exposure, and in attempts to identify components of CO2/sugar signaling pathways. The phenotypic differences, however, between high CO2 and carbohydrate exposed plants, as well as the significant hormonal role during germination and development in the presence of exogenous carbohydrates, suggest that the two treatments are not equivalent. For these reasons, the transcriptome response of Arabidopsis exposed to exogenous glucose or elevated CO2 concentrations for varying periods of time has been examined. In particular, a comparison has been made between plants germinated and grown in the presence of glucose or high CO2, and those transferred to these conditions following phototrophic growth and development at ambient levels of CO2 and in the absence of exogenous carbohydrates. Taking all transcriptome data into consideration, the differing responses would appear to be due primarily to the initial compartmentalization of the organic carbon. Exposure to elevated CO2 results in sequestration of organic carbon as starch in the chloroplast, and controlled production of cytosolic carbohydrates for translocation to sink tissue. Plants grown in the presence of exogenous carbohydrates must also sequester excess carbon in an attempt to decrease signaling responses, as well as osmotic and nutritional effects. Conversely, the pathway to glucose sequestration requires the transport of glucose from the cytosol into plastids, a process that induces a transcriptional response for increased nitrogen assimilation, ABA signaling, and other stress responses. These data suggest that photosynthetically derived carbohydrates elicit a different transcriptional response compared to plants exposed to exogenous carbohydrates. |
