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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

CO₂ Sensing at Ocean Surface Mediated by cAMP in a Marine Diatom¹

Department of Bioscience, School of Science and Technology, Kwansei-Gakuin University, Sanda, Hyogo 669–1337, Japan

Marine diatoms are known to be responsible for about a quarter of global primary production and their photosynthesis is sustained by inorganic carbon-concentrating mechanisms and/or C₄ metabolism. Activities of the inorganic carbon-concentrating mechanism are attenuated under enriched [CO₂]; however, impacts of this factor on primary productivity and the molecular mechanisms of CO₂ responses in marine diatoms are unknown. In this study, transgenic cells were generated of the marine diatom Phaeodactylum tricornutum by the introduction of a -glucuronidase reporter gene under the control of an intrinsic CO₂-responsive promoter, which is the sequence between –80 to +61 relative to the transcription start site of a chloroplastic-carbonic anhydrase gene, ptca1, obtained from P. tricornutum. The activity of the ptca1 promoter was effectively repressed in air-level CO₂ by treating cells with a 1.0 mM cAMP analog, dibutyryl cAMP, or a cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Deletion of the intrinsic cAMP-response element from the ptca1 promoter caused a lack of repression of the reporter gene uidA, even under elevated [CO₂] and a null phenotype to the strong repressive effects of dibutyryl cAMP and 3-isobutyl-1-methylxanthine on the ptca1 promoter. Deletion of the cAMP-response element was also shown to cause derepression of the uidA reporter gene in the dark. These results indicate that the cytosolic cAMP level increases under elevated [CO₂] and represses the ptca1 promoter. This strongly suggests the participation of cAMP metabolism, presumably at the cytosolic level, in controlling CO₂-acquisition systems under elevated [CO₂] at the ocean surface in a marine diatom.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Yusuke Matsuda (yusuke{at}ksc.kwansei.ac.jp).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.086561

^* Corresponding author; e-mail yusuke{at}ksc.kwansei.ac.jp; fax 81–79–565–8542.

Received July 11, 2006; accepted September 21, 2006; published September 29, 2006.

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