Expression analysis of the Arabidopsis CP12 gene family suggests novel roles for these proteins in roots and floral tissues

The chloroplast protein CP12 has been shown to regulate the activity of two Calvin cycle enzymes, phosphoribulokinase (PRK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), by the reversible formation of a multiprotein complex. In Arabidopsis there are three CP12 genes, CP12-1, CP12-2, and CP12-3, and expression analysis suggested that the function of these proteins may not be restricted to the Calvin cycle. Reverse transcription-PCR analysis was used here to investigate further the expression patterns of the three CP12 Arabidopsis genes together with the genes encoding plastid GAPDH (GAPA-1 and GAPB), PRK (PRK), and plastid NAD-dependent GAPDH (GAPCp1 and GAPCp2) during development, in response to changes in light, temperature, and anaerobic conditions. Expression of the CP12-2 gene was similar to that of the Calvin cycle enzymes PRK and GAPDH. However, this was not the case for CP12-1 and -3 which were both expressed in roots. Analysis of transgenic Arabidopsis lines expressing CP12::GUS fusion constructs revealed that the CP12 genes display different spatiotemporal expression patterns. The CP12-1 gene was expressed in root tips whilst CP12-3::GUS expression was evident throughout the root tissue. The most unexpected finding was that all three CP12 genes were expressed in floral tissues; CP12-1 and CP12-2 expression was detected in the sepals and the style of the flower, while in contrast CP12-3::GUS expression was restricted to the stigma and anthers. Taken together, the data suggest that the redox-sensitive CP12 proteins may have a wider role in non-photosynthetic plastids, throughout the plant life cycle.