Functional analysis of the CP12 gene family in Arabidopsis

The chloroplast protein CP12 is present in almost all photosynthetic organisms. This protein has been shown to regulate the activity of two enzymes of the Calvin-Benson cycle, namely glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK). The regulation of these enzymes is achieved by the reversible formation of this multiprotein complex in response to a change in light intensity. In Arabidopsis, there are three CP12 genes, CP12-1, CP12-2 and CP12-3. Expression analysis of these genes suggested that they may have a wider role in non-photosynthetic plastids through the plants’ life cycle and that their function may not be restricted to the Calvin-Benson cycle. The main aim of this study was to determine the functional significance of having three CP12 isoforms and to explore the importance of each individual isoform in vivo. This was done by using Arabidopsis thaliana T-DNA mutant and RNAi transgenic lines with a reduced level of CP12. Our results revealed that single mutant lines did not develop a severe growth phenotype. However, a reduction in the transcript of more than one CP12 gene, in a number of multiple lines, led to a significant reduction in photosynthetic capacity at early stages of development and a severe growth phenotype, including reduced fresh and dry weight, number of leaves and seed yield, as well as affected lateral roots formation. Complementation analysis of CP12-1 in the triple mutant revealed that two out of the three lines rescued the phenotype by showing normal growth and development, confirming the importance of CP12. Our results suggest that the CP12 protein family is essential for normal growth and development and that these proteins are likely to have additional functions apart from the regulation of Calvin-Benson cycle enzymes.