PARENT SESSION

Symposium S5B Light, redox and metabolic regulation: Dark reactions
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 510B
Chair: Peter Schürmann
Co-Chair: Toru Hisabori

The physiological role of CP12 in the regulation of photosynthetic carbon flux. Christine Raines^*,1, Norbert Wedel², Michael Fryer¹, Thomas Howard¹, Julie Lloyd¹, ¹ Biological Sciences Department, Colchester, UK² Plant Physiology, Osnabrueck, Germany

ABSTRACT- A novel chloroplast protein, CP12, has been shown to regulate the activity of both NADP-GAPDH and PRKase in vitro by light-driven reversible dissociation of a complex involving PRK/CP12 and GAPDH. To investigate the physiological role of CP12 in chloroplast primary metabolism we have produced CP12 antisense plants. Unexpectedly, in these antisense plants a severe growth phenotype was observed. The leaf area was significantly reduced and the morphology of the leaves was abnormal. In addition apical dominance was reduced and aberrant floral morphology and sterililty occurred in a number of lines. Although photosynthetic carbon assimilation rates were reduced by a maximum of 25% this could not be the sole cause of this dramatic change in growth. To investigate the reason for these changes in growth we have measured the in vivo activities of PRKase and NADP-GAPDH. An increase in the the light/dark ratio of in vivo chloroplast GAPDH activity was observed in the most severe transgenic lines which could result in an imbalance in the NADP/NADPH ratio. The activation state of malate dehydrogenase was used as an indicator of the stromal NADPH/NADP ratio and in the CP12 antisense plants a significant decrease was observed. Furthermore we have observed a substantial increase (up to 50%) in the activation state of plastid glucose-6-phosphate dehydrogenase in the light. These data suggest that photosynthetic carbon metabolism in the CP12 antisense plants is perturbed and that a futile cycle involving the OPP pathway may be occurring in the light. This would have the effect of reducing the availability of photosynthetic carbon for export for the synthesis of end products such as sucrose, starch and also the sythesis of compounds via the isoprenoid and shikimic acid pathways. Levels of sucrose, starch and phenolics are all reduced in the CP12 antisense plants supporting this hypothesis. Our data provide in vivo evidence that CP12 has an important role in regulating the allocation of carbon from the Calvin cycle.

KEY WORDS: redox, light activation, CP12, photosynthetic carbon flux