PARENT SESSION
Posters P6C Photosynthesis, respiration and alternative electron sinks. Abstracts (660-680)

Role of ascorbic acid during the interactions between mitochondrial metabolism, photosynthetic carbon assimilation and photoinhibition. Agepati Raghavendra^*,1, L Padmavathi¹, ¹ University of Hyderabad, Hyderabad, AP, India

ABSTRACT- Ascorbic acid is the most abundant antioxidant in plant cells. Most of the biosynthetic reactions of ascorbate are in the cytosol. But the last step of L-galactono-1, 4-lactone (GAL) conversion to ascorbate occurs in the mitochondria. Since, ascorbate is the major scavenger for H₂O₂ in chloroplasts, we examined if ascorbate is a factor mediating the cross-talk between chloroplasts and mitochondria, using leaf discs of pea (Pisum sativum). Inhibition of mitochondrial electron transport and ATP synthesis resulted in a drastic decrease in the ascorbate/dehydroascorbate (Asc/Dha) ratio, indicating oxidative stress in the leaf discs. There was a marginal increase in the total ascorbate pool, indicating an enhanced ascorbate synthesis, in response to the stress imposed by the inhibitors. Feeding GAL increased the total ascorbate pool and its redox status. Treating leaf discs with GAL and mitochondrial inhibitors, resulted in a further increase in total ascorbate pool and Asc/Dha ratio. After treatment with GAL + mitochondrial inhibitors, their photosynthesis and respiration decreased when assayed at optimal bicarbonate, but increased when assayed at limiting bicarbonate. It is possible that despite high Asc/Dha ratio (oxidative stress), the increased total ascorbate pool might be also down regulating photosynthesis. However, an increase in ascorbate content in GAL fed leaf discs helped to sustain photosynthesis and respiration under high light. This observation is also supported by light response curves of photosynthesis in wild type and ascorbate deficient (vtc1) mutant of Arabidopsis thaliana. At lower intensities, the vtc1 mutant showed higher rates of photosynthesis, but the wild type performed better at higher light intensities. Thus, elevation of ascorbate helped to sustain photosynthesis in leaf discs at limiting bicarbonate and preventing photoinhibition at high light. Our results indicate that ascorbate could be a signal that mediates stress perception and the interactions between chloroplasts and mitochondria.

KEY WORDS: chloroplasts, interorganelle interaction, mitochondria, photoinhibition