Optimization by mitochondrial metabolism of photosynthetic carbon assimilation

Abstract

The occurrence and importance of respiration in light has been a matter of long debate. However, mitochondrial oxidative metabolism is essential for photosynthetic carbon assimilation. The dependence of chloroplasts on mitochondria is intriguing and has been studied using the experimental systems of mesophyll protoplasts and leaves. Mitochondrial decarboxylation reactions are inhibited in light, but the oxidative electron transport and oxidative phosphorylation are kept up. Chloroplasts and mitochondria stimulate each other¿s activities. For e.g., even after a few minutes of photosynthetic activity, the respiratory O2 uptake is stimulated, resulting in light enhanced dark respiration. The mitochondrial oxidative metabolism, particularly the electron transport, not only optimizes photosynthetic carbon assimilation but also protects chloroplasts against photoinhibition. Both the cytochrome and alternative pathways of mitochondrial electron transport are important in such interaction. The beneficial interaction of chloroplasts and mitochondria occurs at both limiting and optimal CO2. The function of chloroplasts is optimized by the complementary nature of mitochondrial metabolism in multiple ways: facilitation of the export of excess reduced equivalents from chloroplasts, shortening of photosynthetic induction, maintenance of photorespiratory activity and supply of ATP for sucrose biosynthesis. Further, mitochondria cooperate in optimizing nitrogen metabolism, which is closely linked to photosynthesis. These reactions often involve peroxisomes and cytosol. The beneficial interaction between chloroplasts and mitochondria therefore extends invariably to peroxisomes and cytosol. While the metabolite exchange between chloroplasts, mitochondria, peroxisomes and cytosol appears to be the main basis of interaction, efforts are on to identify other biochemical signals between these organelles.