It is logical that different organelles within the plant cell interact in a way that optimizes cellular functions (Figs. 1 and 2). The dependence of chloroplast photosynthesis on mitochondrial metabolism is therefore not surprising. However, the relative importance of different pathways of mitochondrial electron transport, and the flexibility of switching between coupled and non-coupled pathways of electron transport, remain to be clearly established. Preliminary evidence from the use of metabolic inhibitors suggested that the balance between coupled and non-coupled pathways is important for the functioning of chloroplast metabolism. The use and specificity of metabolic inhibitors are debatable due to the possible unspecificity and limited permeability of inhibitors. Further experiments are needed on the interaction between various components, particularly between chloroplasts and the alternative pathway of mito-chondrial electron transport. Transgenic plants and mutants deficient in specific proteins/enzymes would be useful tools with which to test key concepts.
There must be a network of signals between organelles that triggers and coordinates changes in their respective metabolic status. Metabolite concentration is one such possible type of signal. It has already been shown that the relative ratios of TP/ PGA and malate/OAA could be important in mediating the interaction of mitochondria and chloroplasts (Padmasree and Raghavendra, 1999c). There could, however, be additional signals such as cytosolic pH, N status, phosphate level, superoxide radicals or even secondary messengers such as calcium.
Nitrogen itself is an important signal for modulating C metabolism and subsequently the functioning of cellular organelles (Champigny, 1995; Stitt, 1999; Lewis et al., 2000). The effects of nitrate or ammonia on leaf tissue are phenomenonal, particularly in the modulation of gene expression and the diversion of C skeletons from carbohydrate into amino acid metabolism. Supply of nitrate or ammonia to N-starved leaves upregulates the biosynthesis not only of nitrate reductase, but also PEPc and carbonic anhydrase. At the same time nitrate down-regulates the activity of sucrose phosphate synthase. Reciprocal changes in the activity of PEPc and sucrose phosphate synthase are linked to the increase in the phos-phorylation status of these two enzymes (Champigny, 1995; Toroser and Huber, 2000).
Plant cells have developed a strategy to meet the demands for energy (ATP) and reducing equivalents (NADH, NADPH) of different compartments. Supply and demand patterns are dynamic depending on the microenvironment of the cell. For example, upon illumination the chloroplasts can generate ATP as well as NADPH in excess of their own need and can export to other compartments. Under limiting light and high C02, the chloroplast may have to supplement its needs by either import or by restricting export. Mitochondria are geared to export ATP and citrate, leading to reduction of NADP in the cytosol. The import of reducing equivalents by peroxisomes from both chloroplasts and mitochondria demonstrates the flexibility of interorganellar dependence within the photosynthetic cell. It is very important to examine the C/N interaction involving multiple organelles, in transgenic plants and mutants deficient in specific reactions in chloroplasts, mitochondria or perox-isomes.
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