2.4.8  Interactions between mitochondria and chloroplasts

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One dominant interaction between mitochondria and chloroplasts involves metabolite exchange as part of the photorespiratory carbon/nitrogen cycle, but more subtle interactions also occur. For example, respiratory ATP production in light is required to maintain maximum photosynthetic activity (Krömer 1995). Experimentally, if oligomycin is used to disrupt oxidative phosphorylation of mitochondria in leaf cells, photosynthesis is inhibited even though oligomycin is specific to mitochondrial ATP synthase at the concentrations applied (Krömer 1995). Clearly, a link must exist between mitochondrial ATP synthesis and photosynthesis via the energy demands of sucrose synthesis (because of sucrose phosphate synthase). A decline in supply of ATP decreases the rate of sucrose synthesis and this affects the rate of photosynthetic metabolism in chloroplasts. Using a different approach to this same issue, Shyam et al. (1993) inhibited production of mitochondrial ATP with azide and/or the uncoupler FCCP and exacerbated photoinhibition in pea leaves. Their results confirm an interplay between chloroplasts and mitochondria, and suggest a protective role for mitochondria, perhaps by provision of energy for chloroplast repair.

Leaf respiration rates are commonly measured in darkness, as either CO2 release or O2 consumption. Such measurements are complicated in light by reverse gas exchange from photosynthesis. However, a family of A : pi curves can be constructed to reveal a convergence point where leaf gas exchange is independent of photon irradiance, leading to a conclusion that CO2 release is inhibited in light relative to that in dark (Brooks and Farquhar 1985). Such inhibition of CO2 release in light involves a decrease in carbon flow through the TCA cycle and does not seem to involve photorespiration. In contrast, respiratory O2 consumption (excluding that associated with glycine decarboxylation) is stimulated in light compared to that in darkness. Put another way, while TCA cycle activity is decreased in light, electron transport may increase.

This apparent paradox can be explained. During active photosynthesis (non-photoinhibitory conditions), mitochondria in a leaf cell are able to oxidise surplus redox equivalents arising from photosynthetic electron transport. Those redox equivalents are then exported from chloroplasts via the malate–OAA shuttle or the DHAP–PGA shuttle (Krömer and Heldt 1991). Under these conditions, mitochondria would oxidise cytosolic NAD(P)H rather than that generated by the TCA cycle.

Coincidentally, the importance of exporting photosynthetic reducing equivalents to the mitochondria appears to increase during cold hardening (Hurry et al. 1995) and may assist in preventing photoinhibition. It is also possible that the export of α-ketoglutarate from the mitochondrion for nitrate reduction (see above) may lead to a decrease in CO2 release from the TCA cycle; nitrate reduction is greater in light because of a need for photosynthetic reduction of nitrite, the next step in this reaction sequence leading to amino nitrogen.