Control of Mitochondrial Activities by Phytochrome in Cotyledons of Dark-Grown Cucumber Seedlings

Mitochondria isolated from cotyledons of dark-grown cucumberseedlings after illumination with white light showed an increasedcapacity for oxidation of malate. This increase is regulatedby phytochrome, since the rate of oxidation was increased bybrief pulses of red light, the effects of which were reversedby brief pulses of far-red light. Similarly, increases in therates of oxidation of citrate and -ketoglutarate were observedin mitochondria isolated from illuminated cotyledons. By contrast,rates of succinate-, exogenous NADH-, durohydroquinone- andascor-bate-dependent consumption of O₂ were little affectedby the light treatment. Exogenous NAD⁺stimulated the oxidationof malate and -ketoglutarate by mitochondria from dark-grown,control cotyledons, while it did not enhance the rates of oxidationof these substrates by mitochondria isolated from light-treatedcotyledons. The activities of mitochondrial enzymes (malateand succinate dehydrogenase, NAD⁺-malic enzyme and cytochromeoxidase) were not different between illuminated and dark-growncotyledons. Neither cycloheximide nor chloramphenicol inhibitedthe increase in the rate of oxidation of malate. (Received October 24, 1988; Accepted May 24, 1989)