On the Role of Mitochondrial Oxidative Phosphorylation in Photosynthesis Metabolism as Studied by the Effect of Oligomycin on Photosynthesis in Protoplasts and Leaves of Barley (Hordeum vulgare)

Abstract

Low concentrations of oligomycin, which strongly inhibit mitochondrial oxidative phosphorylation but do not affect chloroplast photophosphorylation, caused an inhibition of photosynthesis by 30 to 40% in barley (Hordeum vulgare L.) leaf protoplasts. This inhibition is reversed and the full rate of photosynthesis is regained when the protoplasts are ruptured so as to leave the chloroplasts intact. Oligomycin fed into barley leaves by the transpiration stream inhibited photosynthesis in these leaves by up to 60%. The measurement of metabolites in protoplast and leaf extracts showed that oligomycin caused a decrease in the ATP/ADP ratio and an increase in the content of glucose-and fructose 6-phosphate. Subcellular analysis of protoplasts revealed that the decrease in ATP/ADP ratio in the cytosol was larger than in the stroma and that the increase in hexose monophosphates was restricted to the cytosol, whereas the stromal hexosemonophosphates decreased upon the addition of oligomycin. Moreover, oligomycin caused an increase in the triosephosphate-3-phosphoglycerate ratio. It is concluded from these results that during photosynthesis of a plant leaf cell mitochondrial oxidative phosphorylation contributes to the ATP supply of the cell and prevents overreduction of the chloroplast redox carriers by oxidizing reductive equivalents generated by photosynthetic electron transport.It has been a matter of debate if in a plant leaf cell mitochondrial oxidative phosphorylation contributes to the supply of ATP during photosynthesis metabolism. In a previous publication (1 1), we investigated how to inhibit mitochondrial ATP synthesis in plant protoplasts without affecting photophosphorylation. A suitable inhibitor appeared to be oligomycin. It strongly inhibits mitochondrial oxidative phosphorylation (16) and has only a weak effect on photophosphorylation (10).Very low concentrations of oligomycin inhibiting mitochondrial oxidative phosphorylation caused a partial inhibition of protoplast photosynthesis that was observed only when the protoplasts were intact (1 1). When protoplasts that had been inhibited by oligomycin were ruptured by forcing them through a 5 ,um net, a procedure leaving chloroplasts and Supported by the Deutsche Forschungsgemeinschaft. mitochondria intact, photosynthesis of the released chloroplasts was recovered to the rate obtained in the absence of oligomycin.In a chloroplast suspension the volume of the external medium is about 1000 times larger than the stromal space of the chloroplasts. The phosphate in the medium functions as a reservoir for chloroplast photosynthesis and the products DHAP and PGA are diluted in the medium to such an extent that for the duration of the measurements photosynthesis is not affected by an accumulation of external products. In protoplasts, on the other hand, the volume of the cytosolic space is similar to the stromal one. Therefore, in the intact protoplasts photosynthesis can operate only if the products DHAP2 and PGA are further metabolized in the cytosol and th...

An analytical model of non‐photorespiratory CO2 release in the light and dark in leaves of C3 species based on stoichiometric flux balance

“…Several explanations for reduced Rc in the light have been proposed, most of which involve photosynthesis to some degree:(1) ExcessATP or redox equivalents generated by the light reactions of photosynthesis can help satisfy cellular ATP demand, permitting a reduced rate of respiratory substrate oxidation. In full sunlight, chloroplasts may rely on the mitochondrial electron transport chain (mETC) as an electron sink to oxidize photoreductant, limiting photoinhibition (Krömer, Stitt & Heldt 1988;Krömer & Heldt, 1991a;Saradadevi & Raghavendra 1992;Hurry et al 1996); (2) Biosynthesis of fatty acids, amino acids, phenolics and many other metabolites and compounds, requires NADPH, which in darkness is supplied by the chloroplastic oxidative pentose phosphate pathway (OPPP). Photosynthetically derived NADPH could satisfy these anabolic demands, permitting reduced CO2 release in the OPPP; (3) Photorespiration drives NADH production via Gly decarboxylation in mitochondria; subsequent reduction of hydroxypyruvate in peroxisomes also requires NADH, yet mitochondria seemingly meet less than half of that demand (Krömer & Heldt 1991b).…”

Comparison of Methods to Estimate Dark Respiration in the Light in Leaves of Two Woody Species

“…This inhibition appears to be caused by metabolites from photosynthesis (ATP, NADPH) acting on the respiratory enzymes as respiratory regulators (Graham, 1980; McCashing et al, 1988). However, the mechanism of inhibition is not clear and appears to be complex (Gardestrom and Wigge, 1988;Kromer and Heldt, 1991).…”

“…This inhibition appears to be caused by metabolites from photosynthesis (ATP, NADPH) acting on the respiratory enzymes as respiratory regulators (Graham, 1980; McCashing et al, 1988). However, the mechanism of inhibition is not clear and appears to be complex (Gardestrom and Wigge, 1988;Kromer and Heldt, 1991).The instantaneous evolution of COz in the light is a result of at least four processes, which take place at different rates: photosynthesis, photorespiration, dark respiration in the light (Rd), and refixing of co2 from respiration (Graham, 1980 response is linear at low levels of irradiation, but near the light compensation point there is a break in the linear response, increasing markedly the slope of the light curve due to a decrease in A. This has been interpreted as a result of an increase in the respiration rate due to a progressive disappearance of the light-induced inhibition of dark respiration (Kok, 1948;Sharp et al, 1984).…”

“…Although the ATP and reducing power generated in the photosynthetic process may follow many different pathways (Gardestrom and Wigge, 1988;Kromer and Heldt, 1991), the utilization of ATP and NADPH produced directly from photosynthesis may decrease the requirements for ATP and NADH having catabolic origin and thus decrease respiration rates, resulting in a significant increase in growth efficiency (Raven, 1976).…”

Light Regulation of Leaf Mitochondrial Pyruvate Dehydrogenase Complex

“…The controversy of whether or not mitochondrial respiration is occurring during photosynthesis has been long standing (16), and recent reports by Kromer and colleagues (20,21) working with barley (Hordeum vulgare) leaf protoplasts provide convincing evidence that mitochondrial ATP production is required for optimal photosynthesis. However, Budde and Randall (7,8) recently reported that the pea leaf mtPDC is primarily in an inactivated form in illuminated leaves and that photosynthesis was required for this inactivation to occur.…”