Importance of the cytochrome pathway of mitochondrial electron transport over the alternative pathway during the Kok effect in leaf discs of pea (Pisum sativum)

Is the Kok effect a respiratory phenomenon? Metabolic insight using 13 C labeling in Helianthus annuus leaves

The Kok effect is a well-known phenomenon in which the quantum yield of photosynthesis changes abruptly at low light. This effect has often been interpreted as a shift in leaf respiratory metabolism and thus used widely to measure day respiration. However, there is still no formal evidence that the Kok effect has a respiratory origin. Here, both gas exchange and isotopic labeling were carried out on sunflower leaves, using glucose that was ¹³ C-enriched at specific C-atom positions. Position-specific decarboxylation measurements and NMR analysis of metabolites were used to trace the fate of C-atoms in metabolism. Decarboxylation rates were significant at low light (including above the Kok break point) and increased with decreasing irradiance below 100 µmol photons m^-2  s^-1 . The variation in several metabolite pools such as malate, fumarate or citrate, and flux calculations suggest the involvement of several decarboxylating pathways in the Kok effect, including the malic enzyme. Our results show that day respiratory CO₂ evolution plays an important role in the Kok effect. However, the increase in the apparent quantum yield of photosynthesis below the Kok break point is also probably related to malate metabolism, which participates in maintaining photosynthetic linear electron flow.

A reporter gene system used to study developmental expression of alternative oxidase and isolate mitochondrial retrograde regulation mutants in Arabidopsis

Perturbation of mitochondrial function causes altered nuclear gene expression in plants. To study this response, called mitochondrial retrograde regulation, and developmental gene expression, a transgenic Arabidopsis thaliana (Col-0) line containing a firefly luciferase gene controlled by a promoter region of the Arabidopsis alternative oxidase 1a gene (AtAOX1a) was created. The transgene and the endogenous gene were developmentally induced in young cotyledons to a level higher than in older cotyledons and leaves. Analysis of transgene expression suggests that this is true for emerging leaves as well. Antimycin A (AA), a mitochondrial electron transport chain inhibitor, and monofluroacetate (MFA), a TCA cycle inhibitor, induced expression of the transgene and the endogenous gene in parallel. The following comparative responses of the transgene to inhibitors were observed: (a) the response in cotyledons to AA treatment differed greatly in magnitude from the response in leaves; (b) the induction kinetics in cotyledons following MFA treatment differed greatly from the kinetics in leaves; and (c) the induction kinetics following MFA treatment differed from the kinetics of AA in both leaves and cotyledons. The transgenic line was used in a genetic screen to isolate mutants with greatly decreased transgene and AtAOX1a induction in response to AA. Some of these mutant lines showed greatly decreased induction by MFA, but one did not. Taken altogether, the data provide genetic evidence that suggests that induction of the AtAOX1a gene by distinct mitochondrial perturbations are via distinct, but overlapping signaling pathways that are tissue specific.