Interaction between respiratory carbon flow and photosynthetic light harvesting in the green alga, selenastrum minutum
Creators: name: Holmes, Jody J.
Publisher: name: University of British Columbia
Year: 2008

description: Although the regulation of respiration by photosynthesis has been extensively studied, very little is known about the regulation of photosynthesis by respiration. In this thesis, it was proposed that changes in respiratory carbon flow would affect the ratio of reduced/oxidized pyridine nucleotides and affect reduction of the PQ(cyt b6f) pool via a thylakoid-bound NAD(P)H-PQ oxidoreductase. In turn, reduction of the PQ(cyt b6f) pool would result in a state 1 to 2 transition which could poise the photosynthetic electron transport chain for a decrease in NADPH production. The purpose of the present study was to rigorously test the hypothesis that increased respiratory carbon flow, which increased reduced/oxidized pyridine nucleotide ratios, would increase PQ reduction and result in a state transition. The corollary of this hypothesis was that increased respiratory carbon flow, which did not affect the ratio of reduced/oxidized pyridine nucleotides, would not affect PQ(cyt b6f) reduction or result in a state transition. In the green alga, Selenastrum minutum, five treatments were shown to increase dark respiratory carbon flow as measured by CO2 release and/or starch degradation. These treatments were further subdivided on the basis of their ability to cause a state transition. Class 1 treatments included NH4+ assimilation by N-limited cells, anaerobic treatment, and uncoupling with CCCP and all resulted in large perturbations in room temperature and 77Kfluorescence emission indicative of a state 1 to 2 transition. These changes were correlated with reduction of the PQ pool as measured by changes in the kinetics of time-resolved fluorescence decay and induction. Class 2 treatments included NO3- assimilation by N-limited cells and Pi assimilation by Pi-limited cells. Both of these treatments resulted in only small changes in fluorescence emission suggesting that a state 1 to 2 transition had not occurred. Both NO3-and Pi treatment had only minor effects on PQ(cyt b6f) reduction as measured by fluorescence decay kinetics. However, in cells treated with NO3-, measurement of PQ reduction made using fluorescence induction kinetics was not consistent with the original hypothesis. This inconsistency was proposed to result from actinic effects of signal averaging. Increases in the NADH/NAD but not NADPH/NADP ratio were correlated with class 1 treatments while class 2treatments resulted in small (Pi) or intermediate (NO3-) changes in NADH/NAD ratios. Key respiratory metabolites were examined for each of the five treatments. An examination of the combined mass action ratio for TP to PGA conversion indicated that carbon flow, via NAD-GAPDH and PGA kinase, was significantly enhanced by class 1 treatments while no significant change was noted for class 2 treatments. NAD-GAPDH has been shown to be 86% localized in the chloroplast in Chlamydomonas reinhardtii (Klein, 1986 Planta 167:81). It was proposed that chloroplastic NAD-GAPDH activity was responsible for an increase in chloroplastic NADH/NAD ratios, reduction of PQ, and a state transition. The results of the present study suggest that respiratory carbon flow can regulate the poising of the photosynthetic electron transport chain for the NADPH/ATP production ratio via the state transition. This may have ramifications for interactions between respiration and photosynthesis in both the dark and the light. The physiological significance of this interaction was discussed.