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Title: Plastid redox state and sugars as interactive regulators of nuclear photosynthetic gene expression
Author: Oswald, Oliver
ISNI:       0000 0001 3460 9293
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2001
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Feedback control of photosynthesis by carbon metabolites has long been recognised but the underlying cellular mechanisms that regulate this process remain unclear. As part of this thesis project it was demonstrated that in an Arabidopsis cell culture a block in photosynthetic electron flux prevents the increase in transcript levels of chlorophyll a/b binding protein (CAB) and the small subunit of Rubisco (RBCS) that typically occurs when intracellular sugar levels are depleted. In contrast, the expression of the nitrate reductase (NR) gene, which is induced by sugars, was not changed. These findings were confirmed in planta using Arabidopsis carrying the firefly luciferase reporter gene (LUC) fused to the plastocyanin (PC) and CAB2 photosynthetic gene promoters. Transcription from both promoters increased upon carbohydrate deprivation. Blocking photosynthetic electron transport with 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) prevented this increase in transcription. This work led to the conclusion that plastid derived redox signalling can over-ride the sugar regulated expression of nuclear encoded photosynthetic genes. In the sugar response mutant sun6 (sucrose uncoupled) PC-LUC transcription actually increased in response to exogenous sucrose rather than decreasing as in the wild type. Interestingly, plastid derived redox signals do not influence this defective pattern of sugar regulated gene expression in the sun6 mutant. A model, which invokes a positive inducer originating from the photosynthetic electron transport chain, was proposed to explain the nature of the plastid-derived signal. Previous studies in algal systems have shown that light and temperature are critical regulators of plastid redox state. When the redox state of the photosynthetic electron transport chain in the Arabidopsis culture was altered by changing temperature and light conditions differential effects were observed in carbon starved and carbon supplied cells. These results confirm that various environmental parameters can affect the plastid redox state and consequently nuclear gene expression in higher plants. Over the last few years a growing number of plant 14-3-3 binding proteins, including DNA binding proteins and key proteins of carbon metabolism, have been identified. The binding of these proteins to 14-3-3 proteins affects their phosphorylation state and consequently cellular activity. Sugar deprivation has recently been shown to have a dramatic effect on 14-3-3 binding protein abundance in the same cell culture that was used in this thesis work. However, the plastid redox signal as identified in this thesis work did not result in an altered regulation of 14-3-3 binding proteins by sugars, providing evidence that at least this proposed part of the sugar signalling pathway is not regulated by the redox state of the plastid.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Binding proteins; Photosynthesis