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Sandra Cortès

Control of carbon metabolism in heterotrophic plant cells: Analysis of sugar sensing-function of hexokinases

Published on 28 October 2004

Thesis presented October 28, 2004

In plants, sugars constitute the main energy source for respiration and growth. By modifying gene expression, they are also involved in signaling pathways. In cells, sugar perception occurs at several levels during the phosphorylation​ step of the catalysis of hexoses by hexokinase. The involvement of hexokinase in signaling by sugars was demonstrated by way of glucose analogues that were subsequently metabolized in the cell to different extents. We have used this metabolic approach on both maize root tips and heterotroph cells of Arabidopsis thaliana. We monitored the effects of different carbon substrates (3-O-methyglucose, dihydroxyacetone and glycerol) on the proteolysis and regulation of endoproteases (RSIP) and on cell respiration; its energy state and their metabolic profiles.
These studies show that 3-O-methyglucose (3-OMG) is not a substrate for respiration and does not contribute to biosynthesis. Proteolysis and lipolysis are induced in 3-OMG-fed maize roots. Contrary to prevailing opinion, we show by NMR spectroscopy, th​at 3-OMG is phosphorylated to 3-OMG-6P by the hexokinases. Nevertheless, the rate of catalysis of hexokinase is 100 00 less for 3-OMG than for glucose or mannose. To interpret these observations, we hypothesize that intensity of the sugar signal is related to the flux of carbon through the hexokinase. The results obtained using dihydroxyacetone and glycerol show that both RSIP expression and proteolysis are regulated in a different manner. RSIP expression is regulated at the level of hexokinases, whereas proteolysis is regulated by the supply of carbon substrates to mitochondria. Variations in the flux of carbon through hexokinases seem to be an essential part of the mechanism by which sugars participate in signaling. The last part of the manuscript describes the characterization of a new biological material, heterotroph cell cultures of Arabidopsis thaliana together with the development of experimental procedures for measuring the rate of metabolic flux.

Arabidopsis thaliana, glucose analogues, NMR spectroscopy, carbon flux modulation, heterotroph cell cultures, metabolic flux

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