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JXB Advance Access originally published online on November 28, 2003
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Journal of Experimental Botany, Vol. 55, No. 394, pp. 69-75, January 1, 2004
© 2004 Oxford University Press

Cell and Molecular Biology, Biochemistry and Molecular Physiology

The rate of CO2 assimilation controls the expression and activity of glutamine synthetase through sugar formation in sunflower (Helianthus annuus L.) leaves

Received 17 September 2003; Accepted 3 October 2003

B. Larios1, E. Agüera1, P. Cabello1, J. M. Maldonado2,* and P. de la Haba1

1 Departamento de Biología Vegetal, División de Fisiología Vegetal, Facultad de Ciencias, Universidad de Córdoba, Avda. San Alberto Magno s/n, 14004 Córdoba, Spain
2 Departamento de Biología Vegetal y Ecología, Unidad de Fisiología Vegetal, Facultad de Biología, Universidad de Sevilla, Avda. Reina Mercedes 6, 41012 Sevilla, Spain

* To whom correspondence should be addressed. Fax: +34 954 55 70 05. E-mail: maldonado{at}us.es
Abbreviation: GS, glutamine synthetase.

The expression and activity of glutamine synthetase (GS, EC 6.3.1.2) were examined in relation to the rate of CO2 assimilation in sunflower (Helianthus annuus L.) leaves. Intact plants were kept in the dark for 72 h and subsequently exposed to light under different atmospheric CO2 concentrations (100, 400 and 1200 µl l–1) for 6 h. The in vivo rates of net CO2 assimilation correlated with atmospheric CO2 concentrations. Stomatal conductances and transpiration rates remained largely unaffected by CO2 levels. Exposure of the plants to increasing CO2 concentrations in the light caused concomitant increases in the contents of starch and soluble sugars and a decrease in the nitrate content in leaves. Both cytosolic and chloroplastic (GS2) GS activities were higher at elevated CO2. A greater accumulation of GS2 mRNA was also observed under high CO2. Exogenous supply of sucrose to detached leaves greatly increased the levels of GS enzyme activity and of mRNA for chloroplastic GS in the dark. These results indicate that GS expression and activity in sunflower leaves are modulated by the rate of CO2 assimilation, and that photosynthesized sugars are presumably involved as regulatory metabolites.

Key words: Carbohydrates, CO2 assimilation, gene expression and enzyme activity, glutamine synthetase, Helianthus annuus.


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