PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Plastocyanin contributes to the photosynthetic flux control in tobacco leaves. Mark Aurel Schöttler^*,1, Helmut Kirchhoff¹, Engelbert Weis¹, ¹ Institute of Botany, University of Münster, Schlossgarten 3, Münster, Germany

ABSTRACT- Photosynthetic flux was measured in wild type (wt) and transgenic tobacco plants with overexpressed yeast invertase in the apoplast. In wt, photosynthetic O₂ evolution decreased during senescence from 250 micromoles O₂ mg Chl^-1 h^-1 in young mature leaves (5th to 10th leaf generation) to 50 micromoles O₂ mg Chl^-1 h^-1 (20th leaf generation). In transgenic plants, the photosynthetic rate of young mature leaves could decrease to 50 micromoles O₂ mg Chl^-1 h^-1, depending on the expression level of invertase. The electron transport capacity in isolated thylakoids from these plants, with methylviologen (MV) as an acceptor, always closely matched the assimilation rate in leaves. All plants, irrespective of their photosynthetic flux capacities, exhibited a constant number of Photosystems (2.2/2.5 mmoles PSII/PSI mol Chl.^-1) and plastoquinone (9 mmoles PQ mol Chl.^-1), and a constant chl. a/b ratio. However, the level of photooxidable plastocyanin (PC) decreased with photosynthetic flux rates from about 12 mmoles PC mol Chl.^-1 in young wt leaves to 2 mmoles mol Chl.^-1 in older leaves, or in young leaves from transgenics. PC and fluxes correlated in a linear manner. In wt leaves, the level of cytochrome bf closely followed the PC level (about 8 PC/cyt.bf). In plants overexpressing apoplastic invertase, cytochrome bf stayed relatively constant while PC and photosynthetic flux were depressed. In all plants, the rate of the high-potential electron transport chain (duroquinol to MV) closely matched that of the whole chain transport (H₂O to MV). We suggest that the concentrations of both, cytochrome bf and PC are important factors in the photosynthetic flux control. In leaves overexpressing yeast invertase (exhibiting high apoplastic sugar) the cytochrome bf seems to be expressed in excess, relative to PC, and the flux is mainly controlled by PC. The PC gene expression is known to be modulated by a number of environmental and metabolic (including sugar) signals (Pfannschmidt 2003: TIPS 8, 33-41). Under conditions of sugar-controlled, sink-limited photosynthesis, PC may actually be a major control step to adjust the rate of electron donation to PSI to the rate of electron consuming metabolic reactions. Matching of electron donation and consumption at PSI may be essential to avoid reactive oxygen production.

KEY WORDS: flux control, sink limitation, plastocyanin, electron transport