Journal of Experimental Botany, Vol. 52, No. 362, pp. 1785-1803,
September 1, 2001
© 2001 Oxford University Press
Original Papers |
Single and double overexpression of C4-cycle genes had differential effects on the pattern of endogenous enzymes, attenuation of photorespiration and on contents of UV protectants in transgenic potato and tobacco plants
1 Botanisches Institut der Universität zu Köln, Gyrhofstraße 15, D-50931 Köln, Germany
2 Institut für Biologie I (Botanik/Molekulargenetik), Rheinisch-Westfälische Technische Hochschule, Worringer Weg 1, D-52074 Aachen, Germany
To improve the efficiency of CO2 fixation in C3 photosynthesis, C4-cycle genes were overexpressed in potato and tobacco plants either individually or in combination. Overexpression of the phosphoenolpyruvate carboxylase (PEPC) gene (ppc) from Corynebacterium glutamicum (cppc) or from potato (stppc, deprived of the phosphorylation site) in potato resulted in a 3–6-fold induction of endogenous cytosolic NADP malic enzyme (ME) and an increase in the activities of NAD-ME (3-fold), NADP isocitrate dehydrogenase (ICDH), pyruvate kinase (PK), NADP glycerate-3-P dehydrogenase (NADP-GAPDH), and PEP phosphatase (PEPP). In double transformants overexpressing cppc and chloroplastic NADP-ME from Flaveria pringlei (fpMe1), cytosolic NADP-ME was less induced and pleiotropic effects were diminished. There were no changes in enzyme pattern in single fpMe1 overexpressors. In cppc overexpressors of tobacco, the increase in endogenous cytosolic NADP-ME activity was small and changes in other enzymes were less pronounced. Determinations of the CO2 compensation point (
*) as well as temperature and oxygen effects on photosynthesis produced variational data suggesting that the desired decline in photorespiration occurred only under certain experimental conditions. Double transformants of potato (cppc/fpMe1) exhibited the most consistent attenuating effect on photorespiration. In contrast, photorespiration in tobacco plants appeared to be diminished most in single cppc overexpressors rather than in double transformants (cppc/fpMe1). In tobacco, introduction of the PEP carboxykinase (PEPCK) gene from the bacterium Sinorhizobium meliloti (pck) had little effect on photosynthetic parameters in single (pck) and double transformants (cppc/pck). In transgenic potato plants, increased PEPC activities resulted in a decline in UV protectants (flavonoids) in single cppc or stppc transformants, but not in double transformants (cppc/fpMe1). PEP provision to the shikimate pathway inside the plastids, from which flavonoids derive, might be restricted only in single PEPC overexpressors.
Key words: C4-cycle, malic enzyme, phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, transgenic potato/tobacco.
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