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Plant and Cell Physiology, 2002, Vol. 43, No. 11 1293-1301
© 2002 Oxford University Press

Photosynthesis with Single-Rooted Amaranthus Leaves. II. Regulation of Ribuelose-1,5-Bisphosphate Carboxylase, Phosphoenolpyruvate Carboxylase, NAD-Malic Enzyme and NAD-Malate Dehydrogenase and Coordination between PCR and C4 Photosynthetic Metabolism in Response to Changes in the Source-Sink Balance

Shinichi Sawada1,3, Takeshi Sakamoto1, Makiko Sato1, Minobu Kasai1 and Hideaki Usuda2

1 Department of Biofunctional Science, Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki, 036-8561 Japan
2 Laboratory of Chemistry, Faculty of Medicine, Teikyo University, Ohtsuka, Hachioji, Tokyo, 192-0395 Japan

We have studied source-sink relationships with a model consisting of single-rooted leaves without petioles. We previously reported that the rate of photosynthesis decreased when C4 model plants prepared from Amaranthus cruentus leaves were subjected to sink-limited conditions by exposure to continuous light for a few days. It was suggested that the inhibition is due to a coordinated decrease in the activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) and phosphoenol-pyruvate carboxylase (PEPcase), both essential enzymes for photosynthesis in C4 plants. We further investigated the mechanisms behind the decreased activity of RuBPcase, PEPcase, NAD-malic enzyme and NAD-malate dehydrogenase. The results suggested that (1) the initial activity of RuBPcase is suppressed by a lowering of the Pi level in chloroplasts, (2) the inhibition of PEPcase is due to dephosphorylation of the enzyme via the inhibition of PEPcase kinase and PEPcase phosphatase, (3) the inhibition of NAD-malic enzyme and NAD-malate dehydrogenase is derived from the oxidation of these enzymes, and (4) some proteinous factor(s) may be involved in the inhibition of the activity of these latter three enzymes. The significance of a coordinated decrease in these enzymes in response to a change in the source-sink balance is discussed.

3 Corresponding author: E-mail, shinichi@cc.hirosaki-u.ac.jp; Fax, +81-172-39-3583.


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