Induction of a C4-Like Mechanism of CO2Fixation in Egeria densa, a Submersed Aquatic Species

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Abstract
The expression of phosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme (NADP-ME) in Egeria densaleaves was studied under low temperature and light (LTL) following incubation under high temperature and light (HTL), conditions previously shown to induce high and low CO2 compensation points, respectively. Transfer from LTL to HTL conditions induced increases in the activities and amounts of both enzymes. One NADP-ME isoform was observed in induced and uninduced samples. Two isoforms of PEPC were expressed, with the lower M risoform being induced by HTL. NADP-ME showed properties similar to those of the isoform in C3 species. The inducible PEPC isoform has a low K m for both substrates. PEPC kinetic and regulatory properties (V maxand K m for phosphoenolpyruvate, and I 50for l-malate) are different in samples taken in the dark from those in the light, indicating that some modification of PEPC may be occurring during the day. Finally, abscisic acid induced the expression of PEPC and NADP-ME in a manner similar to temperature induction, except that the activities of both PEPC isoforms were increased. A different signaling system may exist in this species in response to high temperature or abscisic acid, both of which induce changes in photosynthetic metabolism.