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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Synthesis of the Arabidopsis Bifunctional Lysine-Ketoglutarate Reductase/Saccharopine Dehydrogenase Enzyme of Lysine Catabolism Is Concertedly Regulated by Metabolic and Stress-Associated Signals¹

Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100 Israel

In plants, excess cellular lysine (Lys) is catabolized into glutamic acid and acetyl-coenzyme A; yet, it is still not clear whether this pathway has other functions in addition to balancing Lys levels. To address this issue, we examined the effects of stress-related hormones, abscisic acid (ABA), and jasmonate, as well as various metabolic signals on the production of the mRNA and polypeptide of the bifunctional Lys-ketoglutarate reductase (LKR)/saccharopine dehydrogenase (SDH) enzyme, which contains the first two linked enzymes of Lys catabolism. The level of LKR/SDH was strongly enhanced by ABA, jasmonate, and sugar starvation, whereas excess sugars and nitrogen starvation reduced its level; thus this pathway appears to fulfill multiple functions in stress-related and carbon/nitrogen metabolism. Treatments with combination of hormones and/or metabolites, as well as use of ABA mutants in conjunction with the tester sugars mannose and 3-O-methyl-glucose further supported the idea that the hormonal and metabolic signals apparently operate through different signal transduction cascades. The stimulation of LKR/SDH protein expression by ABA is regulated by a signal transduction cascade that contains the ABI1-1 and ABI2-1 protein phosphatases. By contrast, the stimulation of LKR/SDH protein expression by sugar starvation is regulated by the hexokinase-signaling cascade in a similar manner to the repression of many photosynthetic genes by sugars. These findings suggest a metabolic and mechanistic link between Lys catabolism and photosynthesis-related metabolism in the regulation of carbon/nitrogen partitioning.

¹ This work was supported by the FrameWork Program of the Commission of The European Communities, and by The Israel Academy of Sciences and Humanities, National Council for Research and Development. G.G. is an incumbent of the Bronfman Chair in Plant Sciences.

^* Corresponding author; e-mail gad.galili{at}weizmann.ac.il; fax 972–8–9344181.

Received May 2, 2003; returned for revision June 2, 2003; accepted July 15, 2003.

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