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Lysine catabolism is one of the most highly regulated metabolic pathways in plants. Lysine catabolism may serve to regulate lysine homeostasis in some tissues, while efficiently converting lysine to glutamate and then to other stress-related metabolites in response to stress and in certain developmental processes. The first two enzymatic steps of the pathway are catalyzed by lysine-?-ketoglutarate reductase (LKR) and saccharopine dehydrogenase (SDH), which are separate domains of a bifunctional polypeptide LKR-SDH (Gonçalves-Butruille et al., 1996). LKR-SDH gene has been reported to produce transcripts for the bifunctional enzyme and separate monofunctional transcripts. Despite the physical linkage, LKR and SDH possess significantly different pH values for their activity. The structure and regulatory properties of the gene for LKR?SDH reveal that it involves a putative internal promoter in the middle of the gene so that encode both bifunctional LKR?SDH and monofunctional SDH. Bifunctional and monofunctional isoforms is species specific and enzyme has different physiological roles in different taxa. The level of LKR in plants was shown to be significantly up-regulated in inflorescence tissues and developing seeds, as well as in response to osmotic stress (Karchi et al., 1994; Karchi et al., 1995; Tang et al., 1997; Deleu et al., 1999).
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