Molecular genetic dissection and potential manipulation of lysine metabolism in seeds

Lysine is among the most important essential amino acids in the diet of human and livestock because it is presented in severely limiting amounts in cereals and other important crops. Attempts to increase lysine levels in plants were made by reducing the sensitivity of the key enzyme in lysine biosynthesis, namely dihydrodipicolinate synthase, to feedback inhibition by lysine. However, these studies showed that in plant seeds, lysine accumulation is determined not only by the rate of its synthesis, but also by the rate of its catabolism via the α-amino adipic acid pathway. Our laboratory is currently studying the regulation of lysine catabolism in plants in order to explore potentials of reducing lysine catabolic fluxes in transgenic plants. In plants, like animals, lysine is catabolized via saccharopine by two consecutive enzymes, lysine-ketoglutarate reductase (LKR) and saccharopine dehydrogenase (SDH), which are linked on a single bifunctional polypeptide. Yet SDH activity of the LKR/SDH polypeptide may be limitingin vivo because of its non-physiological pH optimum of activity. In some plants, likeArabidopsis and canola, this is overcome by the presence of an additional monofunctional SDH enzyme, which is encoded by the same locus that encodes the bifunctional LKR/SDH enzyme. Results from our, and other laboratories imply that attempts to generate high-lysine crop plants should take into account a seed-specific reduction of lysine catabolism.