Lysine metabolism in cereal crops.

Cereal seeds are deficient in certain amino acids, in particular lysine. Biochemical, molecular and genetic studies have increased our knowledge concerning the regulation of the aspartate pathway, which synthesises lysine. Among the enzymes involved in lysine metabolism, aspartate kinase (AK) and dihydrodipicolinate synthase (DHDPS) control the regulation of lysine biosynthesis, whereas lysine: 2-oxoglutarate reductase (LOR) and saccharopine dehydrogenase (SDH), have been shown to play a key role in the breakdown of lysine. Lysine overproduction can be obtained by altering the sensitivity of DHDPS to lysine, but accumulation of this amino acid in cereal seeds requires further manipulation of LOR and/or SDH. These latter two enzymes have been shown to be specific to the endosperm tissue and their activities are drastically reduced in the high-lysine mutants of maize. We have isolated, purified and characterized the enzymes AK, HSDH, LOR and SDH from rice and coix seeds. The results indicate that the regulation of the LOR activity is complex and involves a calcium-dependent phosphorylation/dephosphorylation mechanism. We analysed the activity of the enzymes involved in lysine metabolism, characterized the storage proteins and determined the soluble lysine content for several opaque and floury maize mutants and show that only some of the mutants are similar to the well studied opaque-2 mutant. The results suggest that lysine accumulation in maize seeds cannot be explained by an increase in soluble lysine, but may be mainly due to an altered storage protein composition