Two genes for threonine accumulation in barley seeds

Abstract

Cereal seed protein is the major direct source of dietary protein for man and also serves as an indirect source through the feeding of livestock. For non-ruminant animals, such as man, swine and poultry, most cereal seed proteins are deficient in lysine and threonine and much effort has been devoted to their improvement¹. Supplementation of barley diets for pigs with lysine and threonine is sufficient to produce a feed with protein of high biological value². Mutant lines that accumulate these amino acids in the seed could therefore be of agricultural value. The synthesis of lysine, threonine and methionine, which are all derived from aspartic acid, is regulated in plants by a series of feedback loops^3,4. This regulation is such that when barley embryos are germinated on a medium containing lysine plus threonine (LT medium) the plants are starved of methionine and fail to grow⁵. Plants which do grow in these conditions may contain enzymes that are no longer feedback-regulated and that may allow enhanced amounts of end-product amino acids to accumulate. We report here the identification of two genes in barley, mutations in which lead to decreased feedback sensitivity of the enzyme aspartate kinase, resistance to lysine plus threonine and, in some cases, to accumulation of soluble threonine in the seed.