Enhanced levels of free and protein-bound threonine in transgenic alfalfa (Medicago sativa L.) expressing a bacterial feedback-insensitive aspartate kinase gene

Threonine, lysine, methionine, and tryptophan are essential amino acids for humans and monogastric animals. Many of the commonly used diet formulation s, particularly for pigs and poultry, contain limiting amounts of these ami no acids. One approach for raising the level of essential amino acids is ba sed on altering the regulation of their biosynthetic pathways in transgenic plants. Here we describe the first production of a transgenic forage plant , alfalfa (Medicago sativa L.) with modified regulation of the aspartate-fa mily amino acid biosynthetic pathway. This was achieved by over-expressing the Escherichia coli feedback-insensitive aspartate kinase (AK) in transgen ic plants. These plants showed enhanced levels of both free and protein-bou nd threonine. In many transgenic plants the rise in free threonine was acco mpanied by a significant reduction both in aspartate and in glutamate. Our data suggest that in alfalfa, AK might not be the only limiting factor for threonine biosynthesis, and that the free threonine pool in this plant limi ts its incorporation into plant proteins.