Expression of bacterial dihydrodipicolinate synthase in transgenic plants: potentials to improve lysine content in forage, grain and tuber crops

Plants generally contain low proportions of several essential amino acids, particularly lysine. The potential of increasing lysine content in vegetative tissues and storage organs of plants was studied by expression in transgenic tobacco plants of a chimeric gene encoding a bacterial dihydrodipicolinate synthase (DHPS), a key enzyme in lysine biosynthesis. The bacterial enzyme is much less sensitive to feedback inhibition by lysine than its plant counterparts. Constitutive expression of the bacterial enzyme caused a dramatic elevation of free lysine in vegetative tissues, which in some cases amounted up to 40 mol % of total free amino acids. In addition, constitutive lysine overproduction caused an alteration in plant phenotype, including loss of apical dominance, reduced plant height, bushy appearance, altered leaf structure and delayed flowering. In order to study the potential of the bacterial DHPS to increase lysine content in storage organs, we have expressed this bacterial enzyme in a seed-specific manner in transgenic tobacco plants. Although lysine synthesis was enhanced during seed development in the transgenic plants, this amino acid failed to over accumulate in mature seeds. The low lysine levels in mature seeds of the transgenic plants was correlated with a lysine-dependent stimulation of another enzyme, lysine-ketoglutarate reductase, which catabolizes lysine into saccharopine. This stimulation was shown to operate via an intracellular signaling cascade, mediated by Ca2+ and protein phosphorylation. The potential of the bacterial DHPS, either alone or in combination with additional genes, to improve the nutritional quality of forage, grain and tuber crops is discussed.