Increasing lysine levels in pigeonpea (Cajanus cajan (L.) Millsp) seeds through genetic engineering

In higher plants the essential amino acids lysine, threonine, methionine and isoleucine are synthesised through a branched pathway starting from aspartate. The key enzyme of lysine biosynthesis in this pathway-dihydrodipicolinate synthase (DHDPS)-is feedback-inhibited by lysine. The dhdps-r1 gene from a mutant Nicotiana sylvestris, which encodes a DHDPS enzyme insensitive to feedback inhibition, was used to improve the lysine content in pigeonpea seeds. The dhdps-r1 coding region driven by a phaseolin or an Arabidopsis 2S2 promoter was successfully overexpressed in the seeds of pigeonpea by using Agrobacterium transformation and particle bombardment. In 11 lines analysed, a 2- to 6-fold enhanced DHDPS activity in immature seeds at a late stage of maturation was found in comparison to wild type. The overexpression of dhdps-r1 led to an enhanced content of free lysine in the seeds of pigeonpea from 1.6 to 8.5 times compared with wild type. However, this was not reflected in an increase in total seed lysine content. This might be explained by a temporal discrepancy between maximal expression of dhdps-r1 and the rate of amino acid incorporation into storage proteins. Assays of the lysine degradative enzyme lysine-ketoglutarate reductase in these seeds showed no co-ordinated regulation of lysine biosynthesis and catabolism during seed maturation. All transgenic plants were fertile and produced morphologically normal seeds.