Inhibitory effects of aspartate-derived amino acids and aminoethylcysteine, a lysine analog, on the growth of sorghum seedlings; relation with three enzymes of the aspartate-pathway

Abstract

The effects of the aspartate-derived amino acids, lysine and threonine on the root growth of Sorghum bicolor (L.) Moench. seedlings have been investigated. Lysine and threonine, when added separately, are not severe inhibitors. However, when present simultaneously, lysine and threonine (each 2 mM) can inhibit the root growth up to 90%. This growth inhibition can be relieved by methionine (or homoserine). The lysine analog, S-2- aminoethylcysteine (AEC) appears to be a strong inhibitor: the root growth decreases to 14% when 0.2 mM AEC is used. This inhibition can be relieved by adding lysine to the culture medium. These effects can be related to the feedback regulation exerted by lysine (or AEC) and threonine on key enzymes of the aspartate-pathway. The activity of aspartate kinase is inhibited for 78% by 0.5 mM lysine, for 25% by 0.5 mM threonine and by a combination of both these amino acids (each 0.5 mM) up to 90%. Diohydrodipicolinate synthase (DHDPS), the first enzyme unique to the lysine synthesis, is strongly inhibited by lysine (80% at 6 μM). Threonine is found to inhibit homoserine dehydrogenase activity up to 74%, using NADH as cofactor. Threonine has no effect on homoserine dehydrogenase when NADPH is used as cofactor.