Lysine-methionine-threonine interactions in growth and development of Mimulus cardinalis seedlings.

Plants of Mimulus cardinalis Douglas (Scrophulariaceae) were grown in axenic culture from seed for 28 days on a minimal salts medium supple-mented with L-lysine, L-methionine, L-threonine, L-isoleucine, DL-or L-homoserine or DL-homocysteine, alone or in combinations ranging from 5 to 1000 μM. Abnormal growth was observed at the higher concentrations of all these aminoacidsexcept homocysteine. The lysine inhibition was significantly reduced by methionine, homocysteine or isoleucine. The threonine inhibition was significantly reduced by methionine or homocysteine. A combination of lysine and threonine at 1 MM was lethal. This synergistic effect was prevented when methionine, homoserine or homocysteine were added to the lysine-threonine combination. These results can be explained in terms of end-product control of aspartokinase and homoserine dehydrogenase by lysine and threonine, respectively, in the biosynthetic pathway to these aspartic-acid-derived amino acids.