Effect of Aspartate‐derived Amino Acids and Aminoethyl Cysteine on Growth of Excised Mature Embryos of Maize1

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Abstract
Twenty‐one amino acids were tested individually for their effects on growth of roots from excised mature embryos of maize (Zea mays L.). Among these, tryptophan, tyrosine, threonine, and homoserine were partially inhibitory to root growth at 1 mM concentrations. Length and dry weight data showed that maize roots both from embryos and from kernels were synergistically inhibited by lysine + threonine but not other paired combinations of aspartate‐derived amino acids. Roots from embryos, however, were more sensitive to inhibition. The synergistic inhibition of embryo root length was near maximal at 1 mM lysine + 1 mM threonine. This synergism was not observed with other paired combinations of aspartatederived amino acids. Methionme and homoserine specifically relieved lysine + threonine inhibition, whereas 17 other amino acids were ineffective. These results are compatible with a model and published enzymology studies which suggest that exogenously added lysine and threonine may act as feedback inhibitors of aspartokinase and homoserine dehydrogenase, respectively, early enzymes in the aspartate biosynthetic pathway.The lysine analog, S‐2‐aminoethyl‐L‐cysteine (AEC) was a potent inhibitor of root growth in maize seedlings. Among 21 amino acids tested, lysine was most effective in relieving AEC inhibition while arginine and methionine were partially effective.