Lysine biosynthesis and utilization during seed development of normal and opaque-2 Zea mays L

This investigation deals with the metabolism of the amino acids from the aspartate family in an attempt to correlate the high free lysine content and the increase of lysine-containing storage proteins in mature opaque-2 mutant seeds. During seed development the changing levels of free aspartate, lysine, methionine, and threonine in endosperm as well as in embryos followed a bi-phasic pattern characterized by a minimum at 35 d post pollination. Up to that time, no striking differences were observed in any of these amino acids between normal and opaque-2 mutant tissues. However, during the second phase, increased levels of these amino acids in mutant endosperm- and to a certain extent in mutant embryos -indicate a feedback deregulation of one or several of the enzymes involved in the biosynthesis of amino acids originating from aspartate. The increases in these amino acids correlated with the demonstrated increased protein synthesis consisting mainly of endosperm and embryo globulins. Diaminopimelate decarboxylase (EC 4.1.1.20) activity was found to be equal in normal and mutant endosperm until 35 d post pollination, whereafter the activity declined sharply and appeared instead in embryos, especially in those of the mutant. Since lysine did not accumulate to any extent in mutant embryos, it is concluded that it was transferred to the endosperm, possibly for globulin synthesis. The data indicate that the embryo may play a specific role in controlling tissue levels of amino acids and thus be involved in the regulation of protein synthesis.