Enzymes of nitrogen assimilation during seed development in normal and high lysine mutants in maize (Zea mays, W64A)

Abstract

Enzymes involved in nitrogen metabolism in endosperms of a normal variety of maize (W64A) and isogenic high lysine mutants (opaque-2 and floury-2) were examined. Glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), asparaginase, asparagine synthetase, and glutamine synthetase were present in the immature endosperm in all three genotypes; increased in activity just prior to the onset of zein biosynthesis; and remained at maximal levels during the period of rapid accumulation of nitrogen. With the exception of GOGAT trends and levels of activities were similar in all cases. Opaque-2 mutants had higher levels of GOGAT (29 ± 0.5 nmol∙min−1 endosperm−1 at day 20 postpollination) than floury-2 (19 ± 0.07) or W64A (13 ± 0.6). Levels of aspartate, asparagine, glutamate, and glutamine were also higher in the high lysine mutants throughout the developmental sequence. NH4+ in the endosperm rose from low levels at day 5 (0.1 μmol∙endosperm−1) to significant levels (1.3, 1.7, and 1.98 μmol∙endosperm−1 in normal, floury-2, and opaque-2, respectively) between 20 and 25 days and then declined. The decline was less apparent in the mutants. Levels of an endopeptidase increased initially in the control and then declined. In the mutants the decline in activity was less apparent and this resulted in higher levels of protease activities at later stages of development. RNAase activities were higher in the mutants throughout the developmental sequence. Where differences were observed, they were more apparent in the opaque-2 than in the floury-2 mutants.