Plant Physiology 94:647-656 (1990)
© 1990 American Society of Plant Biologists
Metabolism and Enzymology
Kinetics of 15NH4+ Assimilation in Zea mays1
Preliminary Studies with a Glutamate Dehydrogenase (GDH1) Null Mutant
José R. Magalhães,
Grace C. Ju,
Patrick J. Rich and
David Rhodes
EMBRAPA—Centro Nacional de Pesquisa de Hortaliças-CNPH, Brasilia-D.F., Brazil,
Center for Plant Environmental Stress Physiology, Department of Horticulture, Purdue University, West Lafayette, Indiana 47907
In higher plants it is now generally considered that glutamate dehydrogenase (GDH) plays only a small or negligible role in ammonia assimilation. To test this specific point, comparative studies of 15NH4+ assimilation were undertaken with a GDH1-null mutant of Zea mays and a related (but not strictly isogenic) GDH1-positive wild type from which this mutant was derived. The kinetics of 15NH4+ assimilation into free amino acids and total reduced nitrogen were monitored in both roots and shoots of 2-week-old seedlings supplied with 5 millimolar 99% (15NH4)2SO4 via the aerated root medium in hydroponic culture over a 24-h period. The GDH1-null mutant, with a 10- to 15-fold lower total root GDH activity in comparison to the wild type, was found to exhibit a 40 to 50% lower rate of 15NH4+ assimilation into total reduced nitrogen. Observed rates of root ammonium assimilation were 5.9 and 3.1 micromoles per hour per gram fresh weight for the wild type and mutant, respectively. The lower rate of 15NH4+ assimilation in the mutant was associated with lower rates of labeling of several free amino acids (including glutamate, glutamine-amino N, aspartate, asparagine-amino N, and alanine) in both roots and shoots of the mutant in comparison to the wild type. Qualitatively, these labeling kinetics appear consistent with a reduced flux of 15N via glutamate in the GDH1-null mutant. However, the responses of the two genotypes to the potent inhibitor of glutamine synthetase, methionine sulfoximine, and differences in morphology of the two genotypes (particularly a lower shoot:root ratio in the GDH1-null mutant) urge caution in concluding that GDH1 is solely responsible for these differences in ammonia assimilation rate.
1 Purdue University Agricultural Experiment Station Journal Article No. 12182. This work was supported by Purdue University Agricultural Experiment Station funds, and by a grant from the Corporation for Science and Technology in Indiana.
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