Glutamic dehydrogenase (GDH) in maize

The role of GDH in nitrogen assimilation is not certain in plants, and this hasstimulated our interest in this enzyme system. Previous work (starch gel electrophoresis) has shown that three isozymes are specified by alleles of the gene Glutamic dehydrogenase: Gdh-F, Gdh-N and Gdh-S. Field observations and growth chamber experiments suggested that plants carrying the Gdh-S allele were more sensitive to low night temperatures than were plants carrying the normal or common allele, Gdh-N. While most enzyme activity occurs in root or mesocotyl mitochondria significant levels can be recognized in pollen, and it seemed possible that a differential transmission of the alleles through the pollen after pollination at low temperatures might provide a sensitive measure of the effects of temperature. Plants from seeds of the F6 generation of Gdh-N/Gdh-S heterozygotes were used for the assay. Plants homozygous Gdh-N/Gdh-N were grown at 27 C day and 22 C night (27-22 C) until the silks emerged and were then placed in the test temperature for 24 hr prior to pollination. Five days after pollination the plants were returned to 27-22 C until the kernels matured. Seedlings were assayed for GDH isozymes as previously described (Heredity 32:397-401), and the results (Table 1) demonstrate that the pollen transmission of Gdh alleles was not influenced by temperature.

Table 1. Transmission of Gdh-S and Gdh-N alleles in pollinations at different temperatures.

Temperature

27-22 C

21-16 C

18-13 C

15-10 C

Ears

2

4

4

2

Progeny: Gdh-N

59

187

161

91

Gdh-S

42

215

172

94

S

101

402

333

185

X2[1:1]

0.44

1.95

0.36

0.05

P

n.s.

n.s.

n.s.

n.s.

There are many obvious weaknesses in this sort of experiment; it is not known whether the GDH that is present in pollen grains is in any way active during the germination and growth of pollen. Our future work will concentrate more on the effects of the different Gdh alleles on root growth at different temperatures. Preliminary characterization of the GDH isozymes indicates that all three have the same pH optima (about pH 8.5); the apparent Michaelis constants are Km[NADH], 7.1x10-5M; Km[a-ketoglutarate], 2.5x10-3; and Km[NH4+], 5x10-2M.

Tony Pryor


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