Characteristics of a glutamate dehydrogenase mutant of maize
Abstract
In higher plants the major pathway of ammonia assimilation is the glutamine synthetase:glutamate synthase (GS/GOGAT) cycle. Although there is potential for assimilation of ammonia via glutamate dehydrogenase (GDH), the precise contribution of the latter pathway to net ammonia assimilation has not been unequivocally established. Some reports suggest no role of GDH in ammonia assimilation while others suggest a significant contribution. To investigate the precise role of GDH, this study focuses on the genetic and biochemical characterization of a mutant of Zea mays (maize) which is deficient in one of the two genes which encode GDH. The study was undertaken to (1) examine the kinetics of $\sp{15}$NH$\sb4\sp{+}$ assimilation in the GDH deficient mutant and its related wildtype; (2) develop near isogenic lines differing for GDH and glycinebetaine and test linkage relationships between genes determining GDH and glycinebetaine levels; (3) characterize GDH isozymes and purify the GDH1 and GDH2 homo-hexamers; and (4) examine the enzyme kinetics of GDH1 and GDH2 homo-hexamers. The GDH-null mutant of maize was found to exhibit a 40-50% lower rate of root ammonia assimilation than the wildtype at the seedling stage in hydroponic culture. However because the mutant exhibited a 30% lower shoot/root ratio than the wildtype, it is unclear whether the lower rate of ammonia assimilation is a direct result of GDH1 deficiency per se. Genetic studies indicated that the Gdh1 gene is unlinked to the gene determining betaine accumulation capacity. In the process of establishing tissue cultures of maize, the GDH1 null mutant of maize was found to regenerate at a significantly higher rate than the GDH1 wildtype. The development of near isogenic lines differing for GDH1 should now enable more definitive $\sp{15}$NH$\sb4\sp{+}$ tracer studies and investigations of the role of GDH in the regeneration process. Purification of GDH1 homo-hexamer away from GDH2 homo-hexamer allowed a comparison of the kinetic properties of the two isozymes and will facilitate preparation of antibodies to the homo-hexamers for further study of the Gdh1Null mutation in maize.
Degree
Ph.D.
Advisors
Rhodes, Purdue University.
Subject Area
Botany|Plant propagation
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