Asparagine biosynthesis in soybean nodules

Area of Science:

• Plant Biochemistry
• Nitrogen Metabolism
• Soybean Physiology

Background:

• Asparagine biosynthesis in soybean nodules is poorly understood due to precursor conversion issues and enzyme instability.
• The enzyme asparagine synthetase is crucial for asparagine synthesis but has been difficult to study in vivo.

Purpose of the Study:

• To investigate the effects of ammonium assimilation inhibitors on (14)CO(2) metabolism to (14)C-asparagine in soybean nodules.
• To demonstrate the existence and characterize the activity of asparagine synthetase in soybean nodules.

Main Methods:

• Detached soybean nodules were incubated with (14)CO(2) in the presence or absence of methionine sulfoximine or azaserine.
• Radioactivity in asparagine and other amino acids was quantified.
• Asparagine synthetase enzyme was isolated and its activity, stability, and kinetic properties were determined.

Main Results:

• Radioactivity in asparagine increased significantly over 4 hours of (14)CO(2) incubation.
• Ammonium assimilation inhibitors reduced dark fixation and label incorporation into amino acids, including aspartate and asparagine.
• A glutamine-dependent asparagine synthetase was isolated, showing peak activity during early flowering and higher localization in nodule cytosol and infected zones.

Conclusions:

• The results support a glutamine-dependent pathway for asparagine synthesis from aspartate in soybean nodules.
• Asparagine synthetase is present and active in soybean nodules, with its activity fluctuating during plant development.
• The enzyme exhibits specific kinetic properties and preferential localization within the nodule.