Soybean mutants lacking constitutive nitrate reductase activity : I. Selection and initial plant characterization

Area of Science:

• Plant Physiology
• Biochemistry
• Genetics

Background:

• Nitrate reductase (NR) is a key enzyme in plant nitrogen assimilation.
• Understanding NR regulation is crucial for improving crop nitrogen use efficiency.

Purpose of the Study:

• To select and characterize soybean mutants with reduced nitrate reductase activity.
• To investigate the nature of NR activity in soybean leaves and roots.

Main Methods:

• Chlorate screening of mutagenized soybean seedlings to identify low nitrate reductase (LNR) mutants.
• Growth chamber and field studies to analyze NR activity in leaves and roots under different nutrient conditions (nitrate vs. urea).
• In vivo NR assays and analysis of oxides of nitrogen (NO(x)) evolution.

Main Results:

• Three LNR mutant lines (LNR-2, LNR-3, LNR-4) were identified, exhibiting approximately 50% of wild-type NR activity in nitrate-grown leaves.
• Mutant leaves showed no NR activity when grown on urea, indicating the absence of constitutive NR activity, unlike wild-type plants.
• Root NR activity was comparable between wild-type and mutant plants, and constitutive NR activity was absent in roots of both types when grown on urea.
• Constitutive NR activity in wild-type plants was localized to the youngest fully expanded leaves and associated with NO(x) evolution, which was absent in LNR-2 mutants.

Conclusions:

• Soybean leaf NR activity comprises both constitutive and inducible components.
• The selected LNR mutants (LNR-2, LNR-3, LNR-4) are deficient in the constitutive NR activity.
• Constitutive NR activity is primarily found in young leaves and linked to NO(x) evolution, suggesting a role in nitrogen metabolism within specific plant tissues.