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Effects of aerobic versus anoxic conditions on glutamine synthetase activity in eelgrass (Zostera marinaL.) roots: regulation of ammonium assimilation potential

Authors
Journal
Journal of Experimental Marine Biology and Ecology
0022-0981
Publisher
Elsevier
Publication Date
Volume
311
Issue
1
Identifiers
DOI: 10.1016/j.jembe.2004.04.013
Keywords
  • Ammonium Assimilation
  • Anoxia
  • Eelgrass
  • Glutamine Synthetase
  • Roots
  • Zostera Marina
Disciplines
  • Biology

Abstract

Abstract Root glutamine synthetase (GS; EC 6.3.1.2) activity was measured daily (0 to 4 days) for eelgrass ( Zostera marina L.) plants held under continuous darkness rooted in sediments, continuous darkness without sediments, continuous light without sediments, and control light/dark cycle (Control L/D). Roots experiencing prolonged aerobiosis exhibited lower activity in vitro than controls, whereas roots experiencing prolonged anoxia exhibited increased activity. Plants held in darkness without sediments had activity intermediate between controls and anoxic roots. One-hour pretreatment of root extracts with ATP slightly reduced in vitro glutamine synthetase activity, whereas pretreatment with ADP and AMP increased activity ≈50%. While glutamine synthetase activity increased with higher adenylate energy charge (AEC) in the reaction mixture, pretreatment of enzyme extracts at high adenylate energy charges decreased subsequent activity relative to pretreatment at lower energy charges. One-hour pretreatment with l-alanine (Ala) had little effect on enzyme activity. Pretreatment with l-glutamine (Gln), l-glutamate (Glu), and γ-amino butyric acid (GABA) increased activity ≈75%. Incubation of excised roots under anoxic conditions for 24 h nearly doubled enzyme activity. However, addition of cycloheximide to anoxic root incubations lessened or prevented the increase in activity. It appears that enhanced glutamine synthetase activity following periods of root anoxia results from interactions with metabolites that fluctuate between aerobic and anoxic conditions, particularly adenylates, and from de novo synthesis of glutamine synthetase or some other protein synthesis-dependent process.

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