Glutamine Synthetase Activity, Ammonium Accumulation and Growth of Callus Cultures of Asparagus officinalis L. Exposed to High Ammonium or Phosphinothricin

Abstract

Summary Glutamine synthetase (GS) activity, ammonium accumulation and growth responses of callus cultures of Asparagus officinalis L. were investigated following 4 weeks exposure to media with added ammonium, and again after a further 4 weeks on a modified basal medium (MBM) containing no added ammonium. Calli grown on MBM supplemented with 40 or 160 mM ammonium for 4 weeks had reduced GS activity, greatly enhanced ammonium content and reduced growth compared with calli exposed to 10 mM added ammonium. When calli were transferred back to MBM, GS activity increased, ammonium content decreased, and growth was enhanced. Phosphinothricin (PPT) was used to endogenously alter the ammonium content of callus tissue. Exposing calli to 10 or 100 μM PPT for 4 weeks reduced GS activity, enhanced ammonium accumulation and reduced growth compared with calli not exposed to PPT, and markedly enhanced callus glutamine content. Calli exposed to 100 pM PPT did not regrow when transferred back to basal media (BM) without added PPT for an additional 4 weeks. We also separated the effect of PPT-induced ammonium accumulation from alterations in tissue amino acid concentrations by adding up to 25 mM glutamine in addition to PPT during the initial 4-week treatment period. Glutamine supplementation overcame the PPT-induced reduction in growth even though GS activity was severely reduced and ammonium accumulated to high concentrations in calli exposed to 100 μM PPT. All calli continued to grow vigorously when transferred back to BM for an additional 4 weeks. The results demonstrate that ammonium accumulation per se was not lethal to asparagus callus tissue, and suggest other effects of using PPT as a selective inhibitor of GS.