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Photorespiratory NH(4)(+) Production in Leaves of Wild-Type and Glutamine Synthetase 2 Antisense Oilseed Rape

Exposure of oilseed rape (Brassica napus) plants to increasing leaf temperatures between 15°C and 25°C increased photorespiratory NH(4)(+) production from 0.7 to 3.5 μmol m(−2) s(−1). Despite the 5-fold increase in the rate of NH(4)(+) production, the NH(4)(+) concentration in root and leaf tissue water and xylem sap dropped significantly, whereas that in the leaf apoplastic fluid remained constant. The in vitro activity of glutamine synthetase (GS) in both leaves and roots also increased with temperature and in all cases substantially exceeded the observed rates of photorespiratory NH(4)(+) production. The surplus of GS in oilseed rape plants was confirmed using GS2 antisense plants with 50% to 75% lower in vitro leaf GS activity than in the wild type. Despite the substantial reduction in GS activity, there was no tendency for antisense plants to have higher tissue NH(4)(+) concentrations than wild-type plants and no overall correlation between GS activity and tissue NH(4)(+) concentration was observed. Antisense plants exposed to leaf temperatures increasing from 14°C to 27°C or to a trifold increase in the O(2) to CO(2) ratio did not show any change in steady-state leaf tissue NH(4)(+) concentration or in NH(3) emission to the atmosphere. The antisense plants also had similar leaf tissue concentrations of glutamine, glycine, and serine as the wild type, whereas glutamate increased by 38%. It is concluded that photorespiration does not control tissue or apoplastic levels of NH(4)(+) in oilseed rape leaves and, as a consequence, that photorespiration does not exert a direct control on leaf atmosphere NH(3) fluxes

Molecular characterization of two ammonium transporters from the ectomycorrhizal fungus Hebeloma cylindrosporum.

Heterologous expression of the yeast triple Mep mutant has enabled the first molecular characterization of AMT/MEP family members in an ectomycorrhizal fungus. External hyphae, which play a key role in nitrogen nutrition of trees, are considered as the absorbing structure of the ectomycorrhizal symbiosis and therefore molecular studies on ammonium transport in hyphae are urgently needed. The kinetic properties of AMT2 and AMT3 from Hebeloma cylindrosporum were studied in Saccharomyces cerevisiae. Expression of HcAmts in the yeast triple Mep mutant restored ammonium retention within cells. The HcAmts did not complement the ammonium sensing defect phenotype of Mep2Delta cells during pseudohyphal differentiation. Northern blot analysis in H. cylindrosporum showed that the HcAMTs were up-regulated upon nitrogen deprivation and down-regulated by ammonium.Journal ArticleResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe