title: Reassimilation of Photorespiratory Ammonium in Lotus japonicus Plants Deficient in Plastidic Glutamine Synthetase.
abstract: It is well established that the plastidic isoform of glutamine synthetase (GS2) is the enzyme in charge of photorespiratory ammonium reassimilation in plants. The metabolic events associated to photorespiratory NH4(+) accumulation were analyzed in a Lotus japonicus photorespiratory mutant lacking GS2. The mutant plants accumulated high levels of NH4(+) when photorespiration was active, followed by a sudden drop in the levels of this compound. In this paper it was examined the possible existence of enzymatic pathways alternative to GS2 that could account for this decline in the photorespiratory ammonium. Induction of genes encoding for cytosolic glutamine synthetase (GS1), glutamate dehydrogenase (GDH) and asparagine synthetase (ASN) was observed in the mutant in correspondence with the diminishment of NH4(+). Measurements of gene expression, polypeptide levels, enzyme activity and metabolite levels were carried out in leaf samples from WT and mutant plants after different periods of time under active photorespiratory conditions. In the case of asparagine synthetase it was not possible to determine enzyme activity and polypeptide content; however, an increased asparagine content in parallel with the induction of ASN gene expression was detected in the mutant plants. This increase in asparagine levels took place concomitantly with an increase in glutamine due to the induction of cytosolic GS1 in the mutant, thus revealing a major role of cytosolic GS1 in the reassimilation and detoxification of photorespiratory NH4(+) when the plastidic GS2 isoform is lacking. Moreover, a diminishment in glutamate levels was observed, that may be explained by the induction of NAD(H)-dependent GDH activity.
authors.affiliation: Departamento de Bioquímica Vegetal y Biología Molecular, Facultad de Química, Universidad de Sevilla, Calle Profesor García González, Sevilla, Spain.
authors.forename: Carmen M
authors.initials: CM
authors.lastname: Pérez-Delgado
authors.affiliation: Departamento de Bioquímica Vegetal y Biología Molecular, Facultad de Química, Universidad de Sevilla, Calle Profesor García González, Sevilla, Spain.
authors.forename: Margarita
authors.initials: M
authors.lastname: García-Calderón
authors.affiliation: Departamento de Bioquímica Vegetal y Biología Molecular, Facultad de Química, Universidad de Sevilla, Calle Profesor García González, Sevilla, Spain.
authors.forename: Antonio J
authors.initials: AJ
authors.lastname: Márquez
authors.affiliation: Departamento de Bioquímica Vegetal y Biología Molecular, Facultad de Química, Universidad de Sevilla, Calle Profesor García González, Sevilla, Spain.
authors.forename: Marco
authors.initials: M
authors.lastname: Betti
mesh_terms: D064751:Ammonium Compounds / Q000378:metabolism*; D001217:Aspartate-Ammonia Ligase / Q000378:metabolism; D005969:Glutamate Dehydrogenase / Q000378:metabolism; D005974:Glutamate-Ammonia Ligase / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D000070116:Lotus / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010788:Photosynthesis; D018515:Plant Leaves / Q000201:enzymology / Q000378:metabolism; D018087:Plastids / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D018749:RNA, Plant / Q000302:isolation & purification / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction
chemical_list: D064751:Ammonium Compounds; D018749:RNA, Plant; D005969:Glutamate Dehydrogenase; D001217:Aspartate-Ammonia Ligase; D005974:Glutamate-Ammonia Ligase