Nitrogen metabolism in citrus based on expressed tag analysis

Nitrogen metabolism in citrus arouses special interest among other plant nutrients due to its important role in plant growth and fruit production. Nitrogen is first assimilated in organic forms as glutamine, glutamate, aspartate, and asparagine from ammonium or nitrate. This vital process integrates pathways from energetic, central intermediary, and biosynthetic metabolism routes, culminating in the translocation of compounds by all parts of the plant. From a number of essential enzymes involved in this process, we depicted seven enzymes named nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), aspartate aminotransferase (AspAT), and asparagine synthetase (AS) for gene prospection based on EST sequencing of Citrus sinensis, C. reticulata, C. aurantifolia, C. latifolia, C. aurantium, and Poncirus trifoliata. These species are part of the CitEST Brazilian program which is focusing on efforts to obtain a great number of EST (expressed sequence tags) related to different citrus species and genera at different developmental stages or under biotic or abiotic stresses. Irrespective of origin, conditions, and nucleotide similarities, citrus species in general conserve a precise set of enzymes for nitrogen metabolism.