title: Amino acid contents and transport in oilseed rape (Brassica napus L.) under different nitrogen conditions.
abstract: Oilseed rape (Brassica napus L.) needs very high nitrogen fertilizer inputs. Significant amounts of this nitrogen are lost during early leaf shedding and are a source of environmental and economic concern. The objective of this study was to investigate whether the remobilization of leaf amino acids could be limiting for nitrogen use efficiency. Therefore, amino acid concentrations were analyzed in subcellular compartments of leaf mesophyll cells of plants grown under low (0.5 mM NO3-) and high (4 mM NO3-) nitrogen supply. With high nitrogen supply, young leaves showed an elevated amino acid content, mainly in vacuoles. In old leaves, however, subcellular concentrations were similar under high and low nitrogen conditions, showing that the excess nitrogen had been exported during leaf development. The phloem sap contained up to 650 mM amino acids, more than four times as much than the cytosol of mesophyll cells, indicating a very efficient phloem-loading process. Three amino acid permeases, BnAAP1, BnAAP2, and BnAAP6, were identified and characterized. BnAAP1 and BnAAP6 mediated uptake of neutral and acidic amino acids into Xenopus laevis oocytes at the actual apoplastic substrate concentrations. All three transporters were expressed in leaves and the expression was still detectable during leaf senescence, with BnAAP1 and BnAAP2 mRNA levels increasing from mature to old leaves. We conclude that phloem loading of amino acids is not limiting for nitrogen remobilization from senescing leaves in oilseed rape.
authors.affiliation: Biochemie der Pflanze, Göttinger Zentrum für Molekulare Biowissenschaften, Universität Göttingen, Justus-von-Liebig-Weg 11, 37077, Göttingen, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Tilsner
authors.forename: Nina
authors.initials: N
authors.lastname: Kassner
authors.forename: Christine
authors.initials: C
authors.lastname: Struck
authors.forename: Gertrud
authors.initials: G
authors.lastname: Lohaus
mesh_terms: D026905:Amino Acid Transport Systems / Q000235:genetics / Q000378:metabolism; D000596:Amino Acids / Q000032:analysis / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D029688:Brassica napus / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D002451:Cell Compartmentation; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics; D005260:Female; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009566:Nitrates / Q000378:metabolism / Q000494:pharmacology*; D009865:Oocytes / Q000378:metabolism; D018515:Plant Leaves / Q000166:cytology / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D013379:Substrate Specificity; D015854:Up-Regulation; D014982:Xenopus laevis
chemical_list: D026905:Amino Acid Transport Systems; D000596:Amino Acids; D018076:DNA, Complementary; D007527:Isoenzymes; D009566:Nitrates