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.

Reproductive organs regulate leaf nitrogen metabolism mediated by cytokinin signal

“…In the reproductive stage, these nitrogen compounds are depredated and the amino acids transported to sink tissues. Because the production of GS1 and AAPs, which are involved in whole plant partitioning of nitrogen, is induced in the reproductive stage (Miflin and Habash 2002;Tilsner et al 2005), it is possible that expression of the genes encoding these enzymes is affected by the development of sink tissues. In this report, we observed a reduction in amino acid content during leaf-stage progression and an increase in amino acid content after removal of the reproductive structures, supporting these possibilities.…”

Nitrogen use efficiency in rapeseed. A review

“…Moreover, it was shown that while the N content in stems varied at flowering between the fertilization levels, it was similar at maturity (Stahl et al 2015). Tilsner et al (2005) showed that the phloem loading of amino acids was not limiting for nitrogen remobilization efficiency, and they hypothesized that sink capacity rather than source strength was the main bottleneck for N remobilization in rapeseed. The final sink capacity of rapeseed can be enhanced primarily through the improvement of the yield components per se.…”

Contents of Amino Acids in Grains of Different Bread Wheat Genotypes

“…Nitrogen fertilisation can be used for nutritional improvement of the human diet by increasing and maintaining protein and essential amino acid contents (Thanapornpoonpong et al, 2008). According to Tilsner et al (2005) under low N supply, the amino acid contents are comparable at all leaf ages and decrease slightly from young to mature leaves, while higher N supply also causes an overall increase in amino acid content (Atanasova, 2008).…”