Forms of retranslocated nitrogen during leaf senescence and nitrogen deficiency
Final Report Abstract
Nitrogen (N) retranslocation from senescing leaves to the grains is one of the limiting factors for an efficient utilization of N in plants. Since the catabolism of nitrogenous compounds produces large quantities of urea during senescence, we investigated the roles of DUR3 and of urea in N remobilization. During natural leaf senescence urea concentrations and DUR3 transcript levels showed a parallel increase with senescence markers like ORE1 in a plant age- and leaf age-dependent manner. Deletion of DUR3 decreased urea accumulation in leaves, whereas the fraction of urea lost to the leaf apoplast was enhanced. Under natural and N deficiency-induced senescence DUR3 promoter activity was highest in the vasculature. An analysis of petiole exudates from wildtype leaves revealed that N from urea accounted for >13% of amino acid N. Urea export from senescent leaves further increased in ureG deletion mutants lacking urease activity. In the dur3,ureG double insertion line the absence of DUR3 reduced urea export from leaf petioles. These results indicated that urea can serve as an early metabolic marker for leaf senescence, and that DUR3-mediated urea retrieval contributes to the retranslocation of N from urea during leaf senescence. Although the DUR3 homolog in yeast is also able to transport polyamines, a function of DUR3 in polyamine transport was not found. Moreover, polyamine profiling of different plant organs indicated that polyamine levels increase with the N nutritional status but are not typically enriched during senescence in a plant age-dependent manner, but rather decrease with the progressing age of the organ. In cooperation with Project 4 we identified an unexpected role of the nitrate transporter NRT1.5 in xylem loading of K+. Growth experiments under defined conditions and ionome profiling revealed that NRT1.5 is required for the long-distance transport of K+ from roots to shoots and thus for preventing premature leaf senescence. In barley, a prominent role of N in iron retranslocation was characterized. Subjecting barley plants with senescent leaves to different N regimes for subsequent metabolic and elemental analysis showed that a low N nutritional status is required for the synthesis of iron chelators and for an efficient iron retranslocation from senescent source leaves into sink organs. In an attempt to characterize the source function of roots during leaf senescence in barley, histological, metabolic, elemental and gene expression studies were conducted in a time course. It turned out that several different processes indicative for senescence or aging processes in roots were temporally coordinated, such as degradation of the root cortex, urea accumulation, leakage of mineral elements, decrease in nutrient uptake activity and the upregulation of genes known to regulate leaf senescence. The time point of these changes fell together with a peak in root ABA concentrations. A putative function of ABA in triggering root senescence processes is currently verified.
Publications
- (2012) Senescenceinduced iron mobilization in source leaves of barley (Hordeum vulgare) plants. New Phytol. 195: 372-383
Shi R., Weber G., Köster J. Hajirezaei M., Zhou C., Zhang F., von Wirén N.
(See online at https://doi.org/10.1111/j.1469-8137.2012.04165.x) - (2015) Extraction of apoplastic wash fluid and leaf petiole exudates from leaves of Arabidopsis thaliana. Bio-protocol 5(24): e1691
Araya, T., Bohner A., von Wirén N.
(See online at https://doi.org/10.21769/BioProtoc.1691) - (2015) Nitratedependent control of shoot K homeostasis by NPF7.3/NRT1.5 and SKOR in Arabidopsis. Plant Physiol. Oct 27. pp. 01152.2015
Drechsler N., Zheng Y., Bohner A., Nobmann B., von Wirén N., Kunze R., Rausch C.
(See online at https://doi.org/10.1104/pp.15.01152) - (2015) Urea retranslocation from senescing Arabidopsis leaves is promoted by DUR3-mediated urea retrieval from the leaf apoplast. Plant J. 81: 377-387
Bohner A., Kojima S., Hajirezaei M., Melzer M., von Wirén N.
(See online at https://doi.org/10.1111/tpj.12740)
