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Aims

Establishment of environmentally-sound agriculture raises demands for a reduction of N budget surpluses in crop production through a more efficient use of soil and fertilizer nitrogen (N). This requires the breeding and cultivation of genotypes with more efficient use of N, allowing a reduction in N supply without yield penalty. Nitrogen efficiency of crops mainly depends on i) maintenance of the N uptake capacity in the roots during reproductive growth and ii) efficient retranslocation of N from vegetative to reproductive plant organs during the seed-filling.

Since leaf senescence leads to decreased allocation of photosynthates to roots (reducing N uptake) and seeds (reducing grain yields) a fine tuning of leaf senescence regarding the mobilization of N for reproductive growth and the maintenance of photosynthetic capacity is necessary. The overall hypothesis of this research project is that an in-depth understanding of the physiological and molecular processes controlling leaf senescence in relation to N mobilization and allocation to reproductive growth will allow developing traits for the breeding of N-efficient crop genotypes.

The main objectives of the experimental complementary approaches of the participating research groups are:
  • to characterize and compare the physiological processes and molecular signals governing developmentally regulated senescence versus N deficiency-induced senescence;
  • to determine time-dependent changes of senescence-associated parameters deciphering the potential bottlenecks in N uptake and retranslocation efficiencies;
  • to investigate whether and under which conditions N remobilization during senescence is sink- or source-limited in relation to N uptake capacities of roots under progressing senescence;
  • to determine the potential for increasing N efficiency in crops by manipulating the expression of genes encoding transcription factors, signalling components, enzymes or transporters.

Summary of the most important achievements

A more efficient use of soil and fertilizer nitrogen (N) requires reduction of N budget surpluses in crop production, which can be achieved by breeding and cultivation of genotypes with more efficient use of N. An improved N efficiency mainly relies on a higher total N uptake by roots and an efficient retranslocation of N from vegetative to reproductive plant organs during leaf senescence. To achieve a deeper understanding of physiological and molecular processes controlling leaf senescence as well as N remobilization and allocation to reproductive organs, the research unit FOR948 formulated three major overarching goals which led to the following discoveries:
  • Identification and integration of newly characterized molecular and biochemical components into regulatory networks by comparing N deficiency-induced with developmental senescence:
    The endogenous, developmentally-regulated generation of H2O2 was established as an early signal triggering developmental but not nitrogen deficiency-induced leaf senescence. In Arabidopsis, H2O2 induces a bunch of transcription factors that regulate leaf senescence in an either positive (e.g. ORE1, ORS1, ATAF1) or negative manner (e.g. JUB1). These transcription factors interact either directly with target genes involved in metabolic pathways or with other regulatory genes and proteins regulating abiotic stress tolerance or cellular processes like autophagy. For the regulation of developmental senescence, a network of five transcription factors has been uncovered including some direct regulatory links to phytohormonal signaling. By contrast, N deficiency-induced senescence employs only a part of these regulatory components.
  • Identifying potential bottlenecks in N retranslocation efficiency and developing strategies to overcome these bottlenecks:
    In barley, a major senescence-associated cysteine protease (PAP14) and amino acid transporters (AATs) have been identified that are involved in the breakdown of chloroplast proteins and the retranslocation of N to the sink organs, respectively. Their potential bottleneck function is currently assessed in transgenic barley. Apart from amino acids, urea turned out as a retranslocated N form, also serving as a metabolic marker for leaf senescence. Glumes have been identified as transitory storage organs for N before they undergo transition to export organs delivering N to the seed endosperm. In line cultivars of oilseed rape, N efficiency of field-grown plants was related to higher N uptake during reproductive growth and delayed leaf senescence. By contrast, the superior N efficiency of hybrids was mainly based on higher overall N acquisition and seed production. The onset of leaf senescence under N deficiency was governed by leaf-inherent factors.
  • Exploring the functions of the individual sink and source organs in the regulation of N remobilisation processes during senescence:
    While elevated atmospheric CO2 has been postulated to improve N use efficiency, oilseed rape responded with accelerated leaf senescence weakening source-sink relationships, e.g. by formation of new branches, which is unfavorable for N efficiency. At harvest of field-grown plants, largest pools of N remained in stems and pod walls and remobilization of these N pools was more important for N delivery into sinks than root uptake of N after flowering. Thus, remobilization of pod wall-N is now claimed as a major target for the improvement of N efficiency. Under ample N supply, oilseed rape synthesized storage proteins in leaves which may represent an important pool for N retranslocation during grain filling and thus a potential breeding target.

In total, the research unit FOR948 generated 45 articles, promoted 7 Ph.D. students, brought 2 female scientists to habilitation and organized several topical sessions at international conferences.