Forms of retranslocated nitrogen during leaf senescence and nitrogen deficiency

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Nitrogen (N) retranslocation from senescing leaves to the grains is one of the limiting factors for an efficient utilization of fertilizer N. Our previous studies showed that urea-N can account for >13% of retranslocated amino acid-N and that the urea transporter DUR3 can increase N retranslocation by retrieval of apoplastic urea upstream of phloem loading. The present Project 5 Forms of retranslocated nitrogen 3 project aims at investigating further the contribution of different N forms and their sense-cence-regulated transporters to N retrieval in senescing leaves and to N retranslocation. In barley, the retranslocation efficiency of 15N-labeled urea and other N forms will be assessed in wild-type and transgenic plants with altered expression of membrane transporters or transcription factors. A regulatory link of these N transporters to upstream transcription factors and to salicylic acid as a putative leaf-inherent signal will be investigated in the model species Arabidopsis. To determine the relevance of leaf-inherent signals for N retranslocation in rapeseed, physiological and molecular analyses will be performed in hydroponics and in the field. Emphasis will be placed on rapeseed hybrid lines that have an outstanding N efficiency under field conditions.Keywords: phloem loading, nitrogen retranslocation, AMT, DUR3, nitrogen starvation, nitrogen efficiency.