Nitrogen retranslocation during senescence in plants

Nitrogen remobilization is essential for nitrogen economy at the whole plant level. 15N tracing experiments showed that the Arabidopsis leaf to leaf N-remobilisation at vegetative stage was mainly determined by leaf senescence severity while N-remobilisation from rosette to seeds is depending on harvest index (Diaz et al. 2008). N-remobilisation efficiency to the seeds was higher in N limiting conditions (Lemaître et al. 2008). In order to investigate natural variation of nitrogen uptake and remobilisation in Arabidopsis, a core collection of accessions was grown at low (N-) and high (N+) nitrate supplies. Plants were labelled using 15N nitrate and traits measured (15N partition, nitrogen concentrations, remobilisation, biomass, yield, flowering). Globally, it was observed that the late-flowering plants had higher vegetative biomass, lower seed biomass and heavier seeds at both N+ and N- conditions. As a result, the late-flowering genotypes had lower harvest index (HI), higher nitrogen harvest index (NHI), higher 15N partitioning in seeds (15NHI), and lower fertility. All the accessions studied responded to low nitrogen nutrition by decreasing plant biomass, by lowering the amount of residual nitrogen lost in the dry remains (N%DR), and by increasing N-remobilisation (15NHI) and allocation of post-labelling assimilated nitrogen to the seeds. The most striking results were provided by the 15N tracing experiments, demonstrating that the amount of 15N, first assimilated into the rosette, and further remobilised to the seeds, is highly dependent on sink/source biomass ratio, i.e. on harvest index. Trait variations allowed us to group accessions in different feature-classes and to detect genotypes showing opposite or extreme behaviours. Results provided will facilitate the choice of recombinant inbred line parents for further QTL experiments. N-remobilisation was investigated for mutants of interest, using the same labelling protocol than for accessions. References : Diaz et al. (2008) Plant Physiology 147 : 1437-1449 Lemaître et al. (2008) Plant Cell Physiology 49(7): 1056-106