Leaf senescence and nitrogen remobilisation

Winter oilseed rape is a very nitrogen-fertilizer consuming crop and is characterized by low nitrogen use efficiency (NUE). A high proportion of assimilated nitrogen remains immobilized in senescent leaves and is returned to the soil failing to contribute to seed production. Improvement of N remobilisation efficiency (NRE) during leaf senescence is likely to improve significantly the overall plant NUE. In order to decipher functional traits involved in senescence-regulated N recycling processes and source-sink relationships, searching for genetic variation and nutritional impact on physiological and molecular key determinants such as genes, enzymes and metabolites, we are developing a dual approach based on genetic and functional genomic strategies.

Research and main results

At critical stages of resource remobilisation, metabolomic and transcriptomic fingerprints of source and sink tissues are performed during vegetative sequential senescence to highlight networks discriminating source from sink leaf and during monocarpic senescence to assess leaf draining and seed filling. Works are designed either under semi-controlled (greenhouse) or field conditions to permit discovering essential nodes of regulation and new candidates for both vegetative tissue NRE performance and seed oil accumulation. Efforts are also devoted to the development and the improvement efficient phenotyping tools dedicated to NRE and leaf senescence description.

1) Identification of genes involved in oilseed rape NRE

In order to identify genes involved in leave senescence process and N ressource recycling, either targeted candidate gene expression or high throughput transcriptomic approaches are performed during the sink to source transition in plant leaves with different profiles of N management. Expression patterns are interpreted in regards of leaf remobilisation efficiency and agronomical performance of the different genotypes studied. Gene of interest are also localised on genetic maps in order to enrich a genetic approach and QTL description of NRE and NUE.

2) Assignment of specific metabolic signatures to leaf age and development

Very dense metabolic profiles are established throughout the development cycle of plants in senescing source, intermediate and developing sink leaves with contrasted nitrogen regimes under greenhouse conditions. This work aims at (i) drawing up a primary metabolic profile of oilseed rape leaves consistent with their developmental stage, (ii) discovering biomarkers and functional effectors of the leaf sink and source statues and resource remobilization processes. This metabolic profiling approach is also applied to a large number of genotypes grown during a long time-scaled experiment in field to point out metabolites as key players of N utilization efficiencies liable to genetic variability and that could be suitable for predicting the differential NUE or grain productivities between genotypes at harvest.

Metabolic  signature and profile evolution of old (# 7), intermediate (# 10) and young (# 13) leaves during vegatative growth of two oilseed rape genotypes (from Albert, 2012)

3) Development of original tools and methodologies for convenient phenotyping of leaf senescence

In addition to conventional tools for rapid assessment of nitrogen status of plants and their physiological functioning (chlorophyll contents, NIRS, PSII functioning, photosynthetic activity,…) we attempt to describe leaf senescence using non destructive NMR Relaxometry (collaborative work with IRSTEA), based on water status and estimation of water distribution at the sub-cellular level. Differences of water status and distribution occur at the tissue and cellular levels according to the leaf age and senescence evolution and links are sought between changes in the NMR signal, the structural modifications that occur in the parenchymal cells and the intensity of the metabolic processes of carbon and nitrogen recycling.

NMR relaxometry applied to oilseed rape leaf water status analysis and measurement of structural changes at the sub-cellular levels during senescence (from Sorin, 2012 ; collab. IRSTEA)

Fundings/Projects

ARCOLE (ANR Genoplante 2006-2008) « Efficacité de la nutrition azotée du colza au printemps : identification de cibles de sélection » (coordination X. Pinochet, CETIOM)

GENERGY (ANR Genoplante 2008-2012): « Improvement of the oil yield of the rapeseed crop in the context of bio fuel production » (coordination N. Nesi, INRA Rennes)

N PROJECT

RAPSODYN (Investissement d’Avenir 2012-2019) : « Optimisation of the rapeseed oil content and yield under low imput : improving breeding of adapted varieties using genetics and genomics” (coordination N. Nesi, INRA rennes)

Main references

Desclos M., Dubousset L., Etienne P., Le Cahérec F., Satoh H., Bonnefoy J., Ourry A. and Avice JC (2008) A Proteomic Profiling Approach to reveal a novel role of Brassica napus drought 22 kD/water-soluble chlorophyll-binding protein in young leaves during nitrogen remobilization induced by stressful conditions, Plant Physiol., 147, 1830–1844

Masclaux-Daubresse C., Reisdorf-Cren M., Orsel M. (2008) Leaf nitrogen remobilisation for plant development and grain filling. Plant Biology, 10, 23-36.

Albert B., Le Cahérec F., Niogret M.F., Faes P., Avice J.C., Leport L., Bouchereau A. (2012) Nitrogen availability impacts oilseed rape (Brassica napus L.) plant water status and proline production efficiency under water-limited conditions. Planta, 236, 659-676.

Collaborations

• UMR1069 Sol Agronomie Spatialisation (SAS), INRA-Agrocampus Ouest, France.
• UMR1091 Environnement et Grandes Cultures (EGC), INRA-AgroParisTech, France.
• UMR950 Ecophysiologie Végétale, Agronomie et Nutritions N, C, S (EVA), INRA-UCBN, France.
• IJPB,  INRA Versailles, France
• CNRGV Centre National de Ressources en Génomique Végétale, INRA Toulouse, France.
• CETIOM, Grignon, France.
• Biogemma, Mondonville and Clermont-Ferrand, France.
• IRSTEA, UR TERE, Rennes, France