The metabolic, biochemical and molecular events occurring during tobacco (Nicotiana tabacum) leaf ageing are presented, with a particular emphasis on nitrogen metabolism. An integrated model describing the source/sink relationship existing between leaves of different developmental stages along the main plant axis is proposed. The results of our study show that a tobacco plant can be divided into two main sections with regards to sink/source relationships. Sink-to-source transition occurs at a particular leaf stage in which a breakpoint corresponding to an accumulation of carbohydrates and a depletion of both organic and inorganic nitrogen is observed. The sink/source transition is also marked by the appearence of endoproteolytic activities and the induction of both cytosolic glutamine synthetase and NAD(H)-dependent glutamate dehydrogenase transcripts, proteins and activities. The role of the newly induced enzymes and the nature of the potential metabolic and developmental signals involved in the regulation of their expression during leaf senescence are discussed.

Comparison of the extent of leaf senescence depending on the genetic background of different recombinant inbred lines (RILs) of Arabidopsis (Arabidopsis thaliana) is described. Five RILs of the Bay-0 3 Shahdara population showing differential leaf senescence phenotypes (from early senescing to late senescing) were selected to determine metabolic markers to discriminate Arabidopsis lines on the basis of senescence-dependent changes in metabolism. The proportion of g-aminobutyric acid, leucine, isoleucine, aspartate, and glutamate correlated with (1) the age and (2) the senescence phenotype of the RILs. Differences were observed in the glycine/serine ratio even before any senescence symptoms could be detected in the rosettes. This could be used as predictive indicator for plant senescence behavior. Surprisingly, late-senescing lines appeared to mobilize glutamine, asparagine, and sulfate more efficiently than early-senescing lines. The physiological basis of the relationship between leaf senescence and flowering time was analyzed.Leaf senescence is a key developmental step in the life of annual plants. During growth, green leaves accumulate nutrients. The main purpose of senescence is the mobilization and recycling of these nutrients to the developing seeds to prepare the next generation. Developmental signals, aging, or stress can induce leaf senescence. The final stage of this process is death, but cell death is actively delayed until nutrients have been removed (Buchanan-Wollaston et al., 2003).During senescence, cell constituents are dismantled in an ordered progression. Chlorophyll degradation is the first visible symptom of senescence, but by the time yellowing can be seen, some senescence has already occurred. Chlorophyll, protein, and lipid degradation processes have been largely investigated (Hö rtensteiner and Feller, 2002). Protein and mRNA degradation parallels the loss in photosynthetic activity and participates in nutrient mobilization. Nucleic acids, especially RNA, form a valuable source of phosphorus in mature leaves. Total RNA levels fall rapidly during senescence, whereas nuclear DNA is maintained to allow gene expression to continue, until late in the process. Nutrients such as sulfur, phosphorus, metal ions, and minerals are also transferred out of the leaves (Himelblau and Amasino, 2001). Accelerated metabolism of membrane lipids results in a decline in the structural and functional integrity of cellular membranes. Thylakoid membranes provide an abundant source of carbon that can be mobilized for use as an energy source during senescence. Rubisco is one of the major sources of nitrogen for mobilization. A major question in leaf senescence is how leaf proteins, up to 75% of which are located within the chloroplast, are degraded and mobilized (Mae, 2004).The molecular events that induce and contribute to the senescence process have recently been extensively investigated. The genomic resources that are now available for Arabidopsis (Arabidopsis thaliana) have allowed the rapid identification of ...

For the first time in Arabidopsis thaliana, this work proposes the identification of quantitative trait loci (QTLs) associated with leaf senescence and stress response symptoms such as yellowing and anthocyanin-associated redness. When Arabidopsis plants were cultivated under low nitrogen conditions, we observed that both yellowing of the old leaves of the rosette and whole rosette redness were promoted. Leaf yellowing is a senescence symptom related to chlorophyll breakdown. Redness is a symptom of anthocyanin accumulation related to whole plant ageing and nutrient limitation. In this work, Arabidopsis is used as a model system to dissect the genetic variation of these parameters by QTL mapping in the 415 recombinant inbred lines of the Bay-0xShahdara population. Fifteen new QTLs and two epistatic interactions were described in this study. The yellowing of the rosette, estimated by visual notation and image processing, was controlled by four and five QTLs, respectively. The visual estimation of redness allowed us to detect six QTLs among which the major one explained 33% of the total variation. Two main QTLs were confirmed in near-isogenic lines (heterogenous inbred family; HIF), thus confirming the relevance of the visual notation of these traits. Co-localizations between QTLs for leaf yellowing, redness and nitrogen use efficiency described in a previous publication indicate complex interconnected pathways involved in both nitrogen management and senescence- and stress-related processes. No co-localization between QTLs for leaf yellowing and redness has been found, suggesting that the two characters are genetically independent.

To investigate the role of stress in nitrogen management in plants, the effect of pathogen attack, elicitors, and phytohormone application on the expression of the two senescence-related markers GS1 (cytosolic glutamine synthetase EC 6.3.1.2) and GDH (glutamate dehydrogenase, EC 1.4.1.2) involved in nitrogen mobilization in senescing leaves of tobacco (Nicotiana tabacum L.) plants, was studied. The expression of genes involved in primary nitrogen assimilation such as GS2 (chloroplastic glutamine synthetase) and Nia (nitrate reductase, EC 1.6.1.1) was also analysed. The Glubas gene, coding a beta-1,3-glucanase, was used as a plant-defence gene control. As during natural senescence, the expression of GS2 and Nia was repressed under almost all stress conditions. By contrast, GS1 and GDH mRNA accumulation was increased. However, GS1 and GDH showed differential patterns of expression depending on the stress applied. The expression of GS1 appeared more selective than GDH. Results indicate that the GDH and GS1 genes involved in leaf senescence are also a component of the plant defence response during plant-pathogen interaction. The links between natural plant senescence and stress-induced senescence are discussed, as well as the potential role of GS1 and GDH in a metabolic safeguard process.