In plants, the process of aging, as well as many environmental constraints, may lead to the death of leaves. This particular type of cell death is often referred to as leaf senescence and can have a profoundly negative impact on crop yields and post-harvest shelf-life.

Olivier Keech 1150

: Leaves are essential plant structures and their well-being is crucial for plant development and survival. When a stress is applied, a plant has two options: try to cope with it or induce senescence and reallocate valuable nutrients towards new, developing or storage organs. A mutual antagonistic relationship can summarize this phenomenon as shown in fig 1. Our aim is to understand how the plant validates senescence over an adaptation strategy in response to stress. This work mainly covers two aspects: 1) to unveil the communication and signalling mechanisms controlling the induction of leaf senescence and 2) to characterize the metabolic regulation that occurrs in response to stress and during leaf senescence.

Figures research profile cropped 1Figure 1. Mutual antagonistic relationship between adaptation and induction of senescence in response to a stress.

1. Using dark-induced senescence as a proxy to decipher signalling pathways controlling the induction of leaf senescence

An individually-darkened leaf (IDL) for 6 days will undergo an accelerated senescence, whereas leaves from a plant entirely darkened (DP) for the same period of time will exhibit a sustained maintenance of their physiological functions and a subsequent repression of the process of senescence (Fig 2).
Figures research profile cropped 2Figure 2. Experimental setup for the two darkening treatments (Weaver and Amasino 2001; Keech et al 2007; Keech et al 2010).
Great differences between transcriptomes and metabolomes of IDL and DP are observed, and highlight the different metabolic strategies between the two darkening treatment (Fig 3).

Figures research profile cropped 3Figure 3. Visualization of transcript and metabolic variations in IDL and DP during a time course from 0 to 6 days. At the transcriptomic level, (A) 3D PCA and (B) 2D PCA highlight the progressive separation of the transcriptomes during the two darkening treatments. At the metabolic level, (A) PCA and (B) a supervised method OPLS-DA (Bylesjö et al 2006) show primarily the distinct metabolic profile between light and darkened samples, and secondly the progressive separation of the metabolic profiles between the two darkening treatments, over the duration of the time course

In order to identify the key players involved in the induction of senescence, we undertook a genetic screen allowing isolation of functional stay-green mutants. We are currently unveiling the function of these mutants.

2. Regulation of metabolism during leaf senescence
In a green leaf, the three energy organelles (peroxisome, mitochondrion and chloroplast) work in synergy to sustain an efficient assimilation of carbon while constantly maintaining the essential functions of the cell. However, when a leaf undergoes senescence ("yellowing"), whole cell-metabolism is drastically modified, and as chloroplasts are rapidly getting impaired, the remaining organelles acquire novel functions, particularly the mitochondrion. In animals, mitochondria have been shown to integrate various signals and to subsequently modulate cell death processes whereas in plants, the contribution of mitochondria in cell death regulation remains unclear, particularly during leaf senescence.
Therefore, we are currently investigating in more details the role of mitochondria during natural leaf senescence (i.e. aging).Figures research profile cropped 4Figure 4. Overview of the mitochondrial transcript expression during leaf senescence. We are particularly interested in unknown genes which encode products predicted to be targeted to mitochondria (unknowns).