The terminal phase of leaf senescence is gated by the cytosolic arginine pool

Leaf senescence aims at degrading cell components to recover valuable nutrients, and reallocate them towards other organs2 (Fig 1a). Once this remobilisation is complete, cells undergo a vacuolar-type of programmed cell death2, ultimately leading to the death of the entire organ. But how do cells from a senescing leaf “know” when to die? If the cell death process per se is initiated too early, the remobilisation might not be complete and, hence futile. This suggests the presence of a “sensing” mechanism that coordinates the remobilization phase with the onset of cell death during leaf senescence (Fig 1a). Using functional stay-green mutants, we show that senescing cells are wired to metabolically dissipate the cytosolic arginine pool, which otherwise represses the progression of leaf senescence. We propose a model in which a senescing cell uses this pool as a proxy for the completion of the remobilisation of nitrogen, and to time accurately the subsequent induction of cell death.