Evidence for programmed cell death during leaf senescence in plants

Cell death caused by senescence of leaves has been thought to be a type of programmed cell death (PCD or apoptosis) for many years, however, no studies at the nuclear level associated with PCD have been reported. In this study, leaf tissue from five different plant species, Philodendron hastatum, Epipremnum aureum, Bauhinia purpurea, Delonix regia, and Butea monosperma was used to detect the evidence of the PCD. Here, we report the detection of PCD in senescent leaf tissue. DNA ladders, resulting from the cleavage of nuclear DNA into oligonucleosomal fragments in apoptotic cells, were detected by gel electrophoresis and southern hybridization only in senescent (but not in non-senescent) leaves in all five plant species. DNA fragmentation and nuclear DNA condensation were further confirmed by using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end in situ labeling (TUNEL) method. Fluorescence was clearly detected in the nuclei of mesophyll cells in senescent leaves. By contrast, there was no TUNEL staining in green healthy leaves. Our results provide direct evidence to support the notion that natural senescence of the leaves is indeed an apoptotic process during plant development.