Programmed cell death of the mung bean cotyledon during seed germination

The cotyledon of the mung bean (Vigna radiata) undergoes programmed cell death (PCD) during seed germination. Death of the cotyledon storage parenchyma cells was preceded by reserve mobilization and vacuolation. During the dismantling phase of PCD, the storage parenchyma cells exhibited progressive autolysis, resulting in the formation of a large central vacuole. Complete lysis of the cytoplasm and nucleus occurred, and condensation of the protoplast away from the cell wall appeared at the final stage of cell death. Cytochrome c translocation from the mitochondria to the cytosol and internucleosomal DNA fragmentation were also observed in cotyledon senescence during seed germination. Nuclease activity was detected in vitro during mung bean cotyledon PCD, and the activity of this nuclease was repressed by the caspase-3 inhibitor Ac-DEVD-fmk. Caspase-like protease (CLP) activity was detected in cotyledons during seed imbibition and after seed germination. CLP activity reached a peak in day 3 cotyledons. These CLP activities were inhibited by a caspase-3-specific inhibitor but were resistant to caspase-l-specific inhibitors. This study demonstrates that the programmed cell death in mung bean cotyledons involves nuclease induction, nuclear DNA fragmentation, cytochrome c releasing and CLP participation. These PCD hallmarks establish the important paradigm for further understanding of PCD in higher plant.