A gibberellin-induced nuclease is localized in the nucleus of wheat aleurone cells undergoing programmed cell death

The aleurone layer of cereal grains undergoes a gibberellin-regulated process of programmed cell death (PCD) following germination. We have applied a combination of ultrastructural and biochemical approaches to analyze aleurone PCD in intact wheat grains. The terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay revealed that PCD was initiated in aleurone cells proximal to the embryo and then extended to distal cells. DNA fragmentation and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling analysis revealed PCD of aleurone cells in maize grains, although the process was delayed as compared with wheat. Aleurone cells undergoing PCD showed a rapid vacuolation with high lytic activity in the cytoplasm, whereas the nucleus, which adopted an irregular shape, appeared essentially intact and showed symptoms of degradation at the end of the process. A nuclease activity was identified localized in the nucleus of aleurone cells undergoing PCD, just prior to the appearance of DNA laddering. This nuclease was induced by gibberellic acid treatment and was not detected when gibberellin synthesis was inhibited or in gibberellic acid-insensitive mutants. This nuclease was activated by Ca(2+) and Mg(2+), strongly inhibited by Zn(2+), and showed optimum activity at neutral pH, resembling nucleases involved in apoptosis of animal cells.