Programmed cell death of secretory cavity cells in fruits of Citrus grandis cv. Tomentosa is associated with activation of caspase 3-like protease

Previously, we found that secretory cell degradation typically occurred through programmed cell death during secretory cavity development in Citrus sinensis L. (Osbeck). This finding indicated that secretory cavities could be utilized as a new cell biology model for investigating the regulatory mechanisms of plant programmed cell death. To study further the programmed cell death during secretory cavity development in Citrus fruit, we studied the morphogenetic characteristics of secretory cavities during their development in Citrus grandis cv. Tomentosa. Using light microscope- and electron microscope-TUNEL assays, immunohistochemistry and immunocytochemistry, we described the precise spatial and temporal alterations in caspase 3-like distribution, chromatin condensation and DNA fragmentation during the programmed cell death of secretory cavity cells. Caspase 3-like was found to be significantly located in both the cytoplasm and the nucleus of secretory cavity cells undergoing programmed cell death, and caspase 3-like is closely associated with chromatin condensation and DNA fragmentation. Interestingly, both caspase 3-like and DNA fragmentation were detected in the nucleoli. Our findings suggest that caspase 3-like may be involved in the programmed cell death of secretory cavity cells, especially in chromatin condensation, DNA fragmentation, nuclear degradation and the degradation of certain organelles.