Programmed cell death in Ricinus and Arabidopsis: the function of KDEL cysteine peptidases in development

Programmed cell death (PCD) in plants is a prerequisite for development as well as seed and fruit production. It also plays a significant role in pathogen defense. A unique group of papain-type cysteine endopeptidases, characterized by a C-terminal endoplasmic reticulum (ER) retention signal (KDEL CysEP), is involved in plant PCD. Genes for these endopeptidases have been sequenced and analyzed from 25 angiosperms and gymnosperms. They have no structural relationship to caspases involved in mammalian PCD and homologs to this group of plant cysteine endopeptidases have not been found in mammals or yeast. In castor beans (Ricinus communis), the CysEP is synthesized as pre-pro-enzyme. The pro-enzyme is transported to the cytosol of cells undergoing PCD in ER-derived vesicles called ricinosomes. These vesicles release the mature CysEP in the final stages of organelle disintegration triggered by acidification of the cytoplasm resulting from the disruption of the vacuole. Mature CysEP digests the hydroxyproline (Hyp)-rich proteins (extensins) that form the basic scaffold of the plant cell wall. The KDEL CysEPs accept a wide variety of amino acids at the active site, including the glycosylated Hyp residues of the extensins. In Arabidopsis, three KDEL CysEPs (AtCEP1, AtCEP2 and AtCEP3) are expressed in tissues undergoing PCD. In transgenic Arabidopsis plants expressing β-glucuronidase under the control of the promoters for these three genes, cell- and tissue-specific activities were mapped during seedling, flower and seed development. KDEL CysEPs participate in the collapse of tissues in the final stage of PCD and in tissue re-modeling such as lateral root formation.