Ricinosomes: an organelle for developmentally regulated programmed cell death in senescing plant tissues

This review describes aspects of programmed cell death (PCD). Present resea rch maps the enzymes involved and explores the signal transduction pathways involved in their synthesis. A special organelle (the ricinosome) has been discovered in the senescing endosperm of germinating castor beans (Ricinus communis) that develops at the beginning of PCD and delivers large amounts of a papain-type cysteine endopeptidase (CysEP) in the final stages of cel lular disintegration. Castor beans store oil and proteins in a living endos perm surrounding the cotyledons. These stores are mobilized during germinat ion and transferred into the cotyledons. PCD is initiated after this transf er is complete. The CysEP is synthesized in the lumen of the endoplasmic re ticulum (ER) where it is retained by its C-terminal KDEL peptide as a rathe r inactive pro-enzyme. Large number of ricinosomes bud from the ER at the s ame time as the nuclear DNA is characteristically fragmented during PCD. Th e mitochondria, glyoxysomes and ribosomes are degraded in autophagic vacuol es, while the endopeptidase is activated by removal of the propeptide and t he KDEL tail and enters the cytosol. The endosperm dries and detaches from the cotyledons. A homologous KDEL-tailed cysteine endopeptidase has been fo und in several senescing tissues; it has been localized in ricinosomes of w ithering day-lily petals and dying seed coats. Three genes for a KDEL-taile d cysteine endopeptidase have been identified in Arabidopsis. One is expres sed in senescing ovules, the second in the vascular vessels and the third i n maturing siliques. These genes open the way to exploring PCD in plants.