Induction of a ricinosomal-protease and programmed cell death in tomato endosperm by gibberellic acid

Several examples of programmed cell death (PCD) in plants utilize ricinosomes, organelles that appear prior to cell death and store inactive KDEL-tailed cysteine proteinases. Upon cell death, the contents of ricinosomes are released into the cell corpse where the proteinases are activated and proceed to degrade any remaining protein for use in adjacent cells or, in the case of nutritive seed tissues, by the growing seedling. Ricinosomes containing pro-SlCysEP have been observed in anther tissues prior to PCD and ricinosome-like structures have been observed in imbibed seeds within endosperm cells of tomato. The present study confirms that the structures in tomato endosperm cells contain pro-SlCysEP making them bona fide ricinosomes. The relative abundance of pro- versus mature SlCysEP is suggested to be a useful indicator of the degree of PCD that has occurred in tomato endosperm, and is supported by biochemical and structural data. This diagnostic tool is used to demonstrate that a sub-region of the micropylar endosperm surrounding the emerged radical is relatively long-lived and may serve to prevent loss of mobilized reserves from the lateral endosperm. We also demonstrate that GA-induced reserve mobilization, SlCysEP accumulation and processing, and PCD in tomato endosperm are antagonized by ABA.