Involvement of caspase-like activity in Z. aethiopica spathe senescence

Abstract

There are numerous examples of cell death during plant development that conforms to the general definition of programmed cell death (PCD). Although several morphological similarities were found between animal cells undergoing apoptosis and dying plant cells, the identity of most of the key elements involved in plant PCD remain unknown. Proteolytic activity associated with caspase-like proteins is well known to be involved in animal and yeast apoptosis, but their role in plant PCD and in particular in plant senescence is still not clear. We have been using the development of Zantedeschia aethiopica floral spathe as a natural model for studying the physiological and biochemical features of senescence. Soon after its formation, this photosynthetic leaf-like organ undergoes senescence, which leads to the organ death if pollination does not occur. Fruiting inhibits the ongoing spathe senescence and the regreening of its abaxial parenchyma cells is observed. Previously, in a cDNA macroarray analysis we have identified several proteolytic enzyme genes that were up-regulated during the spathe whitening process. Although these genes are most probably involved in the execution of senescence program, it could be also possible that early induced genes could code for a signal protease activity. In this work, the progression of typical morphological and biochemical features of PCD, such as fragmentation of the nucleus and internucleosomal cleavage of DNA, will be evaluated during spathe development. In order to address the question of whether caspase-like proteolytic activity is involved in the regulation of the senescence process we have determined the activity of the most common caspases in plants during spathe development. Activity for caspase-6 (VEIDase) was higher than that of other caspases studied, suggesting that VEIDase is the principal caspase-like activity in Z. aethiopica.