Participation of chloroplasts in plant apoptosis

摘要

Milochondria are known to participate in the initiation of programmed cell death (PCD) in animals and in plants. The role of chloroplasts in PCD is still unknown. We describe a new system to study PCD in plants; namely, leaf epidermal peels. The peel represents a monolayer consisting of cells of two types: phototrophic (guard cells) and chemotrophic (epidermal cells). The peels from pea (Pisum sativum L.) leaves were treated by cyanide as an inducer of PCD. We found an apoptosis-enhancing effect ofillumination on chloroplastcontaining guard cells, but not on chloroplastless epidermal cells. Antioxidants and anaerobiosis prevented the CN~--induced apoptosis of cells of both types in the dark and in the light. On the other hand, methyl viologen andmenadione known as ROS-generating reagents as well as the Hill reaction electron acceptors (BQ, DAD, TMPD, or DPIP) that are not oxidized spontaneously by O_2 were shown to prevent the CN~--induced nucleus destruction in guard cells. Apoptosis of epidermal cells was potentiated by these reagents, and they had no influence on the CN effect. The light-dependent activation of CN~--induced apoptosis of guard cells was suppressed by DCMU, stigmatellin or DNP-INT, by a protein kinase inhibitor staurosporineas well as by cysteine and serine protease inhibitors. The above data suggest that apoptosis of guard cells is initiated upon a combined action of two factors, i.e., ROS and reduced plastoquinone of the photo synthetic electron transfer chain. As to reduction of ubiquinone in the mitochondrial respiratory chain, it seems to be antiapoptotic for the guard cell.