Regulation of viability and cell death by hormones in cereal aleurone

Aleurone cells are essential for the mobilization of reserves stored in the endosperm of cereals but are not required after seedling establishment. Following imbibition the embryo produces gibberellins (GAs) that initiate a program in the aleurone cell that results in the synthesis and secretion of a broad spectrum of acid hydrolases. When the aleurone layer has fulfilled its role as a secretory gland and the contents of its cells are depleted, the aleurone cell dies. Because aleurone cell death is part of the normal development of the grain and is initiated by endogenous hormones, we classify it as a form of programmed cell death (PCD). Abscisic acid (ABA) is produced in dormant and quiescent grain and this hormone prevents the initiation of the GA-triggered cascade that leads to enzyme production and cell death. Based on these data we argue that ABA is a key factor in maintaining aleurone cell viability and preventing PCD in dormant and quiescent grain. Aleurone cells die when ROS accumulate and cause oxidative damage to cell membranes. Aleurone cells store large amounts of triglycerides and the conversion of these lipids to sugars by P-oxidation produces H2O2 and contributes to oxidative stress in the aleurone cell. ROS are normally metabolized by enzymes, including ascorbate peroxidase, catalase and superoxide dismutase. We hypothesize that a reduction in the activities of ROS-metabolizing enzymes and a decrease in the number of mitochondria following GA treatment prevents the aleurone cell from effectively metabolizing ROS. This in turn leads to oxidative damage and cell death.