Reactive Oxygen Species and Programmed Cell Death

Reactive oxygen species (ROS) are increasingly recognized as important signaling molecules modulating numerous vital cellular processes. They are also highly toxic byproducts of aerobic metabolism, resulting from Department of Plant Physiology and Molecular Biology, University of Plovdiv, 24 Tsar Assen str., Plovdiv 4000, Bulgaria, Fax: 00359 32 629495, E-mail: tsangech@uni-plovdiv.bg *Corresponding author excitation or incomplete reduction of molecular oxygen (Apel and Hirt 2004). In addition to normal metabolic processes, increased production of ROS, referred to as oxidative stress, is observed during various types of abiotic and biotic stress (Gechev et al. 2006). Plants as sessile organisms are particularly vulnerable to oxidative stress. In order to use ROS as signaling molecules and to cope with excess ROS production, they have developed an extensive and redundant antioxidant system consisting of antioxidant molecules, ROS-producing and detoxifying enzymes. A recent review outlined 289 genes related to ROS homeostasis in Arabidopsis thaliana (Gechev et al. 2006). This complex genetic network has allowed plants not only to protect themselves against oxidative stress but also to utilize ROS as signaling molecules controlling various aspects of development, adaptation to abiotic stress and proper responses to biotic stress. The diverse biological outcomes are determined by multiple factors, including ROS chemistry, intensity of the signal, site of ROS production, developmental stage of the plant and stress history (Gadjev et al. 2008). In addition, ROS interact with other signaling molecules such as nitric oxide, lipid messengers and plant hormones.