Reactive Oxygen Species in Plant Cell Death

Paradoxically, death is an integral part of life. Cell death is essential for growth and development of eukaryotes, by maintaining tissue and organ homeostasis in concert with cell proliferation, growth, and differentiation. Until recently, the wide variety of cell death types reported in the literature was mostly caged in two semantic categories: apoptosis and necrosis. Discrimination between these two forms was based on the presence or absence of specific biochemical and molecular hallmarks, such as DNA laddering, cytochrome c release, caspase involvement, ATP depletion, cytoplasmic swelling, and loss of membrane integrity (Pennell and Lamb, 1997). However, over the last decade this arbitrary division had clearly become too simplistic and a more accurate description of plant cell death needed to be established (van Doorn and Woltering, 2005). To avoid a Babel confusion of languages, we propose necrosis to depict accidental cell death caused by extrinsic factors, such as phytotoxic accumulation of specific molecules after a traumatic stress event. Thus, plant cell death through necrosis is passive, indiscriminate, and often follows irreversible injury. It is characterized by a progressive loss of membrane integrity that results in swelling of the cytoplasm and release of cellular constituents. Accordingly, the terms active or programmed cell death (PCD) define any form of cell death involving a single or a series of molecular and cellular orderly processes mediated by intracellular death programs, regardless of the (external) trigger or the hallmarks it exhibits (Jacobson et al., 1997; Dangl et al., 2000). Although both cell death events are quite well defined in animals, in plants there seems to be much more overlap between the phenotypic and molecular hallmarks of necrosis and PCD, thus making it harder to discriminate between the two events. In plants, PCD can occur during many developmental processes and abiotic stress conditions. For instance, PCD has been described during hypoxia stress (Drew et al., 2000), extremes in temperature (Vacca et al., 2004), and ozone (Langebartels et al., 2002). Developmental PCD has been reported during tracheary element formation (Kuriyama and Fukuda, 2002), seed development, germination (Souter and Lindsey, 2000; Young and Gallie, 2000), and senescence(definition) processes (Gunawardena et al., 2004). However, cell death events remain best described during incompatible plant-pathogen interactions that form the basis for the hypersensitive response (HR; Pennell and Lamb, 1997). This multitude of plant PCD events clearly illustrates a functional analogy between cell death across kingdoms: sculpting and deleting structures, eliminating cells to control cell quality and quantity after trauma, and producing differentiated cells without organelles (Jacobson et al., 1997). But, is there more than just a functional similarity between PCD in plants and animals? In animals, reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), superoxide ion, and nitric oxide (NO) are well-recognized triggers of cell death (Jabs, 1999). In contrast, involvement of these molecules during plant PCD was, for a long time, rather hypothetical. In addition to the lack of accurate and quantifiable detection methods, the inherent toxic nature of ROS masked their underlying function in various signaling networks. Only recently, the prominent role of ROS has been revealed in the induction, signaling, and execution of plant cell death. The initial phenotypic and pharmacological evidence for ROS signaling during plant PCD has been confirmed genetically with the identification of Arabidopsis (Arabidopsis thaliana) mutants incapable either of arresting ROS-driven PCD or of undergoing a PCD despite high ROS levels. Recently, the first genes involved in ROS perception and signal transduction have been identified and now we are faced with the challenge of uncovering the other players in the gene regulatory network of ROS-dependent cell death. In this update, we will describe the current knowledge on ROS homeostasis and cell death events in plants. Life under aerobic conditions is intimately linked with ROS production. Indeed, ROS are generated as by-products of the most essential energy-generating processes, photosynthesis and respiration. Together with an extensive battery of oxidases, chloroplasts, peroxisomes, and mitochondria are the main organellar ROS producers (Vranová et al., 2002; Apel and Hirt, 2004). Despite an efficient antioxidant machinery in these organelles, subtle changes in ROS homeostasis are inevitable (Foyer and Noctor, 2005). When the increase in ROS is relatively small, the housekeeping antioxidant capacity is sufficient to reset the original balance between ROS production and scavenging, thus reestablishing redox homeostasis. Under growth-limiting environmental conditions, this delicate redox balance is easily disturbed, potentially leading to a significant ROS accumulation. In fact, a 3- to 10-fold increase in ROS levels has been calculated under stress conditions (Polle, 2001). Thus, it is not surprising that a transient oxidative burst and a subsequent temporary shift in the intracellular redox state are common features of both biotic and abiotic stress responses (Dat et al., 2000; Mittler et al., 2004). Under unfavorable environmental conditions, such as temperature extremes, drought, or salt stress, the rate of carbon fixation is limited, causing an increase in photoinhibition potentially steering the photosystem toward overproduction of superoxide radicals and H2O2 (Foyer and Noctor, 2005). Similarly during ozone exposure, ROS are generated following the entry of ozone through the stomata and its conversion in the leaf apoplast, eventually leading to the formation of HR-like lesions (Pellinen et al., 1999; Rao and Davis, 2001). Furthermore, most incompatible plant-pathogen interactions are typified by a biphasic oxidative burst during which both infected and adja