Reactive Oxygen Species Signaling in Response to Pathogens

Abstract

The production of reactive oxygen species (ROS), via consumption of oxygen in a so-called oxidative burst, is one of the earliest cellular responses following successful pathogen recognition. Apoplastic generation of superoxide (O2 −), or its dismutation product hydrogen peroxide (H2O2), has been documented following recognition of a variety of pathogens (Doke, 1983; Auh and Murphy, 1995; Grant et al., 2000b). Avirulent pathogens, successfully recognized via the action of disease resistance (R) gene products in plant immune system, elicit a biphasic ROS accumulation with a low-amplitude, transient first phase, followed by a sustained phase of much higher magnitude that correlates with disease resistance (Lamb and Dixon, 1997). However, virulent pathogens that avoid host recognition induce only the transient, low-amplitude first phase of this response, suggesting a role for ROS in the establishment of the defenses. In line with this conclusion, elicitors of defense responses, often referred to as microbe-associated molecular patterns (MAMPs), also trigger an oxidative burst. Initial characterization of the oxidative burst left unclear whether ROS acted as executioners of pathogen, host cells (in the form of the familiar hypersensitive response [HR]), or both, or, alternatively, as signaling molecules that were not directly involved in the mechanisms that actually stopped pathogen growth. ROS production and functions in response to pathogens. Several enzymes have been implicated in apoplastic ROS production following successful pathogen recognition. The use of inhibitors pointed to plasma membrane NADPH oxidases (inhibited by diphenylene iodonium [DPI] but not by cyanide or azide; Grant et al., 2000a) and cell wall peroxidases (inhibited by cyanide or azide but not by DPI; Grant et al., 2000a; Bolwell et al., 2002) as the two most likely biochemical sources. The NADPH oxidase, also known as the respiratory burst oxidase (RBO), was initially described in mammalian neutrophils as a multicomponent complex mediating microbial killing (Lambeth, 2004). gp91phox is the enzymatic subunit of this oxidase and transfers electrons to molecular oxygen to generate superoxide. Arabidopsis (Arabidopsis thaliana) has 10 Atrboh (Arabidopsis RBO homolog) genes homologous to gp91phox (Torres and Dangl, 2005). Several recent reports demonstrate that members of the Rboh family mediate the production of apoplastic ROS during the defense responses, as well as in response to abiotic environmental and developmental cues (Torres and Dangl, 2005). However, we know very little about either the precise subunit structure of the plant NADPH oxidase or its activation. Both are likely different than in mammalian neutrophils (Torres and Dangl, 2005). Peroxidases form a complex family of proteins that catalyze the oxidoreduction of various substrates using H2O2. In particular, pH-dependent peroxidases in the cell wall can also be a source of apoplastic H2O2 in the presence of a reductant released from responding cells (Wojtaszek, 1997; Bolwell et al., 1998). The expression of these enzymes is induced following recognition of bacterial and fungal pathogens (Chittoor et al., 1997; Sasaki et al., 2004). A French bean (Phaseolus vulgaris) cationic peroxidase and H2O2 (detected by cerium chloride staining) were colocalized in the vicinity of invading bacteria together with other components of the papillae (Brown et al., 1998). These results suggested that H2O2 generation by this enzyme could lead to the generation of subcellular, polarized physical barriers at infection sites. Although the primary oxidative burst following pathogen recognition occurs in the apoplast, ROS produced in other cellular compartments may also have functions in defense. High levels of ROS can be produced inside the plant cell as by-products of metabolic processes, in particular, light-driven production of ROS as a by-product of photosynthesis (Karpinski et al., 2003; Apel and Hirt, 2004). Uncoupling, or inhibition, of the photosystem machinery in the chloroplast and photorespiration associated with chloroplast and peroxisome function can lead to the formation of high levels of ROS that can dramatically affect cellular homeostasis. It is important to recall the nearly ubiquitous requirement for light in the HR (Goodman and Novacky, 1994), as illustrated by the requirement of high-intensity light for cell death mediated by resistance gene proteins (Tang et al., 1998). Under high-light conditions, photorespiratory ROS mediate different mechanisms of lipid peroxidation leading to cell death than in the dark, underscoring the importance of light during the HR (Montillet et al., 2005). Various ROS-scavenging systems, including ascorbate peroxidases, glutathione, superoxide dismutases, and catalases, maintain ROS homeostasis in different compartments of the plant cell (Mittler et al., 2004). These enzymes could restrict the ROS-dependent damage or finely tune ROS-dependent signal