Reactive oxygen species signaling in response to pathogens.

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. In the plant cell, ROS can directly cause strengthening of host cell walls via cross-linking of glycoproteins (Bradley et al., 1992; Lamb and Dixon, 1997), or lipid peroxidation and membrane damage (Lamb and Dixon, 1997; Montillet et al., 2005). However, it is also evident that ROS are important signals mediating defense gene activation (Levine et al., 1994). Additional regulatory functions for ROS in defense occur in conjunction with other plant signaling molecules, particularly with salicylic acid (SA) and nitric oxide (NO; see Fig. 1). However, ROS also regulate additional plant responses in relation to other signals. Here, we discuss these roles of ROS with a focus on the response to pathogen infection.