Mechanisms for the generation of reactive oxygen species in plant defence - a broad perspective

In response to attempted invasion by a pathogen, plants mount a broad range of defence responses, including the generation of reactive oxygen species (ROS). The most spectacular and one of the earliest observable aspects of this mechanism is the oxidative burst - a rapid and transient production of large amounts of ROS. This review is intended to provide a broad perspective on the multiplicity of mechanisms and signal transduction pathways leading to a single phenomenon of ROS generation in plant defence reaction to microbial infection. Thus, two major models describing the origin of ROS in the oxidative burst are presented, namely: the NADPH oxidase system analogous to that of mammalian phagocytotic cells, and the pH-dependent generation of hydrogen peroxide by exocellular peroxidases. Additionally, the involvement of other plasma membrane-bound proteins, utilizing MADH or NADPH, in ROS generation is also demonstrated. Furthermore, other exocellular enzymes, like germin/oxalate oxidases and amine oxidases producing ROS in response to pathogen infection are described. The potential participation of protoplastic ROS-generating systems residing in mitochondria, peroxisomes, and glyoxysomes constitutes the next part of this review. Finally, the potential elements of the signal transduction pathways leading to the activation of various mechanisms of ROS production are indicated. (C) 1997 Academic Press Limited.