MECHANISMS FOR THE GENERATION OF REACTIVE OXYGEN SPECIES IN PLANT DEFENSE - A BROAD PERSPECTIVE

In response to attempted invasion by a pathogen, plants mount a broad range of defence responses, including the generation of reactive oxyge n species (ROS). The most spectacular and one of the earliest observab le aspects of this mechanism is the oxidative burst - a rapid and tran sient production of large amounts of ROS. This review is intended to p rovide a broad perspective on the multiplicity of mechanisms and signa l transduction pathways leading to a single phenomenon of ROS generati on in plant defence reaction to microbial infection. Thus, two major m odels describing the origin of ROS in the oxidative burst are presente d, namely: the NADPH oxidase system analogous to that of mammalian pha gocytotic cells, and the pH-dependent generation of hydrogen peroxide by exocellular peroxidases. Additionally, the involvement of other pla sma membrane-bound proteins, utilizing MADH or NADPH, in ROS generatio n is also demonstrated. Furthermore, other exocellular enzymes, like g ermin/oxalate oxidases and amine oxidases producing ROS in response to pathogen infection are described. The potential participation of prot oplastic ROS-generating systems residing in mitochondria, peroxisomes, and glyoxysomes constitutes the next part of this review. Finally, th e potential elements of the signal transduction pathways leading to th e activation of various mechanisms of ROS production are indicated. (C ) 1997 Academic Press Limited.