Annu. Rev. Plant Biol. 2004. 55:373–99 doi: 10.1146/annurev.arplant.55.031903.141701 Copyright c  2004 by Annual Reviews. All rights reserved First published online as a Review in Advance on January 12, 2004 REACTIVE OXYGEN SPECIES: Metabolism, Oxidative Stress, and Signal Transduction Klaus Apel 1 and Heribert Hirt 2 1 Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH) Universit¨ atstr. 2, 8092 Z¨ urich, Switzerland 2 Max F. Perutz Laboratories, University of Vienna, Gregor-Mendel-Institute of Molecular Plant Sciences, Austrian Academy of Sciences, Vienna Biocenter, Dr. Bohrgasse 9, 1030 Vienna, Austria; email: heribert.hirt@unvie.ac.at Key Words programmed cell death, abiotic stress, pathogen defense ■ Abstract Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Whereas plants are surfeited with mechanisms to combat increased ROS levels during abiotic stress conditions, in other circumstances plants appear to purposefully generate ROS as signaling molecules to control various processes including pathogen defense, programmed cell death, and stomatal behavior. This review describes the mechanisms of ROS generation and removal in plants during development and under biotic and abiotic stress conditions. New insights into the complexity and roles that ROS play in plants have come from genetic analyses of ROS detoxifying and signaling mutants. Considering recent ROS-induced genome- wide expression analyses, the possible functions and mechanisms for ROS sensing and signaling in plants are compared with those in animals and yeast. CONTENTS INTRODUCTION ..................................................... 374 GENERATION OF ROS ................................................ 374 Biotic Strategies to Generate ROS ....................................... 375 Abiotic Strategies to Generate ROS ..................................... 377 ROS DETOXIFICATION ............................................... 380 Nonenzymatic ROS Scavenging Mechanisms ............................. 380 Enzymatic ROS Scavenging Mechanisms ................................. 381 THE ROLE OF ROS IN SIGNALING ..................................... 382 ROS Sensing by Histidine Kinases ...................................... 383 ROS Activation of Mitogen-Activated Protein Kinase (MAPK) Signaling Pathways ................................................. 383 ROS Inhibition of Protein Phosphatases .................................. 384 ROS Activation of Transcription Factors ................................. 384 1543-5008/04/0602-0373$14.00 373