ISSN 1021-4437, Russian Journal of Plant Physiology, 2012, Vol. 59, No. 2, pp. 141–154. © Pleiades Publishing, Ltd., 2012. Original Russian Text © V.D. Kreslavskii, D.A. Los, S.I. Allakhverdiev, Vl.V. Kuznetsov, 2012, published in Fiziologiya Rastenii, 2012, Vol.59, No. 2, pp. 163–178. 141 INTRODUCTION Plants and cyanobacteria require light and oxygen for their normal life. In the process of plant metabo- lism, reactive species of oxygen and also nitrogen and sulfur are generated [1]. Unfavorable (stressful) envi- ronmental conditions shift the balance between oxi- dants and antioxidants toward oxidants, which becomes the reason for the development of intracellu- lar oxidative stress (OS). The size of ROS pool depends on the relative rates of ROS generation and destruction and on the life time of their main species. The activities of antioxidant enzymes (superoxide dis- mutase (SOD), catalase, peroxidase, and some others) and also the content of low-molecular antioxidants (ascorbic acid, glutathione, tocopherols, carotenoids, anthocyanins) play a key role in the control of ROS level and the content of lipid peroxidation (POL) products in the cell [2]. Plants grow under variable environmental condi- tions and are subjected to various stressor actions. During their evolution, plants developed various mechanisms of acclimation, including various regula- tory pathways for overcoming stress induced by unfa- vorable environmental factors. Among these defense mechanisms are a decrease in the rate of ROS generation, an increase in the rate of ROS scavenging, acceleration of recovery of damaged cell structures, an enhancement of absorbed energy heat dissipation, and some others. It should be also noted that, along with ROS, reactive nitrogen and sul- fur species play an essential role in the OS develop- ment in the cell. Investigations of ROS activity and role in plants have a long history. One of the key moments of this history was detection of superoxide anion and hydro- gen peroxide generation in the process of pseudocyclic electron transport (in the Mehler reaction) resulting not in the NADP + reduction but in О 2 absorption [3]. One more important moment was revealing ROS involvement in the hormonal regulation of defense gene expression protecting plants against pathogen infection [4]. In addition, it was shown that activation of the redox system in the plasmalemma and enhanced ROS generation in the apoplast are among universal plant cell responses to stress [5, 6]. In the recent time, ROS are increasingly frequently considered as very important signal molecules involved in the transduc- tion of intracellular signals controlling gene expression and activity of anti-stress systems [2, 7–11]. Frequently, ROS fulfill these functions using other molecules as mediators. There are many products of metabolism induced by reactive oxygen. Among ROS effects, POL products take a special place. The signal- ing function of ROS and POL products is realized via the regulation of cell calcium status, by direct or medi- ated influence on Са 2+ ion entry into the cytosol, where they play the role of second messengers [9, 12], via hormonal signaling [1, 12], MAPK activity [13], REVIEWS Signaling Role of Reactive Oxygen Species in Plants under Stress V. D. Kreslavskii a , D. A. Los b , S. I. Allakhverdiev a, b , and Vl. V. Kuznetsov b a Institute of Fundamental Problems of Biology, Russian Academy of Sciences, Pushchino branch, Institutskaya ul. 2, 142290 Pushchino, Moscow oblast, Russia; e-mail: vkreslav@rambler.ru b Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia; Received July 23, 2011 Abstract—The review considers the role of H 2 O 2 , 1 O 2 , , and the products of lipid peroxidation as sig- naling molecules in the processes of stress signal transduction in plants. The data concerning possible ROS participation in transduction of stress signals from chloroplasts to the nuclear genome, H 2 O 2 involvement in transduction stress signals in cyanobacteria, and also the interactions between ROS and other signaling sys- tems within the cell are presented. It is suggested that redox regulators, protein kinases/protein phosphatases, and transcription factors play crucial role in the functioning of ROS-dependent signaling systems in the plant cell. Keywords: plants, reactive oxygen species, stress, signaling system, chloroplast. DOI: 10.1134/S1021443712020057 O 2 •– Abbreviations: IP 3 —inositol 1,4,5-triphosphate; MAPK—mito- gen-activated protein kinase; OS—oxidative stress; POL—per- oxidation of lipids; PQ—plastoquinone pool; PS—photosystem; PUFA—polyunsaturated fatty acids; RC—reaction center; SOD—superoxide dismutase.