Unraveling the Tapestry of Networks Involving Reactive Oxygen Species in Plants

Abstract

The Plant Physiology Special Issue on Reactive Oxygen Species (ROS) published in June 2006 marked the early efforts to resolve the tapestry of mechanisms influenced by ephemeral singlet oxygen, superoxide (O2 .−), hydrogen peroxide (H2O2), hydroxyl radical (OH.), peroxynitrite (ONOO−), and nitric oxide (NO) in plant cells. The Updates and research articles in the issue focused on ROS as messengers in signal transduction rather than simply unavoidable toxic by-products of metabolism or environmental perturbation. The issue was inspired by the growing perception that a delicate interplay between localized ROS production and amelioration is fundamental to responses to biotic and abiotic cues as well as development (Mittler et al., 2004). The Special Issue on ROS was the first of the Plant Physiology Focus Collection (Ort, 2006) to feature online papers published in the journal for the 2 years prior to and following the Special Issue (http://www.plantphysiol.org/cgi/collection/reactive_oxygen_species). Since June 2006, more than 50 publications have further explored the regulated production and scavenging of ROS in plants and the contribution of these reactive molecules to cellular processes ranging from response to pathogens to polar cell elongation. The recent investigations on the functions of ROS have been augmented by development of methods that allow direct visualization of ROS formation and artful utilization of genetic and genomic resources from multiple plant models. Here we consider some of the newly visualized fibers of the complex networks that include ROS and ROS signaling in plant cells. We apologize for any work that we were unable to include due to space limitations. The 2006 Special Issue on ROS included the introduction of redox-sensitive GFP as a tool to monitor the redox state of the plant cell (Jiang et al., 2006). Since then researchers have taken advantage of this tool to monitor the redox state involved in glutathione metabolism (Meyer et al., 2007) or the removal of ROS by manganese superoxide dismutase in mitochondria (Morgan et al., 2008), and additional papers are anticipated. Other valuable methods for in vivo monitoring of ROS were also introduced, including autoluminescence (Havaux et al., 2006) and biophotone imaging (Kobayashi et al., 2007) as well as luciferase imaging (Shao et al., 2007). Dichlorofluoroscein dyes have been widely used to monitor ROS levels in plants. Recently, the dyes Amplex Red and Amplex Ultra Red were added to the toolbox for detection/imaging of H2O2 and successfully used to demonstrate ROS production in nuclei (Ashtamker et al., 2007). An interesting application for the measurement of ROS and photosynthesis was reported by Hideg and Schreiber (2007), who used 3-(N-dansyl)aminomethyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole, coupled with fluorescence imaging, to monitor in parallel in vivo photosynthesis and ROS formation. Monitoring intracellular versus extracellular ROS production was recently reported by Monshausen et al. (2007), who used OxyBURST and dichlorofluoroscein to monitor extracellular and intracellular ROS production in root hairs, respectively. These new tools expand the choices available to researchers attempting to monitor ROS in vivo. Localized ROS production in organelles (i.e. plastids, mitochondria, and peroxisomes) and in specific regions (apoplastic space and apex of polarized cells) may trigger different signaling cascades. With a diverse arsenal of ROS-generating enzymes, such as the plant orthologs of the respiratory burst NADPH oxidases (RBOHs), plant cells, like mammalian cells, can initiate and most probably amplify ROS production for the purpose of ROS signaling. A newly identified extracellular peroxidase and two type III peroxidases play an active role in H2O2 production and subsequent cell death in the local and systemic response to pathogen attack (Bindschedler et al., 2006; Choi et al., 2007). During pathogenesis or wounding, ROS production is mainly apoplastic, whereas during salt stress ROS also can be produced from internalized membranes (endosomes) by NADPH oxidase in a phosphatidylinositol 3-kinase-dependent manner (Leshem et al., 2006, 2007). In addition, inhibition of the fusion of these ROS-containing endosomes with central vacuoles leads to increased salt tolerance. Intriguingly, a previously unidentified nuclear source of ROS production has been observed during elicitor treatment (Ashtamker et al., 2007). These reports add two extra subcellular sources of ROS production in plant cells, thereby adding to the complexity of the ROS signaling network. Through its relative stability and ability to diffuse through membranes, H2O2 already had been recognized as the most potent signaling ROS in plants. At present, specific aquaporins have been demonstrated to channel H2O2 actively across membranes (Bienert et al., 2007). Despite the current lack of evidence for an in planta action, these data point to aquaporins as p