The role of reactive oxygen species in hormonal responses.

Reactive oxygen species (ROS) are versatile molecules mediating a variety of cellular responses in plant cells, including programmed cell death (PCD), development, gravitropism, and hormonesignaling. A picture showing how ROS function in signal transduction networks has started to emerge as the result of recent studies providing genetic, cell biological, and physiological evidence describing roles for ROS in signaling (Apel and Hirt, 2004; Laloi et al., 2004; Mittler et al., 2004; Mori and Schroeder, 2004). However, further efforts are necessary to characterize the targets and molecular functions of ROS, as well as the complex interplay of ROS-generating and ROSscavenging mechanisms. Moreover, the interactions of nitric oxide with other ROS species in hormone signaling is a subject of interest (Desikan et al., 2004; Wendehenne et al., 2004; Guo and Crawford, 2005; Bright et al., 2006). Due to limited space, in this Update article we focus on recent progress made in understanding the roles of ROS in hormone signaling.