Singlet Oxygen Generation and Signaling in Higher Plants

Singlet oxygen (¹O₂), the excitation stage of the ground-state molecular oxygen, is a fundamental reactive oxygen species (ROS) with important functions in plant growth, development, and stress responses. In plant cells, ¹O₂ is mainly generated in the chloroplast due to photosensitizing activity of tetrapyrroles. Moreover, ¹O₂ can be generated in non-photosynthetic tissues when plants suffer environmental stresses. Although ¹O₂ was initially considered as a cytotoxin-causing merely photooxidative damages, more recent work suggests that ¹O₂ also acts as a signal that either triggers a programmed cell death response or promotes acclimation. The ¹O₂ signaling pathway is distinct and operates independently of other ROS signaling cascades. In Arabidopsis, EXECUTER1 (EX1) protein has been identified as a crucial signaling component that perceives and relays ¹O₂ signals to the nucleus, thereby initiating extensive transcriptional reprogramming. Additionally, oxidative products of carotenoids, such as β-cyclocitral, are also recognized as ¹O₂-derived signaling molecules. Through specific chloroplast-to-nucleus signaling and cross talk with hormone signaling networks-including jasmonic acid (JA) and salicylic acid (SA)-¹O₂ helps finely coordinate plant growth, defense responses, and cell fate decisions under fluctuating environmental conditions. This review aims to summarize current knowledge on ¹O₂ generation and signaling, ¹O₂-induced chloroplast changes under diverse stress conditions, and cross talk between ¹O₂ and phytohormone signaling.