# Singlet Oxygen Generation and Signaling in Higher Plants

## TL;DR

This paper reviews how singlet oxygen functions as a signaling molecule in plants, influencing growth, stress responses, and communication between chloroplasts and the nucleus.

## Contribution

The paper provides a comprehensive review of singlet oxygen's role in plant signaling and its interaction with hormone pathways.

## Key findings

- Singlet oxygen acts as a signaling molecule, triggering programmed cell death or acclimation in plants.
- The EXECUTER1 protein in Arabidopsis is crucial for relaying singlet oxygen signals to the nucleus.
- Singlet oxygen signaling interacts with jasmonic acid and salicylic acid pathways to regulate plant responses.

## Abstract

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.