Introduction
Singlet oxygen (1O2, or SO) is a reactive oxygen species (ROS) predominantly produced in the chloroplast during photosynthesis at PSII by energy transfer from excited chlorophyll or charged reactions centers to molecular oxygen (ie.3O2 or triplet oxygen) (Dmitrieva et al., 2020). Although it has been studied most extensively in the context of high-light stress, SO accumulates in response to many other abiotic stresses such as heat, heavy metals, mechanical injury, and osmotic stress (Pospíšil & Prasad, 2014; Chen & Fluhr, 2018). As one of the most short-lived ROS, SO is highly unstable and quickly reacts with nearby biological molecules such as lipids, proteins, and carotenoids that can trigger chloroplast-to-nucleus retrograde signaling to influence nuclear gene expression (Triantaphylidès & Havaux, 2009; Galvez-Valdivieso & Mullineaux, 2010). At sublethal doses of SO, retrograde signaling can contribute to adaptation to abiotic stresses by activating hormone signaling and expression of genes involved in detoxification and management of oxidative stress (Ramel et al., 2012a, 2013). Furthermore, many of the responses triggered by SO overlap with disease resistance pathways, including induction of numerous transcription factors in common (Ochsenbein et al., 2006; Mor et al. 2014; Zhang et al., 2014). Thus, SO may have multiple roles in stress-responsive signaling and could potentially contribute to biotic as well as abiotic stress responses. The goals of this review are 1) to provide a brief overview of SO’s chemical properties, synthesis, and signaling in plants; 2) to summarize the current state of knowledge of the role(s) of SO in biotic stress; and 3) to propose a path forward to elucidate these roles.