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.