Intracellular SO Accumulation in Plants
In addition to production of gaseous SO by photosensitizers at the surface of the epidermis, SO may be generated in multiple intracellular locations, often as a byproduct of primary metabolism or other enzymatic reactions. Due to its higher reactivity in solution, SO is estimated to be ~1,000-fold less persistent in cells than in a gas phase (Flors & Nonell, 2006), and measurement in vivo remains challenging (see Dmitrieva et al. 2020; Prasad et al., 2018; and You et al., 2018). However, it has been detected in chloroplasts, mitochondria, peroxisomes, the cytosol and the nucleus (Mor et al., 2014; Koh et al., 2022). Mor and coworkers (2014) reported that SO could be generated in the dark and in non-photosynthetic tissues. In the mitochondria, SO is produced by electron transport-linked phosphorylation, and in the peroxisomes, SO may be generated by Fenton reactions involving iron-containing proteins and ascorbate (Sandalio & Romero-Puertas, 2015). At the plasma membrane or other membranes, SO may result from lipoxygenase activity and decomposition of lipid peroxides. Lipoxygenases for example generate SO in response to osmotic stress and mechanical wounding, and mediate cell death in roots in response to osmotic stress (Prasad et al., 2017; Chen and Fluhr, 2017; Chen et al., 2021). Another light-independent route for intracellular SO production is the Haber-Weiss reaction between superoxide and hydrogen peroxide (Mor et al., 2014). Detection with the fluorescent probe Singlet Oxygen Sensor Green (SOSG) suggested that SO levels in the mitochondria and peroxisomes of dark-adapted root tips increased in response to treatment with the bacterial elicitor flagellin (flg22) (Mor et al., 2014). Therefore, light-independent SO production in these organelles could potentially contribute to plant biotic interactions, and warrants further investigation. However, the majority of SO in plants is produced in a light-dependent fashion the chloroplast, and consequently this organelle is the focus of most research on SO in plant stress responses.
SO is generated in the chloroplast as a byproduct of normal metabolism and in response to stress. Photosystem II (PSII), the predominant source of SO, continually produces this ROS during photosynthesis when excess light energy is passed from the photosystem to nearby ground state atmospheric oxygen (ie. triplet oxygen) (Apel & Hirt, 2004). This can occur from either excited chlorophylls or energy charge separation of the PSII reaction center (Dmitrieva et al., 2020). Energy can also be passed to triplet oxygen at PSII when the electron transport chain between PSII and PSI is over-reduced (Asada, 2006). In addition, PSI can contribute to SO generation in the chloroplast through a process known as the Mehler reaction. In this case, reduced ferredoxin transfers an electron to 3O2 instead of to its principle target NADP+, generating SO and decreasing production of NAPDH to fuel the Calvin-Benson cycle (Mehler, 1951). In addition to generation of SO at PSII and PSI, Dogra and Kim (2020) also hypothesize that SO could potentially be generated at the grana margin of the thylakoid membrane, where damaged PSII components are transported for repair (Dogra & Kim, 2020). Dysregulation of chlorophyll biosynthesis can also cause leakage of chlorophyll intermediates from the chloroplast into the cytosol, and these intermediates can cause light-dependent SO accumulation in the cytosol (Koh et al., 2022).