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).