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to NADPH. However, under various abiotic stresses, the electron transport chain (ETC)
tends to be overloaded and a part of the electron flow is diverted from ferredoxin to O
2
, re‐
ducing it to O
2
•−
. The photoreduction of O
2
at PSI proceeds via Mehler reaction and produ‐
ces O
2
•−
, which is disproportionated to H
2
O
2
and O
2
with the use of superoxide dismutase.
H
2
O
2
is rapidly detoxified to H
2
O by the ascorbate peroxidase pathway (Figure 4A). Because
of the electron flow from water in PSII to water in PSI that occurs in this process, it has been
termed the water-water cycle [77]. This cycle does not only scavenge O
2
•−
and H
2
O
2
, but also
generates a pH gradient across thylakoid membranes which enhances non-radiative dissipa‐
tion of light energy by non-photochemical quenching (see later). Therefore, the water-water
cycle is considered to function as a dissipatory mechanism of the excess energy [77,78].
H
2
O
2
is also produced during a process that proceeds concurrently to the photosynthesis -
photorespiration. During photosynthetic carbon assimilation, ribulose-1,5-bisphosphate car‐
boxylase/oxygenase enzyme (Rubisco) uses CO
2
to carboxylate ribulose-1,5-bisphosphate
(RuBP). CO
2
uptake results in the formation of two molecules of 3-phosphoglycerate (3-
PGA) that are utilized for biosynthetic reactions and the recycling of the RuBP acceptor mol‐
ecule. However, Rubisco can also use O
2
to oxygenate RuBP, forming one molecule of 3-
PGA and one molecule of 2-phosphoglycolate (2-PG). The latter cannot be used for
biosynthetic reactions and is considered as an inhibitor of the chloroplast function. Photores‐
piration functions to convert 2-PG back to 3-PGA and thus to recover carbon. It constitutes a
series of reactions taking place in chloroplasts, peroxisomes, and mitochondria. 2-PG is de‐
phosphorylated to glycolate in the chloroplast and transported to the peroxisome where it is
oxidized to glyoxylate. O
2
is the electron donor in this reaction, which results in H
2
O
2
gener‐
ation. Glyoxylate is transaminated to glycine which is transported to the mitochondrion,
where two molecules of glycine are converted to serine and the remaining carbon and nitro‐
gen are released as CO
2
and NH
3
, respectively. The amine group is used to form a new gly‐
cine from glyoxylate and the resulting hydroxypyruvate is reduced to glycerate. Finally,
glycerate is phosphorylated in the chloroplast to form 3-PGA, which can be fed back to the
Calvin cycle [79,80].
The production of ROS is also an unavoidable consequence of the aerobic respiration. It oc‐
curs under normal respiratory conditions but can be enhanced in response to biotic and
abiotic stress. ROS produced in mitochondria are regarded to be essential in PCD regulation
[81]. In mitochondria, O
2
•−
is mainly produced in complex I, ubiquinone and complex III of
ETC [82]. This O
2
•−
can be further converted into highly toxic
•
OH which may penetrate
membranes and leave the mitochondrion [83]. Hydroxyl radical can also initiate the peroxi‐
dation of mitochondrial membrane polyunsaturated fatty acids (PUFA) that leads to the for‐
mation of cytotoxic lipid aldehydes, alkenals and hydroxyalkenals, such as
malonyldialdehyde (MDA). Lipid peroxidation products may cause cellular damage by re‐
acting with other lipids, proteins and nucleic acids. The mitochondrial ETC produces signifi‐
cant amount of ROS but the mitochondrial enzyme - alternative oxidase (AOX) can prevent
ROS overproduction [84]. Some studies, performed on tobacco plants, have demonstrated
that the lack of AOX induces PCD while the AOX overexpression decreases the lesion size
during HR [85,86].
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