Abstract
Photoinhibition is the popular topic in plant photosynthesis. However,
restricted to experimental systems ofin vitro membranes, knowledge
of photosystem II (PSII) donor-side
photoinhibition remains limited. Here, we report the first in
vivo study of the mechanism in the marine higher plantZostera marina . Preferential
oxygen-evolving complex photoinactivation decreased the light-harvesting
capacity and enhanced photosystem I cyclic electron flow (CEF).
Non-photochemical quenching was inefficient and alternative electron
flows, e.g. chlororespiration, Mehler reaction, malic acid synthesis,
and photorespiration, remained unactivated, thereby reducing the
unnecessary consumption of limited electron resources and maintaining a
well carbon assimilation level.
At
variance with the PSII
acceptor-side photoinhibition, the
PSII photodamage of Z. marina was not attributed to1O2but was associated with the long-lived P680+ resulted
from the photoinactivated OEC. Furthermore, we
provided the novel insights into the
PSII donor-side photoinhibition that
rare PSII-CEF and ascorbate assumed
photoprotective roles in Z. marina , which could donate electrons
to the PSII reaction center to prevent the oxidative damage by
P680+. This study addressed
an
important knowledge gap in PSII
donor-side photoinhibition, providing a novel understanding of
photosynthetic regulation mechanism responding to light stress.
Keywords: alternative electron donor; oxygen-evolving complex;
P680+; PSII donor-side photoinhibition; Zostera
marina
Introduction
Photosystem
II (PSII) photoinhibition is commonly expected to occur when
photosystems cannot sufficiently
utilize the energy absorbed by their antenna system, its
occurrence depends on the redox
state of PSII acceptor components (Gururani et al.,
2015).
Strong illumination creates excessive excitation, which in turns leads
to over-reduction of plastoquinone (PQ) acceptors, thus rendering the
PSII reaction center (RC) inactive (Barber and Andersson, 1992; Melis,
1999). In contrast to acceptor-side
photoinhibition, PSII donor-side
photoinhibition
is caused
by
water-splitting
dysfunction,
e.g., chemical damage like the
removal of the Mn cluster by NH2OH, Tris or high-salt
washing (Callahan and Cheniae, 1985; Yadav and Pospíšil, 2012),
and light damage like the
photoinactivated oxygen-evolving complex (OEC) caused by direct UV
illumination absorption (Yadav and Pospíšil, 2012; Havurinne and
Tyystjärvi,
2017).
These can be observed early in the photosynthetic membrane systemin vitro . Recently, vulnerable OEC under visible wavelengths have
also been reported in vivoin specific unicelluar algae including the diatom Phaeodactylum
tricornutum and the cyanobacteriaSynechocystis , which are
likely the result of a lack of
photoprotective sunscreen compounds such as non-photosynthetic pigments
or divinyl chlorophyll (Havurinne and Tyystjärvi, 2017; Soitamo et al.,
2017). So far,in vivo PSII
photoinhibition, derived
from
photoinactivated OEC,
has not been demonstrated with
direct experimental evidence in higher plants under visible light.
Several explicit characteristics of
the mechanism can be summarized as
follows: (1) the mechanism even
occurs under low light conditions (Keren and Krieger-Liszkay, 2011); (2)
non-photochemical quenching (NPQ) has
low protective efficiency
(Tyystjärvi, 2013); (3) the
photoinhibition rate constant (KPI) and light intensity
are directly proportional (Tyystjärvi and Aro, 1996); (4) light leads to
primary inactivation of OEC and secondary damage of PSII RC (Hakala et
al., 2005; Tyystjärvi, 2008). The in vitro occurrence of
donor-side photoinhibition, although mimicking, might not completely
reflect in vivo phenomena
(Dall’Osto et al.,
2017)
because of the incomplete photosynthetic apparatus. Restricted to
imperfect experimental systems of photosynthetic membranesin vitro ,
knowledge of
the mechanism remains limited and
fragmented (Zavafer et al., 2015,
2017).
During PSII donor-side photoinhibition, the PSII RC may be susceptible
to damage by both
P680+ and 1O2 (Aro
et al., 1993; Vass, 2011). When the electron donation of OEC is inferior
to the rate of electron withdrawal by P680+• (Vass,
2012), the long-lived
P680+, a strong oxidant damaging component
of PSII RC,
is assumed to be a source of damage
in this mechanism, which appears to be uncontroversial. However, whether
the highly reactive 1O2 is generated
in the mechanism remains unclear, since the relevant results are mainly
based on speculation and in vitro experiments (Nishiyama et al.,
2011; Yadav and Pospíšil, 2012; Pospíšil,
2016). It has been considered that
peroxidation of lipids by the
highly oxidizing P680+ and TyrZ+could cause 1O2 formation (Yadav and
Pospíšil, 2012; Pospíšil, 2016). The
impaired OEC might damage specific oxygen channels that block oxygen
molecules to P680 and conduct formative oxygen molecules outward, thus
resulting in an accumulation of 1O2(Nishiyama et al.,
2011).
However, it has also been proposed that1O2formation only occurs in
preparations that contain functional OEC (Hideg et al., 1994; Johnson et
al., 1995). When OEC is damaged, the
redox potential of the
QA/QA− pair shifts to
a higher value, thus decelerating the conversion from
P680+QA− to3[P680+Phe−]
(Ivanov et al., 2008).
Even
though the inactive OEC first promotes1O2 generation, conformational changes
induced by the persistent OEC inactivity protects against1O2 formation (Tyystjärvi, 2008).
Ascorbate (AsA), as an alternative PSII electron donor, exerts a
photoprotective role by continually supporting electron transport
through PSII (Tóth et al., 2011). Although the amount of AsA and its
affinity to PSII varies with species, this alternative electron
transport appears to be ubiquitous
in both plants and green alga, and
serves a more vital protection in heat-stressed plants (Tóth et al.,
2009). When the OEC in thylakoids
is impaired, either by acidic pH or
by UV-B exposure, AsA is also photooxidized at the donor side of PSII
(Mano et al., 2004). During normal OEC function, no electrons are
donated from AsA to PSII. It has thus been considered that the AsA acts
as emergency electron donor when
water oxidation is impaired (Mano
et al., 2004).
As
another electron donation event,
PSII cyclic electron flow
(PSII-CEF),
a pathway by which electrons on the QB site of PSII are
returned to P680+ via cytb559, has been reported
to exert a
species-dependent role in
photoprotection (Ananyev et al.,
2017).
Typically, strong light would lead
to the over-reduction of QA through linear electron
transfer, thereby enhancing the activity of PSII-CEF to consume the
excess energy (Feikema et al., 2006; Lavaud,
2007). Other factors, such as
nitrogen limitation which caused over-reduction of PQ pool, can
stimulate the electron flow, thus preventing PSII RC photodestruction
(Wagner et al., 2016). PSII-CEF also
plays a vital role in drought-tolerant species via proton gradient
formation as a contribution to ATP production and photoprotection
without consuming the limited water supply (Ananyev et al., 2016,
2017). Electron transport from
PSII-CEF to PSII RC occurs when
suppressed transfer of electrons from OEC to P680 extend the life time
of P680+ (Thompson and Brudvig, 1988; Miyake,
2002).
Hence, it is reasonable
to assume that both AsA and
PSII-CEF
can donate electrons to
prevent the accumulation of the
long-lived P680+, exerting photoprotective roles
during the PSII donor-side photoinhibition.
Zostera
marina (Zosteraceae), a widespread
seagrass species throughout the temperate northern hemisphere, playing
ecological service function, evolved from a freshwater ancestor of
terrestrial monocots and successfully adapted to a fully submerged
marine environment (Wissler et al., 2011) where it must deal with
shifted spectral composition,
characterized by a high penetration
of blue-green light (Olsen et al.,
2016).
The lack of cryptochrome blue-light
receptors demonstrated by genome-based research inZ. marina (Olsen et al.,
2016) would lead to insufficient anthocyanin levels
(Li
et al., 2013), thus weakening the screening of high-energy blue-green
wavelengths (Hughes et al., 2010). Accordingly, Z. marina would
possess more susceptible OEC with absorption peaks ranging between UV
and blue-green light wavelengths (Tyystjärvi, 2008), providing the
condition for the occurrence of PSII donor-side
photoinhibition.
Our recent study (Yang et al.,
2017) showed that: (1)
photoinhibition of Z. marinaalso occurred in low light conditions; (2)
photoinhibition is closely relevant
to the inactivation of OEC described by delayed fluorescence (DF); (3)
the time-course changes ofF v/F m exhibited the
deficiency of the P fast component of the Hanelt
photoinhibition model, suggesting the slow NPQ
development. These results agreed
with the characteristics of PSII donor-side photoinhibition. Therefore,Z. marina , a marine
angiosperm with complete photosynthetic
apparatus
and functional differentiation, may
be a valid
model species to
study PSII donor-side
photoinhibition.
In the present study, in vivoPSII
photoinhibition derived from photoinactivated OEC under visible light
was first determined in Z.
marina , by identifying the primary target of the light-induced
impairment. The integrated
characteristics of the PSII photoinhibition associated with light
absorption, electron transfer, and energy conversion were
explored.
Furthermore, damaging
and photoprotective mechanisms were
verified, based on the assumption that
the alternative PSII electron
donation pathways were activated to
promote depletion of the long-lived
P680+.