How Does Chloroplast Protect Chlorophyll Against Excessive Light?

Abstract

Chlorophylls (Chls) are the most abundant plant pigments on Earth. Chls are located in the membrane of thylakoids where they constitute the two photosystems (PSII and PSI) of terrestrial plants, responsible for both light absorption and transduction of chemical energy via photosynthesis. The high efficiency of photosystems in terms of light absorption correlates with the need to protect themselves against absorption of excess light, a process that leads to the so-called photoinhibition. Dynamic photoinhibition consists of the downregulation of photosynthesis quantum yield and a series of photo-protective mechanisms aimed to reduce the amount of light reaching the chloroplast and/or to counteract the production of reactive oxygen species (ROS) that can be grouped in: (i) the first line of chloroplast defence: non-photochemical quenching (NPQ), that is, the dissipation of excess excitation light as heat, a process that takes place in the external antennae of PSII and in which other pigments, that is carotenoids, are directly involved; (ii) the second line of defence: enzymatic antioxidant and antioxidant molecules that scavenge the generated ROS; alternative electron transport (cyclic electron transport, pseudo-cyclic electron flow, chlororespiration and water-water cycle) can efficiently prevent the over-reduction of electron flow, and reduced ferredoxin (Fd) plays a key role in this context.

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Chapter 3
How Does Chloroplast Protect Chlorophyll Against
Excessive Light?
Lucia Guidi, Massimiliano Tattini and Marco Landi
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/67887
Provisional chapter
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
How Does Chloroplast Protect Chlorophyll Against
Excessive Light?
Lucia Guidi, Massimiliano Tattini and Marco
Landi
Additional information is available at the end of the chapter
Abstract
Chlorophylls (Chls) are the most abundant plant pigments on Earth. Chls are located in
the membrane of thylakoids where they constitute the two photosystems (PSII and PSI)
of terrestrial plants, responsible for both light absorption and transduction of chemical
energy via photosynthesis. The high eciency of photosystems in terms of light absorp-
tion correlates with the need to protect themselves against absorption of excess light, a
process that leads to the so-called photoinhibition. Dynamic photoinhibition consists of
the downregulation of photosynthesis quantum yield and a series of photo-protective
mechanisms aimed to reduce the amount of light reaching the chloroplast and/or to coun-
teract the production of reactive oxygen species (ROS) that can be grouped in: (i) the rst
line of chloroplast defence: non-photochemical quenching (NPQ), that is, the dissipation
of excess excitation light as heat, a process that takes place in the external antennae of PSII
and in which other pigments, that is carotenoids, are directly involved; (ii) the second line
of defence: enzymatic antioxidant and antioxidant molecules that scavenge the gener-
ated ROS; alternative electron transport (cyclic electron transport, pseudo-cyclic electron
ow, chlororespiration and water-water cycle) can eciently prevent the over-reduction
of electron ow, and reduced ferredoxin (Fd) plays a key role in this context.
Keywords: antioxidant, carotenoids, excess excitation energy, non-photochemical
quenching, photosystem
1. Introduction
Pigments in plants, cyanobacteria, algae and photosynthetic anoxygenic bacteria are the most
important molecules involved in photosynthesis, the only biological process that tunnels
© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.

energy on Earth. Pigments play two key roles in photosynthesis: they absorb sunlight and
transduce it into chemical energy. The most important pigment is certainly chlorophyll (Chl),
an organic compound that typically shows chlorine, a cyclic tetrapyrrole ring, coordinated to
a central atom of magnesium (Figure 1). This molecular structure is very similar to that found
in the eme group in which the central atom is iron. Diversication of various Chls is due to the
dierent side chains bonded to the chlorine ring (Chl a, b, c, d, e and f).
The process of light absorption consists of a sequence of photophysical and photochemi-
cal reactions that are subdivided into three stages: (i) light absorption, (ii) utilization of this
energy to synthesize ATP and reducing power, reduced ferredoxin (Fd) and NADPH and (iii)
absorption and reduction of atmospheric CO2 into carbon skeleton. However, the most impor-
tant and true light reaction is represented by charge separation that occurs at the reaction
Figure 1. Structures of the chlorophyll molecules.
Chlorophyll22

centres. The process is possible for the presence of organic molecules able to capture sunlight
and transduce it in chemical energy namely photosynthetic pigments and that is chlorophylls
and, carotenoids. These pigments aggregate with proteins and act as an antenna harvesting
the energy of sunlight and tunnelling this energy into the reaction centres located in photosys-
tems. In plants and algae, there are about 200–400 light harvesting molecules. Light harvesting
complexes have evolved many adaptive mechanisms that permit photosynthetic organisms to
thrive in dierent environments. The spectral distribution of sunlight that reaches our planet
largely covers the absorption spectra of photosynthetic pigments utilized in light harvesting
antennas (Figure 2). In a general way, light harvesting antennas have developed the ability to
optimize light capture under both low- and high-intensity light conditions [1].
The optimal absorption wavelength range for light harvesting antennas is in the red region
(680–690 nm), where the energy is utilized by chlorophyll to split water and reduce ferredoxin.
The evolution of the most abundant pigments, chlorophyll a, is probably related to its ecient
absorption in this region in addition to, perhaps, its chemistry and for its redox potential.
All photosynthetic pigments show a chromophore, which possesses two orbitals whose dif-
ference in energy falls within the light spectrum. In consequence, a photon of incident light
is able to excite an electron from its ground-state orbital to the excited state. From a chemical
point of view, the chromophore exists as conjugated π-electron systems or metal complexes.
In a conjugated π system, electron excitation occurs between π orbitals spread across alter-
nating single and double bounds (e.g., carotenoids). The metal complex chromophores share
d orbitals between transition metals and ligands (e.g., chlorophylls). Really, in the antenna
pigments, chromophores are not individual entities, and they synergically interact with each
other and this interaction plays a crucial role in the light harvesting mechanism.
Figure 2. Chlorophyll a, b and carotenoids absorbance spectra.
How Does Chloroplast Protect Chlorophyll Against Excessive Light?
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23

Light-harvesting complex (LHC) is the complex of subunit proteins that may be part of a
larger supercomplex of a photosystem and is the functional unit in photosynthesis, devoted
to the absorption of sunlight. The energy excitation is rst tunnelled among other surround-
ing molecules of the same complex and then from one LHC to another and then funnelled
to reaction centres (RCs), where it is converted into charge separation with 90% quantum
eciency.
The presence of proteins in LHC complexes is aributable to the fact that Chl of RCs cannot
absorb sunlight at an ecient rate that is enough for ecient photosynthesis to occur. In fact,
Chl molecules in RCs absorb only a few photons each second, which are insucient to drive
electron transport into chloroplast membranes (present in 1 RC of about 300 antenna mol-
ecules). To overcome this problem, RCs are associated with antenna pigment-protein com-
plexes that absorb sunlight and very eciently transfer it to RCs. For the importance of the
LHCs in gathering sunlight, they dier in the number of pigments and in their composition
and structure in a way that they are an optimized energy collector system (Figure 3). The
proteins play an important function in the precise position, mutual separation and relative
orientation of antenna.
Figure 3. A schematic representation of the light absorption process of chloroplasts. Antenna complexes, composed of
carotenoids, Chl a and Chl b molecules, absorb photons from sunlight and transfer them to the RC, which consists of a
special couple of molecules of Chl a. Antenna complexes and the RC form a photosystem.
Chlorophyll24

Photosynthetic unit (PSU) represents the basic unit of the light-harvesting apparatus and
consists of a large number of antenna chromophores coupled to a RC. Excitation-transfer
pathways follow a scheme in which dierent chromophores build an energy funnel where
chromophores, which absorb in the blue side of spectrum, transfer excitation energy to more
red-shifted chromophores (Figure 3). Theoretically, the PSUs are considered individual enti-
ties but [2] proposed the lake and puddle model. In the second model, the PSUs do not interact
with each other and the excitation light absorbed by chromophores is always transferred to
the same RC. Dierently, in the lake model, the antenna chromophores form a matrix with
embedded RCs in which there is an unrestricted energy transfer.
2. Charge separation in photosystems and electron transport
Photosynthesis starts with light absorption by the chromophores, which excites the molecules
from the ground state to an electronic excited state. Once sunlight energy is absorbed, pig-
ments in the excited state have a short life and relax to the ground state after about 4 ns [3].
The singlet excited state lifetime of Chl is lower compared with the radiative lifetime, largely
owing to intersystem crossing, which yields triplet excited states of Chl (about 10 ns) [4]. This
electronic excitation must be usefully harvested before the molecules relax, and this happens
when excitons are transferred through space among chromophores until they reach, eventu-
ally, a RC where charge separation occurs. In plants, there are two RCs constituted by two Chl
molecules, P680 and P700, respectively, for PSII and PSI, and Chl with absorbance maxima
corresponding to these wavelengths is proposed as the nal slight sink. These chlorophylls
drive electron transfer by charge separation, a reaction in which P680 and P700 molecules
reduce an acceptor. These driving reactions energetically downhill from the potential that is
more negative to ones that are more positive (Figure 4). All these electron transfer steps in
photosynthesis share a common feature. The loss of an electron from one component, which
remains in an oxidized state, reduces another one. Typically, electron transport carriers are
small molecules or atoms of metallic elements that can exist in a number of valence states.
In photosystem RCs, the light-induced loss of an electron (charge separation) leaves P680
and P700 in an oxidized state (P680+ and P700+) and the respective acceptors, pheophytin
for P680 and A0 (chlorophyll), in a reduced state. P680+ is reduced from an adjacent tyrosine
molecule (TyrZ) in the polypeptide chain of the D1 protein of the PSII complex. In turn, the
oxidized is reduced by electrons from the oxygen-evolution complex (OEC) that oxidized
water. Two water molecules are oxidized to produce oxygen, four protons and four electrons
that are transferred one at a time. These redox reactions are carried out by OEC that consists
of four manganese atoms held in a protein matrix with one atom of calcium and chlorine each
(Figure 4). This process is known as a S-cycle from [5] that provides protons derived from
water oxidation to be released into the lumen of the thylakoid membranes.
In the other set of reactions, reduced pheophytin is oxidized by passing an electron to the
rst of two plastoquinone (PQ) molecules, tightly bound at the site QA of D2 protein in the
PSII. Then, via an iron atom, an electron is transferred to the next PQ at the site QB. Both PQs
require two electrons for their complete reduction; at the QA site, PQ undergoes to a single
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reduction event to the semiquinone state before being re-oxidized by the PQ at QB site. Two
successive reductions occur that fully reduce PQ at QB site, which, for its reduction, requires
also two protons from the stromal side of the membranes and forms PQH2 that leaves PSII
and diuses in the lipid bilayer, representing a mobile carrier of protons and electrons. A new
molecule of PQ (in oxidized form) replaces this plastoquinone in the QB site.
PQH2 formed by the PSI activity represents the substrate of the Q cycle on cytochrome b6 f,
another integral transmembrane protein complex on thylakoid membranes. PQH2 is oxidized
in two steps to PQ. The rst step happens at Qp site, located on the luminal side of cytochrome
b6 f, and the electron is transferred at the end to plastocyanin (PC), a soluble small protein
containing copper. The second electron is transferred until Qn site located on the stromal side
of the cytochrome where it reduces further PQ molecule to semiplastoquinone. Another PQH2
molecule originating from PSII is oxidized in the same two steps at the Qp site, generating
further a reduced plastocyanin and completing the reduction of semiplastoquinone to PQH2.
The oxidation of PQH2 at Qp site determines the release of two protons in the lumen that
represents the most important feature of the Q cycle. In fact, this cycle acts as a proton pump,
essential to generate the transmembrane electrochemical H+ gradient.
After light absorption and charge separation in PSI, P700+ is generated, and it is reduced
back to P700 by direct interaction with reduced PC diusing from cytochrome b6 f complex.
Figure 4. A representation of the linear non-cyclic (solid line) and cyclic electron ow (dashed line) in the chloroplast
membranes. OEC tetranuclear Mn cluster; P680, reaction centre of photosystem II (PSII); P680*, excited electronic state
of P680; Ph, pheophytin; QA and QB, plastoquinone; protein complex containing cytochrome b6 and cytochrome f; PC,
plastocyanin; P700, reaction centre of PSI; P700*, excited electronic state of P700; A0, a special chlorophyll a molecule;
A1, phylloquinone; Fe-S, iron sulphur centres; Fd, ferredoxin; NADP, nicotinamide-adenine dinucleotide phosphate and
FNR, ferredoxin-NADP+ reductase.
Chlorophyll26

Plastocyanin, from its copper atom, reduces directly P700+. The electron ow generated by
charge separation that occurs in P700 determines the reduction of dierent carriers, and
the nal electron acceptor is represented by Fd, a small water-soluble iron-sulphur protein.
Reduced Fd is capable of reducing a variety of molecules. Usually, it reduces NADP+, which
requires two electrons and two protons to yield NADPH in a reaction catalyzed by ferre-
doxin-NADP+ reductase (FNR) (Figure 4), thus completing the so-called Z scheme. The elec-
tron ow generates even chemical energy, that is ATP, by the enzymatic activity of ATP-ase,
a transmembrane complex that utilizes the proton gradient generated by Q cycle and water
oxidation, to synthetize ATP.
3. Excess of excitation energy
In the past, the higher order structure of PSII was thought to be important only to increase
the eciency of light harvesting; nowadays, it has been suggested that it provides the essen-
tial dynamic properties involved in its regulation [6]. When light is low, in a way, extremely
ecient antenna systems absorb light and tunnel it through RC, but when light is in excess,
a large extent of this energy is dissipated, overall as heat, to prevent photo-damage to PSUs.
When plants are exposed to shade or sunlight conditions, dierent mechanisms occur. Shade
leaves are typically larger in area but thinner than sun leaves because they develop shorter
palisade cells. In shade leaves, the chloroplasts move within the cells to take up a position
where they will absorb the maximum light without shading other chloroplasts below. In
addition, shade leaves show a large number of antenna, and usually, the peripheral antenna
are rich in Chl b molecules (Chl a/b = 1.33). All these mechanisms enhance and optimize the
light absorption. However, even shade leaves have adapted mechanisms aimed to regulate
the light absorption, as the state II-I transition (also called spillover process). The aim of this
process is the reduction of light tunnelled to P680 altering the ratio of light energy absorbed
between PSII and PSI. In fact, RCs of the two photosystems have dierent absorption spectra
(high energy is absorbed by P680 as compared with P700), and this determines that when
the energy ow through each is not balanced to the requirement of the Z scheme, an excess
of energy could accumulate in the system. In this way, LHCII trimers represent a feedback
loop that adjusts the amount of antenna Chls, providing energy to each photosystem (state
transition). The excess of light energy owing through PSII RCs is higher than that owing
through PSI RCs, conditions in which an excess of reduced PQ occurs. This activates a kinase
that phosphorylates some LHCII trimers, and this extra charge allows them to dissociate from
the PSII (state II) and migrate towards the stroma lamellae (state I transition) where they bind
to the PSI complex, increasing in this way the ow through the system. The increase of PSI
activity leads to the oxidation of reduced PQ, which activates a phosphatase that removes the
phosphate group to the LHCII trimers that return to PSII (state II transition).
In contrast, sun leaves live in very high radiation levels overall at the top of the canopy. The
light response curve in relation to the light intensity shows that the amount of energy utilized
is lower than that absorbed because the light energy utilized in carbon reduction is mostly
due to the limitation on the rate of CO2 diusing into the leaf (Figure 5). In these conditions,
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the antenna Chls become saturated and tunnel a high ow of the excitation energy to the RC
that cannot be dissipated along the electron ow. The excess of energy must be eciently dis-
sipated through dierent mechanisms in order to avoid photo-damage to PSII.
Photosystem II is particularly sensitive to photoinhibition because the high redox potential of
the oxidized P680 (P680+), on the other hand, necessary for water oxidation. Accumulation of
P680+ leads to dierent types of photoinhibition:
(i) Acceptor-side photoinhibition: when reduced PQ is not re-oxidized, the P680* charge
recombination is inhibited and P680 is expected to lead to the triplet state of P680, TP680*.
This chemical species may react with oxygen and produce harmful singlet oxygen.
(ii) Donor-side photoinhibition: if the OEC is chemically inactivated, the donation of elec-
trons from water does not keep up with the electron transfer from P680 to the accep-
tor side. In this case, an accumulation of P680+ occurs. The high redox potential of this
chemical species induces the oxidation of various organic components such as proteins
or pigments until damage is done to D1 protein of PSII.
Figure 5. Absorbed and utilized energy in response to increasing light intensities. When light absorbed exceed photo-
systems requirement, the ‘excess energy’ can potentially cause photo-oxidative damage if it is not e ciently dissipated.
Chlorophyll28

Dierent mechanisms are present in PSII aimed to dissipate the excess of photons absorbed
by antenna, and dierent defence lines occur into the chloroplast.
4. First line of defence of chloroplast: dissipation of excess excitation
light
First line of chloroplast defence includes suppression mechanisms aimed to reduce or dis-
sipate the excitation light tunnelled in P680. At leaf level, the change in the leaf angle with
respect to the incident light and/or the chloroplast movement into the leaf to self-shading
positions along the sidewalls of cells represent mechanisms by which a decrease in absorbed
light can occur.
In the chloroplast, there are essentially three mechanisms to contrast the high light conditions:
adjustment in synthesis and amount of antenna protein, movement of LHCII (state II-I transi-
tion) and non-photochemical quenching [7]. The rst of these mechanisms is related to the
expression of Lhcb genes, whose expression is downregulated by high light conditions and/or
low CO2 concentration. The sensor mechanism is not known even though one possible candi-
date is the redox potential (i.e., the level of reduced PQ) [8], but also ROS represent possible
signal molecules [9, 10]. Clearly, these slow mechanisms cannot entirely prevent the accumu-
lation of excess of energy in the antenna system. However, photosynthesis in green plants
depends on protective mechanisms that adapt within minutes or seconds to changing light
conditions. Excited Chls return to the ground state either by emiing photons (uorescence)
or by dissipating it as heat. All these mechanisms aimed to remove this trapped energy before
it passed on down the electron transport chain are named non-photochemical quenching (NPQ).
NPQ is heterogeneous and composed by at least three components: the major and rapid com-
ponent is the pH- or energy-dependent component qE, a second component qT, related to the
phenomenon of state transition but negligible in most of plants under excess light and the
third and slow component, qI, related to the photoinhibition of photosynthesis [11].
It has been reported that two distinct qE mechanisms occur, one involving zeaxanthin (Zea)
(quenching type 1) and the other carotenoid lutein (Lut) (quenching type 2) [12]. In qE type
I, three xanthophylls, violaxanthin (Vio), anteraxanthin (Ant) and Zea, are involved in the
well-known xanthophyll cycle in which the epoxidation of Vio to Zea via Ant determines an
ecient dissipation of excess light into heat [13]. Electron ow pumping and generating pro-
tons in the lumen decrease its pH from about 7 to less than 5; this represents a strong signal
that starts a series of quenching processes. The low pH-induced protonation of PsbS peptide,
for its proximity to antenna complexes (CP24, CP26 and CP29), induces in turn in these com-
plexes conformational changes. In the chemical state, antenna complexes bind one molecule
of Zea and one of Chl (Zea-Chl complex = quenching complex) that accept energy transfer from
excited Chls. Zeaxanthins are able to return to their ground state dissipating energy as heat :
LHCII
* + zeaxanthin → LHCII + zeaxanthin * . (1)
Zeaxanthin * → zeaxanthin + heat. (2)
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It has been reported that in the crystal structure of LHCII is present Vio, and its peripheral
localization suggests that it could be de-epoxidized to Zea by Vio de-epoxidase (VDE), an
enzyme that is activated by low lumen pH occurring in high light conditions. The back reac-
tion by Zea epoxidase is slow and causes a sustained quenching that relaxes within 1–3 hours
following light stress and depends on the release of Zea from antenna pigments. In conclu-
sion, Zea is certainly considered a regulator of light harvesting for its role in the xantho-
phyll cycle and carries out three fundamental roles during high light conditions: (i) protection
against photo-oxidation due to radical oxygen’s aack (because it quenches oxygen singlet
energy), (ii) absorption of Chl triplet energy and (iii) absorption of incoming photons and
transferring them to neighbouring Chl molecules increasing in this way the overall absorption
spectrum of the PSs [14]. In addition, it has been reported that this xanthophyll exhibits an
antioxidant function in the thylakoid membrane [15].
In addition, trimeric LHCII binds other types of xanthophylls: two all-trans-luteins and a 9-cis-
noexanthin [16]. The minor monomeric complexes CP24, CP26 and CP29 all bind Lut, and in
addition, CP29 binds two xanthophyll cycle carotenoids and one-half to one neoxanthin (Neo),
CP24 binds two xanthophyll cycle carotenoids and CP26 binds one xanthophyll cycle carot-
enoids and one Neo [17, 18]. In the quenching type 2, qE is an intrinsic LCHII property: protein
conformational changes alter congurations of bound pigment (normally Lut), which become
an ecient quencher of Chl-excited state [12]. A change in the conformational state of another
LHCII-bound xanthophyll, Neo, correlates with the extent of quenching. In the model for type
2 quenching proposed by [19], Zea acts not as a quencher but as an allosteric modulator of
the ΔpH sensitivity of this intrinsic LHCII quenching process. The two types of quenching
involved dierent xanthophylls that operate at dierent sites, but there are some similarities in
the reasons that both involve ΔpH and PsbS-mediated conformational changes [12].
Given that the xanthophyll cycle quenches only 95% of the triplet Chl [20], the unquenched
triplet Chl is the reason for the need of singlet oxygen not only scavenging by carotenoids
bound to LHCII but also by carotenoids free in lipid matrix [21]. Lut has the specic property
of quenching harmful 3Chl* by binding at site L1 of the major LHCII complex and of other
Lhc proteins of plants, thus preventing ROS formation [20]. Neo contributes PSII photoprotec-
tion in a dual way: determins conformational change in trimeric LHCII, which reduces light
absorption and controls the accessibility of the O2 to the inner core of the complex [20, 22].
The trimeric organization of LHCII is, denitively, eective in screening the internal protein
domain from molecular oxygen [23].
5. Second line of defence of chloroplast: antioxidant enzymes and
molecules
As reported above, the excess of excitation energy induces an excess of singlet-excited Chl a that
is de-excited via thermal dissipation. However, the remaining singlet-excited Chl a can convert
to triplet-excited Chl that readily reduces molecular oxygen. This determines the synthesis of
ROS that is potentially dangerous to organic molecules in the chloroplast. In the second line
of defence, antioxidant molecules and enzymes that together scavenge ROS play a key role.
Chlorophyll30

The primary products of molecular oxygen reduction are disproportionate to H2O2 and O2 in
a reaction catalyzed by superoxide dismutase (SOD). H2O2 produced is then reduced to water
with the reducing power of ascorbate (ASA) in a reaction catalyzed by ASA peroxidase (APX),
and ASA is oxidized to monodehydroascorbate (MDHA) that is directly reduced to ASA by
reduced ferredoxin or NADPH by MDHA reductase. Alternatively, MDHA is spontaneously
disproportionated to dehydroascorbate (DHA) and ASA. DHA is then reduced by reduced glu-
tathione (GSH), by the enzyme DHA reductase that produces oxidized glutathione (GSSG) and
ASA. Finally, GSSG is reduced again in GSH by the action of GSH reductase, and the reducing
power is represented by reduced Fd or NADPH, that, in turn, are reduced by PSI activity. This
indicates that any pathway aimed to regenerate ASA utilizes electrons derived from water. For
this reason, the previous process is referred as water-water cycle [10].
In addition to the primary antioxidant systems, carotenoids have a protective role against
ROS since they are very ecient physical and chemical quenchers of singlet oxygen and
potent scavengers of other free radicals [24]. For example, β-carotene, located in the core
complex of both PSII and PSI, plays a role as a quencher of Chl triplet and singlet oxygen [25],
and the products generated from the oxidation of β-carotene by singlet oxygen represent pri-
mary sensor signalling under oxidative stress [26]. Other carotenoids play an important role
as antioxidants in the chloroplast. Lut is the most abundant carotenoid in the chloroplast and
is required as a quencher [7], while Neo can scavenge superoxide anion [27]. The antioxidant
activity of carotenoids is carried out in combination with other lipophilic antioxidants. In
this way, it has been reported that Zea, in cooperation with tocopherol, prevented photo-
oxidation induced by high light [28], or a strong increase in carotenoids pigment (including
those involved in xanthophyll cycle) is reported together with the activity of SOD enzyme
following oxidative stress [29]. Again, carotenoids can inuence the structure and uidity
of thylakoid membranes [30], that is essential for photosynthetic functions, inuence barrier
status to ions and oxygen, increase thermostability and protect against lipid peroxidation.
In fact, as reported by [30], β-carotene can uidize the membrane because it can move in the
inner hydrophobic part of the membrane, and xanthophyll (and in particular Zea) shows the
polar group that orientates these carotenoids perpendicular to the membrane surface.
6. From PSII repair processes to alternative electron sinks
In the last 30–40 years, the susceptibility of D1 protein to photo-damage has been well known,
and the concept of the replacement of the damaged D1 protein during the repair cycle of PSII
is extensively investigated [13, 31–33]. Moreover, D1 damage has been shown to be directly
proportional to light intensity [34].
The repair process of photo-damaged D1 proteins consists of dierent steps: (i) prompt, par-
tial disassembly of the PSII holocomplex, (ii) exposure of the photo-damaged PSII core to the
stroma of the chloroplast, (iii) degradation of photo-damaged D1, (iv) de novo D1 biosynthesis
and insertion in the thylakoid membrane and (v) re-assembly of the PSII holocomplex, fol-
lowed by activation of the electron-transport process through the reconstituted D1/D2 het-
erodimer [35]. The sequence leading to the recovery of photo-damaged PSII is consistent with
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the frequent D1 turnover in the chloroplast and with the heterogeneity in the conguration
and function of PSII.
In the past, the sensibility of PSII was linked to an inherent defect of photosynthetic apparatus
but now it is clear how this mechanism of damage-repair of PSII is extremely regulated [33]
and protects even PSI from irreversible damage. In fact, the repair mechanisms in PSI are time
and high energy consuming, and it has been suggested that the inhibition of PSII is likely to
protect PSI [33].
Reduced Fd plays an important role in preventing the over-reduction of electron ow, and a
wide range of electron sinks are available in chloroplasts. Electrons are preferentially utilized by
the FNR enzyme that produces NADPH for CO2 photoassimilation or ferredoxin:thioredoxin
reductase that synthesizes thioredoxin responsible for the regulation of some enzymes
of Calvin-Benson cycle [36]. On the other hand, reduced Fd can release electrons also to
ferredoxin:nitrite reductase and sulphite reductase for the reductive assimilation of nitrite [37]
and sulphur [38]. Finally, reduced Fd represents an electron donor for fay acid desaturases
[39] and glutamine:oxoglutarate amino transferase [40]. However, when NADP+ is not avail-
able, reduced Fd releases its electron to dierent acceptors whose function is to avoid an over-
reduction of PSI [41]. It has been discovered that there is an electron transport driven solely
by PSI and scientists called it cyclic electron ow. In this cycle, electrons can be recycled from
reduced Fd to PQ and subsequently, to the cytochrome b6 f complex via the Q cycle [42]. Such
cyclic ow generates ΔpH and thus ATP without the accumulation of reduced species. In addi-
tion, the generated ΔpH may regulate photosynthesis via NPQ (see Section 4). Another elec-
tron acceptor of reduced Fd is molecular oxygen inducing the pseudo-cyclic electron ow. The
reduction of molecular oxygen with one electron generates superoxide anions in the so-called
Mehler reaction, which restores the redox poise when linear electron ow is over-reduced
[43]. The radical oxygen species is eciently removed by water-water cycle. Chlororespiration
is another eective electron sink in which reduced Fd is directly involved. In this process,
two enzymes play the key role: NADH dehydrogenase complex and nucleus-encoded plastid-
localized terminal oxidase (PTOX). The enzyme PTOX catalyzes the reaction in which elec-
trons are transferred from PQH2 to molecular oxygen forming water [44].
Finally, in addition to the above-reported electron ow, photorespiration is another ecient
pathway by which plants adjust the ATP/NADPH ratio and consume the excess of excitation
energy.
7. Conclusions
Certainly, Chls represent the key molecules involved in light energy absorption and transduc-
tion into chemical energy. Chls absorb the light energy that reaches leaves in a very ecient
manner but sometimes, light exceeds photochemistry requirement, and the complexity of pho-
tosystems is essential to modulate and dissipate excess of excitation energy. A wide range of
responses to environmental stimuli thus characterizes the photoprotection of chloroplasts. The
increasing level of complexity from the molecular (pigments and protein) to supramolecular
Chlorophyll32

(photosystems) level mirrors the necessity of dierent time-scale responses (from seconds to
months) to modulate light that is (inevitably) absorbed. In the range of seconds to minutes, mod-
ulation of the redox state of photosynthetic electron transport activates the non-photochemical
quenching of excess of excitation energy not only through xanthophyll cycles [13] but also by II-I
state transition [45]. On a larger scale (minutes to hours), modulation of redox state of electron
transport induces changes in gene expression (organellar and nucleus) through retrograde regu-
lation that changes the structure of the photosynthetic apparatus [46, 47]. On the time scale from
weeks to months, the redox state of electron transport determines changes in plant growth and
morphology [48].
Author details
Lucia Guidi1*, Massimiliano Taini2 and Marco Landi1
*Address all correspondence to: lucia.guidi@unipi.it
1 Department of Agriculture, Food and Environment, University of Pisa, Pisa, Italy
2 National Research Council of Italy, Department of Biology, Agriculture and Food Sciences,
Institute for Sustainable Plant Protection, Sesto Fiorentino, Florence, Italy
References
[1] Perrine Z, Negi S, Sayre RT. Optimization of photosynthetic light energy utilization by
microalgae. Algal Res 2012; 1: 134–142. doi:10.1016/j.algal.2012.07.002
[2] Bernhardt K, Trissl H-W. Theories for kinetics and yields of uorescence and photochem-
istry: how, if at all, can dierent models of antenna organization be distinguished experi-
mentally? Biochim Biophys Acta 1999; 1409: 125–142. doi:10.1016/S0005-2728(98)00149-2
[3] Mullineaux CW, Pascal AA, Horton P, Holzwarth AR. Excitation-energy quenching in
aggregates of the LHC II chlorophyll–protein complex: a time-resolved uorescence
study. Biochim Biophys Acta 1993; 1141: 23–28. doi:10.1016/0005-2728(93)90184-H
[4] Bowers PG, Porter G. Quantum yields of triplet formation in solutions of chlorophyll.
Proc R Soc A Math Phys Eng Sci 1967; 296: 435–441. doi:10.1098/rspa.1967.0036
[5] Kok B, Forbush B, McGloin M. Cooperation of charges in photosynthetic O2 evolution
I. A linear four step mechanism. Photochem Photobiol 1970; 11: 457–475. doi:10.1111/j.
1751-1097.1970.tb06017.x
[6] Horton P. Are grana necessary for regulation of light harvesting? Au J Plant Physiol
1999; 26: 659–669. doi:10.1071/PP99095
[7] Dall’Osto L, Lico C, Alric J, Giuliano G, Havaux M, Bassi R. Lutein is needed for ecient
chlorophyll triplet quenching in the major LHCII antenna complex of higher plants and
How Does Chloroplast Protect Chlorophyll Against Excessive Light?
http://dx.doi.org/10.5772/67887
33

eective photoprotection in vivo under strong light. BMC Plant Biol 2006; 6: 32. doi:10.1186/
1471-2229-6-32
[8] Foyer CH, Neukermans J, Queval G, Noctor G, Harbinson J. Photosynthetic control of
electron transport and the regulation of gene expression. J Exp Bot 2012; 63 (4): 1637–
1661. doi:10.1093/jxb/ers013
[9] Foyer CH, Noctor G. Redox sensing and signalling associated with reactive oxy-
gen in chloroplasts, peroxisomes and mitochondria. Physiol Plant 2003; 119: 355–364.
doi:10.1034/j.1399-3054.2003.00223.x
[10] Foyer CH, Noctor G. Redox regulation in photosynthetic organisms: signalling, acclima-
tion, and practical implications. Antioxid Redox Signal 2009; 11: 861–905. doi:10.1089/
ars.2008.2177
[11] Müller P, Li X-P, Niyogi KK. Non-photochemical quenching. A response to excess light
energy. Plant Physiol 2001; 125: 1558–1566. doi:10.1104/pp.125.4.1558
[12] Ruban AV, Berera R, Ilioaia C, van Stokkum IHM, Kennis JTM, Pascal AA, van Amerongen
H, Robert B, Horton P, van Grondelle R. Identication of a mechanism of photoprotective
energy dissipation in higher plants. Nature 2007; 450: 575–578. doi:10.1038/nature06262
[13] Demmig-Adams B, Adams III WW. Photoprotection and other responses of plants to
high light stress. Annu Rev Plant Physiol Plant Mol Biol 1992; 43: 599–626. doi:10.1146/
annurev.pp.43.060192.003123
[14] Horton P, Ruban A. Molecular design of the photosystem II light-harvesting antenna:
photosynthesis and photoprotection. J Exp Bot 2004; 56 (411): 365–373. doi:10.1093/jxb/
eri023
[15] Brunei C, Guidi L, Sebastiani F, Taini M. Isoprenoids and phenylpropanoids are key
components of the antioxidant defense system of plants facing severe excess light stress.
Environ Exp Bot 2015; 119: 54–62. doi:10.1016/j.envexpbot.2015.04.007
[16] Liu Z, Yan H, Wang K, Kuang T, Zhang J, Gui L, An X, Chang W. Crystal structure of
spinach major light-harvesting complex at 2.72 Å resolution. Nature 2004; 428: 287–292.
doi:10.1038/nature02373
[17] Bassi R, Pineau B, Dainese P, Marquardt J. Carotenoid-binding proteins of photosystem
II. Eur J Biochem 1993; 212: 297–303. doi:10.1111/j.1432-1033.1993.tb17662.x
[18] Wobbe L, Bassi R, Kruse O. Multi-level light capture control in plants and green algae.
Trends Plant Sci 2016; 21: 55–68. doi:10.1016/j.tplants.2015.10.004
[19] Horton P, Wentworth M, Ruban A. Control of the light harvesting function of chloro-
plast membranes: the LHCII-aggregation model for non-photochemical quenching.
FEBS 2005; 579: 4201–4206. doi:10.1016/j.febslet.2005.07.003
[20] Jahns P, Holzwarth AR. The role of the xanthophyll cycle and of lutein in photopro-
tection of photosystem II. Biochim Biophys Acta 2012; 1817: 182–193. doi:10.1016/j.
bbabio.2011.04.012
Chlorophyll34

[21] Havaux M, Niyogi KK. The violaxanthin cycle protects plants from photooxidative
damage by more than one mechanism. Proc Natl Acad Sci U S A 1999; 96: 8762–8767.
doi:10.1073/pnas.96.15.8762
[22] Mozzo M, Dall’Osto L, Hienerwadel R, Bassi R, Croce R. Photoprotection in the antenna
complexes of photosystem II: role of individual xanthophylls in chlorophyll triplet
quenching. J Biol Chem 2000; 283: 6184–6192. doi:10.1074/jbc.M708961200
[23] Krüger TPJ, Ilioaia C, Johnson MP, Belgio E, Horton P, Ruban AV, van Grondelle R.
The specicity of controlled protein disorder in the photoprotection of plants. Biophys
J 2013; 105: 1018–1026
[24] Stahl W, Sies H. Antioxidant activity of carotenoids. Mol Aspects Med 2003; 24: 345–351.
doi:10.1016/S0098-2997(03)00030-X
[25] Cazzaniga S, Li Z, Niyogi KK, Bassi R, Dall’Osto L. The Arabidopsis szl1 mutant reveals
a critical role of β-carotene in photosystem I photoprotection. Plant Physiol 2012; 159:
1745–1758. doi:10.1104/pp.112.201137
[26] Havaux M. Carotenoid oxidation products as stress signals in plants. Plant J 2014; 79:
597–606. doi:10.1111/tpj.12386
[27] Dall’Osto L, Cazzaniga S, North H, Marion-Poll A, Bassi R. The Arabidopsis aba4-1
mutant reveals a specic function for neoxanthin in protection against photooxidative
stress. Plant Cell 2007; 19: 1048–1064. doi:10.1105/tpc.106.049114
[28] García-Plazaola JI, Becerril JM, Hernández A, Niinemets Ü, Kollist H. Acclimation of
antioxidant pools to the light environment in a natural forest canopy. New Phytol 2004;
163: 87–97. doi:10.1111/j.1469-8137.2004.01096.x
[29] Ariz I, Esteban R, García-Plazaola JI, Becerril JM, Aparicio-Tejo PM, Moran JF. High
irradiance induces photoprotective mechanisms and a positive eect on NH4+ stress in
Pisum sativum L. J Plant Physiol 2010; 167: 1038–1045. doi:10.1016/j.jplph.2010.02.014
[30] Domonkos I, Kis K, Gombos Z, Ughy B. Carotenoids, versatile components of oxygenic
photosynthesis. Prog Lipid Res 2013; 52: 539–561. doi:10.1016/j.plipres.2013.07.001
[31] Aro E-M, Virgin I, Andersson B. Photoinhibition of photosystem II. Inactivation, protein
damage and turnover. Biochim Biophys Acta 1993; 1143: 113–134.doi:10.1016/0005-2728
(93)90134-2
[32] Nishiyama Y, Murata N. Revised scheme for the mechanism of photoinhibition and
its application to enhance the abiotic stress tolerance of the photosynthetic machinery.
Appl Microbiol Biotechnol 2014; 98: 8777–8796. doi:10.1007/s00253-014-6020-0
[33] Järvi S, Suorsa M, Aro E-M. Photosystem II repair in plant chloroplasts — regulation,
assisting proteins and shared components with photosystem II biogenesis. Biochim
Biophys Acta 2015; 1847: 900–909. doi:10.1016/j.bbabio.2015.01.006
[34] Tyystjärvi E, Aro E-M. The rate constant of photoinhibition, measured in lincomycin-
treated leaves, is directly proportional to light intensity. Proc Natl Acad Sci U S A 1996;
93: 2213–2218. doi:10.1073/pnas.93.5.2213
How Does Chloroplast Protect Chlorophyll Against Excessive Light?
http://dx.doi.org/10.5772/67887
35

[35] Melis A. Photosystem-II damage and repair cycle in chloroplasts: what modulates the
rate of photodamage in vivo? Trends Plant Sci 1999; 4: 130–135. doi:10.1016/S1360-1385
(99)01387-4
[36] Carrillo N, Ceccarelli EA. Open questions in ferredoxin-NADP+ reductase catalytic
mechanism. Eur J Biochem 2003; 270: 1900–1915. doi:10.1046/j.1432-1033.2003.03566.x
[37] Guerrero MG, Vega JM, Losada M. The assimilatory nitrate-reducing system and its reg-
ulation. Annu Rev Plant Physiol 1981; 32: 169–204. doi:10.1146/annurev.pp.32.060181.
001125
[38] Nakayama M, Akashi T, Hase T. Plant sulte reductase: molecular structure, catalytic
function and interaction with ferredoxin. J Inorg Biochem 2000; 82: 27–32. doi:10.1016/
S0162-0134(00)00138-0
[39] Los DS, Murata N. Structure and expression of fay acid desaturases. Biochim Biophys
Acta 1998; 1394: 3–15. doi:10.1016/S0005-2760(98)00091-5
[40] Sunil B, Talla SK, Aswani V, Raghavendra AS. Optimization of photosynthesis by multi-
ple metabolic pathways involving interorganelle interactions: resource sharing and ROS
maintenance as the bases. Photosynth Res 2013; 117: 61–71. doi:10.1007/s11120-013-9889-z
[41] Arnon DI, Allen MB, Whatley FR. Photosynthesis by isolated chloroplasts. Nature 1954;
174: 394–396. doi:10.1038/174394a0
[42] Munekage Y, Hashimoto M, Tomizawa CK-I, Endo T, Tasaka M, Shikanai T. Cyclic
electron ow around photosystem I is essential for photosynthesis. Nature 2004; 429:
579–582. doi:10.1038/nature02598
[43] Allen JF. Cyclic, pseudocyclic and noncyclic photophosphorylation: new links in the
chain. Trends Plant Sci 2003; 8: 15–19. doi:10.1016/S1360-1385(02)00006-7
[44] Krieger-Liszkay A, Feilke K. The dual role of the plastid terminal oxidase PTOX: between
a protective and a pro-oxidant function. Front Plant Sci 2015; 6: 1147. doi:10.3389/fpls.
2015.01147
[45] Kargul J, Barber J. Photosynthetic acclimation: structural reorganisation of light harvest-
ing antenna – role of redox-dependent phosphorylation of major and minor chlorophyll
a/b binding proteins. FEBS J 2008; 275: 1056–1068. doi:10.1111/j.1742-4658.2008.06262.x
[46] Pfannschmidt T. Chloroplast redox signals: how photosynthesis controls its own genes.
Trends Plant Sci 2003; 8: 33–41. doi:10.1016/S1360-1385(02)00005-5
[47] Fernadez AP, Strand A. Retrograde signaling and plant stress: plastid signals initi-
ate cellular stress responses. Curr Opin Plant Biol 2008; 11: 509–513. doi:10.1016/j.
pbi.2008.06.002
[48] Rosso D, Bode R, Li W, Krol M, Saccon D, Wang S, Schillaci LA, Rodermel SR, Maxwell
DP, Hüner NPA. Photosynthetic redox imbalance governs leaf sectoring in the Arabidopsis
thaliana variegation mutants immutans, spoy, var1, and var2. Plant Cell 2009; 21: 3473–
3492. doi:10.1105/tpc.108.062752
Chlorophyll36