Photoinhibition is the inhibition of photosynthesis by excessive light resulting in the reduction of plant growth. Exposure to additional stress factors during exposure to light increases the potential for photoinhibitory effects. Reversible photoinhibition is indicative of a protective mechanism aimed at dissipating excess light energy, while irreversible photoinhibition indicates damage to the photosynthetic systems. The present review summarizes the physiological mechanisms of photoinhibition and discusses the interaction between light and other stress factors. In addition, some of the features and strategies that help plants avoid or restrict the occurrence of photoinhibition are analyzed. Most of these defense mechanisms are associated with the dissipation of excessive energy such as heat. Understanding these mechanisms can help improve plant cultivation procedures, avoid plant cell death, and increase plant productivity.
Photosynthesis is a series of biochemical processes of the conversion of sun's energy into chemical energy in green plants. Chloroplasts are major cellular organelles in the biochemical processes through light and dark reactions and thereby produce vital molecules including oxygen on which most life on this earth depends and the carbohydrates such as sucrose, glucose and starch, which are converted into the main cellular "fuel" ATP by cellular respiration (Fig. 1). During this process, chloroplasts have mastered the challenge of harvesting light energy efficiently at one moment and then safely disposing of harmful excess excitation energy as a heat (Ohnishi et al. 2005). Without this protection through energy dissipation, photosynthesis could not occur in our relatively oxygen-rich atmosphere. (Demmig-Adams and Adams, 2000). Photosynthesis begins when light is absorbed by an antenna pigment. This pigment can be a chlorophyll (Chl), carotenoid or bilin (open chain tetrapyrrole) depending on the type of organisms. The conversion of light energy into usable chemical energy depends on the activity of pigments; it is converted into oxidation-reduction potential energy, and is stabilized in the reaction center in a form that has a lifetime sufficiently long (milliseconds) to permit electrons to be extracted from the systems (Ruban 2009). When electrons rise to a higher energy level this concomitantly increases the reduction potential of the electron acceptable molecule (Ruban 2009). The photochemical transfer of the electrons to other redox enzymes is used to generate pH gradients that produces the energy for ATP synthesis, as well as NADPH that is used for conversion of atmospheric CO2 (and/or bicarbonates) into the carbohydrates (sugars, starch and other metabolites) (Fig. 1). The electron transport chain of chloroplasts in green plants, which includes several electron acceptors such as pheophytin, quinone, plastoquinone (Pq), cytochrome b6f, and ferredoxin (Fd), results in the reduction of NADP to NADPH (Fig. 2). This process, which is arguably the most vital biochemical pathway because nearly all living organisms on earth either directly or indirectly depends on it (Ohnishi et al. 2005), is a complex process occurring in cyanobacteria, algae and green plants.
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The photosynthetic reaction center
The photosynthetic reaction centers are placed on two photosystems (PS II and PS I) of higher plants as well as cyanobacteria, red and green alga. The photosynthetic bacteria contain unique single photosystem), which are characterized by complex structures involving multisubunit proteins that function as remarkable photochemical devices. All reaction centers have a high similarity in their structure and composition. They consist of two intrinsic membrane proteins whose primary amino acid sequences are quite similar, but not identical at their core. In all organisms, the interactions between these two proteins form a heterodimeric structure that gives the binding sites for the cofactors that participate in the photochemical reactions of charge separation. The cofactors include chlorophylls (Chls) or bacteriochlorophylls, pheophytins and bacteriopheophytins (Chl molecules lacking the central Mg2+ ion), and quinones (vitamin K). The reaction center proteins bind pigments such as Chl and pheophytin molecules, which absorb light (photon) (Fig. 2). The free energy generated is used to reduce a chain of electron acceptors, which subsequently acquire lower redox potentials, and is important for the production of ATP and NADPH during photosynthesis.
Capture of light energy
Light is energy called photons. A reaction center is organized to capture the light energy (photons) using pigment molecules and to turn it into a usable form (Fig. 2). Once the light energy has been captured directly by the Chls, or shuttled by resonance energy transfer from surrounding antenna pigments, two electrons are released to the electron transport chain. If a photon hits an electron, it will raise the electron to a higher energy level within a pigment (Tyystjärvi et al. 2002). The excited electrons can easily return to the most stable at their lowest energy level (ground state). (Nishiyama, Allakhverdiev and Murata 2002). This process is exploited by photosynthetic reaction centers as releasing energy out. In all eukaryotic photosynthetic organisms, some proteins organize chlorophylls light-harvesting complex II (LHC II) associated with PS II and LHC I associated PS I in thylakoids (Paulsen 1995, Green and Durnford 1996). In higher plants, LHC II antenna complex is a trans-membrane pigment protein, with three helical regions that cross the non-polar part of the membrane, about approximately 15 chlorophyll a and b molecules, and several carotenoids. LHC I is not well determined but seems to be similar to that of LHC II. At the molecular level, LHCII plays a major role by finely controlling the amount of light energy delivered to the photosynthetic reaction centers (Ruban 2009).
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Photosystem II (PS II)
PS II, which generates the excited electron that eventually reduce NADP+, is placed on the thylakoid membranes, the site of photosynthesis in chloroplasts (Sonoike 1996). The structure of PS II is remarkably similar to the bacterial reaction center and therefore it is assumed that they originate from a common ancestor. The core of PS II consists of two subunits referred to as D1 and D2. Unlike the bacterial reaction centre, PS II of green plants contains additional subunits that bind Chls to increase the efficiency of light capture. As mentioned above, the photosynthetic reaction begins with the photochemical excitation of a pair of Chl molecules. The presence of Chl a instead of bacteriochlorophyll determines the absorbance of shorter wavelength light by PS II. The pair of Chl molecules at the reaction center is called as P680 (Tyystjärvi et al. 2002). The absorbance of a photon by the reaction center results in the transfer of a high-energy electron to a nearby pheophytin molecule. The excited electron travels from the pheophytin through two plastoquinone (Pq) molecules. Two excited electrons are required to fully reduce the loosely bound Pq to PqH2 concomitant with the uptake of two protons.
Once photo-induced charge separation has taken place, the P680 molecule carries a positive charge. The P680 is a very strong oxidant, and it extracts electrons from two water molecules that are bound directly to the manganese center. This center contains four manganese ions, a calcium ion, a chloride ion, and a tyrosine residue. Manganese exists in four oxidation states (Mn2+, Mn3+, Mn4+ and Mn5+) and forms strong bonds with oxygen-containing molecules such as H2O. The biochemical process of oxidizing two molecules of H2O requires four electrons. The H2O molecules that are oxidized in the manganese center are the direct source of the electrons that reduce two molecules of Pq to PqH2.
Photosystem I (PS I)
Electrons are transferred from PS II to the cytochrome bf complex and then to plastocyanin (PC), a blue copper protein and electron carrier. The PC complex carries the electron that will neutralize an electron pair in the reaction center of PS I. As with PS II, a pair of Chl molecules initiates photo-induced charge separation in PS I. This complex is referred to as P700 in reference to the wavelength at which the Chl molecules capture photon maximally. As soon as the captured photon is initiated electric charge separation, the excited electron travels straight down a biochemical pathway through a Chl molecule situated directly above the P700, and then continues through a quinone situated directly above that, through three 4Fe-4S cluster molecules and reaches finally to an inter-changeable ferredoxin (Fd) complex. The soluble Fd protein contains a 2Fe-2S cluster coordinated by four cysteine residues. The positive charge left on the P700 is neutralized by the transfer of an electron from PC. The biochemical pathway of electric charge separation from PS II (P680) to PS I (P700) creates a linear electron flow from H2O to NADP+. (Nishiyama, Allakhverdiev and Murata 2006).
3. The photoinhibition of photosynthesis
Photoinhibition can decrease productivity and plant growth, therefore avoidance of photoinhibition is critical for the fitness and survival of plants in natural habitats (Kitao et al. 2000). Photoinhibition is the reduction of the photosynthetic rate in response to light exposure at high irradiance (Fig. 3). Although a reduction in photosynthetic rate can also result from photo-oxidation and other causes, photoinhibition refers to the reduction of photosynthetic capacity independent of changes in pigment concentration induced by exposure to high irradiance. The most recent explanation of the mechanisms underlying photoinhibition is a hypothesis known as the manganese mechanism of photoinhibition proposed by the group of Esa Tyystjärvi (Hakala et al. 2005, Ohnishi et al. 2005). Photoinhibition readily and often occurs in photosynthetic organisms, from higher plants to cyanobacteria. In higher plants, ultraviolet light (UV) causes photoinhibition more efficient than wavelengths of visible light, and blue light is also more efficiently than other wavelengths of visible light (Tyystjärvi et al. 2002). Photoinhibition is a series of chemical reactions that inhibit different activities of PS II, but there is no consensus on what these reactions consist of. The activity of the oxygen-evolving complex of PS II is often lost before the rest of the reaction center loses activity (Hakala et al. 2005, Ohnishi et al. 2005, Tyystjarvi 2008). The photoinhibition of PS II membranes under anaerobic conditions however leads primarily to inhibition of electron transfer on the acceptor side of PSII. The PS I is less susceptible to light-induced damage than PS II, but slow inhibition of this photosystem has been observed (Sonoike 1996). Photoinhibition of PSII is easily caused by singlet oxygen produced either by weakly coupled Chl molecules (Santabarbara et al. 2002). Reactive oxygen species (ROS) have then the crucial role in the electron acceptor-side for the photoinhibition (Tyystjärvi 2008). Strong light causes the reduction of the Pq pool, which leads to protonation and double reduction (and double protonation) of the Pq electron acceptor of PS II. The protonated and double reduced forms of Pq do not function in electron transport. Furthermore, charge recombination reactions occurring in the inhibited PS II are more likely to lead to the triplet state of the primary donor (P680) than the same reactions in an active PS II. Triplet P680 can react with oxygen to produce harmful singlet oxygen (Tyystäjrvi 2008). Chemical inactivation of the oxygen-evolving complex causes the remaining electron transfer activity of PS II to become very sensitive to light. It has been considered that, even in a healthy leaf, the oxygen-evolving complex does not always function in all PS II centers, and these reaction centers are prone to rapid irreversible photoinhibition (Anderson et al. 1998). Absorption of a photon by the manganese ions of the oxygen-evolving complex triggers its inactivation as well. Further inhibition of the remaining electron transport reactions occurs by a similar mechanism to that of donor-side photoinhibition of PS II and it is supported by the action spectrum of photoinhibition (Hakala et al. 2005). In the other manner, photoinihbition has been observed to occur by the metabolic energy consumption in guard cells in normal light conditions for stomatal opening (Fig. 4). It is probably due to massive ATP consumption by the plasma membrane K+-channels in light (Goh et al. 2002) and H+-ATPase and anaerobiosis suppresses oxidative phosphorylation in the mitochondria (Goh et al. 1999).