Photosystem (PS) II is a multisubunit thylakoid membrane pigment–protein complex responsible for light-driven oxidation of water and reduction of plastoquinone. Currently more than 40 proteins are known to associate with PSII, either stably or transiently. The inherent feature of the PSII complex is its vulnerability in light, with the damage mainly targeted to one of its core proteins, the D1 protein. The repair of the damaged D1 protein, i.e. the repair cycle of PSII, initiates in the grana stacks where the damage generally takes place, but subsequently continues in non-appressed thylakoid domains, where many steps are common for both the repair and de novo assembly of PSII. The sequence of the (re)assembly steps of genuine PSII subunits is relatively well-characterized in higher plants. A number of novel findings have shed light into the regulation mechanisms of lateral migration of PSII subcomplexes and the repair as well as the (re)assembly of the complex. Besides the utmost importance of the PSII repair cycle for the maintenance of PSII functionality, recent research has pointed out that the maintenance of PSI is closely dependent on regulation of the PSII repair cycle. This review focuses on the current knowledge of regulation of the repair cycle of PSII in higher plant chloroplasts. Particular emphasis is paid on sequential assembly steps of PSII and the function of the number of PSII auxiliary proteins involved both in the biogenesis and repair of PSII.
Photosystem II (PSII) is a multi-subunit thylakoid membrane protein complex that catalyzes the light-driven electron transfer from water to plastoquinone (PQ). PSII is located mostly in the stacked areas of the thylakoid membrane, called grana membranes, and more than 40 proteins have been found associated with PSII, either stably or transiently  and . Recently, the crystallographic structure of cyanobacterial (Thermosynechococcus vulcanus) oxygen evolving PSII complex was determined at 1.9-Å resolution . The minimal reaction center complex of PSII, capable of charge separation, contains five proteins: D1, D2, α and β subunits of Cyt b559, and PsbI subunit. The reaction center proteins D1 and D2 bind all the redox-active cofactors, which are required for PSII electron transport. Excitation energy, captured by light harvesting complex (LHC)II and the PSII internal antenna proteins CP43 and CP47, induces charge separation in PSII, which in turn enables the oxygen-evolving complex (OEC) to oxidize water molecules and provide electrons to the electron transfer chain. Apart from the major PSII core proteins D1, D2, CP43 and CP47, the bulk of other PSII subunits are of low molecular mass and mainly involved in PSII assembly, stabilization and dimerization. Of these, chloroplast-encoded low molecular mass subunits of PSII, including PsbI, PsbJ, PsbL, PsbM and PsbTc, have been shown to be important for the assembly and/or stability of PSII in higher plants , , ,  and . In turn, the nuclear-encoded PsbW is important for the accumulation of the PSII-LHCII supercomplexes . In addition, a high number of auxiliary proteins assist the assembly of PSII.
Physiological significance of PSII damage and repairLiterature during the past 30 to 40 years has well established the susceptibility of the PSII D1 protein to damage upon exposure of plants to light in their natural environments, and similarly, the basic concept for replacement of the damaged D1 protein by a newly-synthetized copy during the repair cycle of PSII has been extensively investigated and reviewed , , ,  and . Moreover, the damage of the D1 protein has been shown to be directly proportional to light intensity  and . With the half-life of 2.4 h, the D1 protein was recently shown to have the fourth fastest turn-over rate of barley (Hordeum vulgare) proteins, when plants were growing under normal growth light intensity (500 μmol m− 2 s− 2) . Also, the D2, CP43 and PsbH subunits show higher degradation rate as compared to the other PSII subunits , ,  and . Indeed, one of the major challenges of oxygenic photosynthetic organisms is to ensure the maintenance of PSII activity.
Originally, the vulnerability of PSII was thought to be an inherent fault of the photosynthetic machinery, but more recently it has been concluded that there is a strong physiological basis for the constant, yet highly regulated, photodamage and repair cycle of PSII. While efficient repair system is available for PSII, such a mechanism has been considered not to exist for PSI and hence the damaged PSI complexes are thought to be irreparable. The degradation of damaged PSI subunits and subsequent de novo synthesis and assembly of PSI proteins and Fe–S clusters are time consuming and energy requiring processes. Indeed, it has been observed that the amount of functional PSI in cucumber (Cucumis sativus) leaves is not fully recovered in six days after the photoinhibitory treatment at chilling temperatures, which causes serious damage to PSI complexes . Consequently, it has been proposed that the role of PSII damage is the protection of PSI . In short, the functional PSII centers are suggested to increase the redox pressure on PSI making PSI more susceptible to photoinhibition. Thus, partial inhibition of the PSII complexes is likely to protect PSI. In line with this hypothesis, a dynamic control of active PSII centers via photoinhibition-repair cycle was recently demonstrated to rescue Arabidopsis (Arabidopsis thaliana) PSI from photodamage under high light conditions, yet under normal growth temperature . Taking together, the photodamage of PSII cannot any more be considered solely as an undesired consequence of the highly oxidizing chemistry of the water splitting PSII, but it is likely to function also as a PSI protection mechanism.
Lateral trafficking along the thylakoid membrane is essential for PSII repair cycleIn plants the thylakoid membrane network is composed of appressed grana stacks interconnected with non-appressed thylakoid domains, the stroma-exposed thylakoids. The diameter of grana is typically 300–600 nm and the extent of grana membrane stacking is dynamically regulated based on the prevailing light environment. In short, shade plants contain broader grana stacks with more membrane layers per granum as compared to sun plants . Electron microcopy has revealed that in spinach (Spinacia oleracea) a 10 min switch to a lower light intensity increased grana size and number per chloroplast by 10–20% and returning of the leaves to the normal growth light for 10 min reversed the phenomenon . The PSII complexes are most active as dimers and supercomplexes , which are densely packed in grana core regions of the thylakoid membrane network ,  and . However, monomerization and migration of PSII complexes to non-appressed thylakoids are a prerequisite for the repair cycle . Indeed, the FtsH and Deg proteases degrading the damaged D1 protein, ribosomes and the SecY translocon responsible for synthesis and thylakoid insertion of the newly-synthetized D1 proteins, respectively, and several auxiliary proteins assisting the PSII assembly (including PSB27, LPA1, CYP38/TLP40, LQY1 and TLP18.3) are all enriched in the non-appressed domains of the thylakoid membrane , ,  and .
The repair cycle depends on
FtsH and Deg proteases degrading the damaged D1 protein
monomerization and migration of PSII complexes to non-appressed thylakoids
ribosomes and the SecY translocon responsible for synthesis and thylakoid insertion of the newly-synthetized D1 proteins
several auxiliary proteins assisting the PSII assembly (including PSB27, LPA1, CYP38/TLP40, LQY1 and TLP18.3)
Several reports on molecular mechanisms related to the lateral trafficking of damaged PSII complexes from the grana domains to the stroma-exposed regions of the thylakoid membrane have been published recently. The phosphorylation of PSII core proteins is strongly linked to the destacking and lateral shrinkage of grana as well as to the mobility of PSII under high light intensity (for details, see Section 5.1). Not only the phosphorylation of PSII core proteins but also the amount of Arabidopsis Curvature Thylakoid 1(CURT1) proteins, which are highly enriched in the grana margins, has been shown to represent an important factor determining the number of membrane layers in grana stacks . More research is, however, required to validate the role of PSII phosphorylation and CURT1 protein during high light induced changes in thylakoid architecture.