Photosynthesis is a necessary process for many plants to produce organics to support their livings.However,the photosynthesis carries great risks to plants beacause the excess energy of light may have a negative effect on the photosynthetic efficiency and photosynthetic apparatus.

If plants are exposed to light intensities that exceed their photosynthetic capacity,photodamage may happen.The light damages the plants mainly by damaging their DNA,various protein complexes in the thylakoid membrane, and by inducing the production of reative oxygen species (ROS) in chloroplasts. Medium or high concentration of ROS causes apoptosis,or even necrosis of plant cells.Therefore,under extreme light intensity conditions, plants may die.

The excess light energy may slow down the rate of photosynthesis,which is known as photoinhibition. Photoinhibition mainly takes place in PhotosystemⅡ(PSⅡ),because PSⅡ is more senstitive to light than the rest of the photosynthetic machinery.PSⅡ，which lies in the thylakoid membrane, together with cytochrome b6/f,PhotosystemⅠand ATP synthase, funtions in harvesting light and transduce solar energy into chemical energy,while PSⅡitself catalyzes the light-driven water oxidation.

The reaction center of PSII contains the D1/D2/cytochrome b559 complex, which houses the cofactors necessary for electron transport, together with the nearest light-harvesting chlorophyll binding proteins CP43 and CP47, form the core complex(core monomer).

PSⅡ is a very fragile photosynthetic apparatus,and plants have evolved diverse photoprotection mechanism,such as the movement of leaves and chloroplasts,screening of photoradiation,ROS scavenging systems ,dissipation of light energy as thermal energy.

Despite all the regulatory mechanisms against high light,the reaction center within PSⅡ is tend to be damaged in all light intensities.Therefore,plant cells deploy a complex PSⅡ repair system which involves disassembly and reassembly of thylakoid membrane protein complexes. When proteins in the PSII reaction center are damaged ,they undergo rapid repair and reassembly to enable photosynthetic electron transport to continue.When photodamage induce a reversible phophorylation and disassembly of Light-harvesting-complexⅡ supercomplex in plants,damaged PSII core monomers migrate from grana stacks to stroma-exposed membranes. Damaged PSII core monomers then partially disassemble and release CP43,and PSⅡ subunits(D1 mainly) are degraded by protease.After that,new PSⅡ subunits are synthesized and they are reassem bled into new PSⅡ core monomers,which later migrate to grana stalks and reform functional PSⅡ light-harvesting complex.

HHL1(Hypersensitive to High Light) is a chloroplast transmembrane protein found in Arabidopsis Thaliana. HHL1 is expressed in response to high light and functions in protecting PSⅡ against photodamage and photoinbition.It is found to be abundant in stroma lamallae when PSⅡ is photodamaged. Research shows that HHL1 helps maintain the accumulation of PSⅡ complexes.

HHL1 helps maintain the formation and stability of the LHCⅡ ,it also affects the reassembly of PSⅡ monomers under high light .According to some research,HHL1 may function in guiding the release of CP43 from PSⅡcore monomers.To do this,HHL1 associate with PSⅡ monomers after disassembly of photodamaged LHCⅡ and monomerization of PSⅡ dimers.It may specially interact with CP47 and CP43 to help form the binding sites for extrinsic proteins to PSII in PSⅡ repair.