Source: School of Life Sciences
Written by: School of Life Sciences
Edited by: Wang Dongmei

Recently, Professor Hongbin Wang’s group from School of Life Sciences in Sun Yat-sen University identified a novel chloroplast protein HHL1 functions in protection of Photosystem II from Photodamage in land plants. This finding has been published in The Plant Cell (Jin et al. 2014, doi:10.1105/tpc.113.122424), a top academic journal published by the American Society of Plant Biologists (ASPB) and ranked first in impact among journals publishing primary research in the plant sciences. Dr. Honglei Jin, who is a Ph.D. student of Prof. Wang, is the first author of this paper. This work was supported by the grants from the Natural Science Foundation of China and Guangdong Provincial Natural Science Foundation of China.

Plant photosystem transduces water and CO2 into organic substance and releases O2 by photosynthesis, which is basis of organism survival, and important medium of carbon cycle on earth. The multi-subunit pigment-protein complex of PSII can catalyze the light-driven water oxidation and reduction of plastoquinone using light energy, which is important to start photosynthesis. However, PSII is the most susceptive photochemical site, and is prone to be damaged by the continuous flux of light conditions. Thus, efficient PSII repair, which requires the sequential disassembly and reassembly of PSII proteins and the ligation of various cofactors, is essential for plant, and many additional auxiliary and regulatory factors are needed to facilitate this complex, multistep process in chloroplasts.

To gain further insight into the maintenance and repair of PSII, Professor Wang’s group screened a pool of Arabidopsis thaliana mutants using the chlorophyll fluorescence system and identified hypersensitive to high light (hhl) mutants, which show lower photosynthesis efficiency under high-light conditions and exhibit high-light-induced defects in PSII photochemistry and accumulation of PSII supercomplexes. HHL1 is exclusive in land plants, and primarily localizes in the stroma-exposed thylakoid membranes and associates with the PSII core monomer complex through direct interaction with PSII core proteins CP43 and CP47. Interestingly, HHL1 also directly interacts, in vivo and in vitro, with LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1), which functions in the repair and reassembly of PSII. Furthermore, the hhl1 lqy1 double mutant show increased photosensitivity compared with single mutants. Taken together, these results suggest that HHL1 forms a complex with LQY1 and participates in photodamage repair of PSII under high light.

Professor Wang’s group is interested in studying the recognition mechanism in plant innate immunity and chloroplast-related responses to stress. Previously, they identified the receptor OsLYP4/6 with dual function in rice, which can trigger innate immunity by perceiving fungal chitin and bacterial PGN (Liu et al., 2012, Plant Cell, ESI hot papers). More recently, they illuminated the roles of redox system in the response to stress in chloroplast, including the biogenesis of photosystem (Wang et al., 2013, Plant Physiology, Cover story) and the regulation of redox status of chloroplast (Liu et al., 2013, Plant Journal, F1000 recommended).

The excess sunlight absorbed by plants will cause inevitable damage to the photosynthetic apparatus, especially PSII. Plant cells therefore deploy a complex PSII repair system to recover the photosynthetic activity of plants. This study indicates that HHL1 functions in protection of PSII from photodamage.