Identification of core subunits of photosystem  II as action sites of HSP 21, which is activated by the GUN 5‐mediated retrograde pathway in Arabidopsis

• Published in: The Plant journal : for cell and molecular biology 2017-03, Vol.89 (6), p.1106-1118
• Main Authors: Chen, Si‐Ting, He, Ning‐Yu, Chen, Juan‐Hua, Guo, Fang‐Qing
• Format: Article
• Language: English
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.13447

Photosystem  II ( PSII ) is the most thermolabile photosynthetic complex. Physiological evidence suggests that the small chloroplast heat‐shock protein 21 ( HSP 21) is involved in plant thermotolerance, but the molecular mechanism of its action remains largely unknown. Here, we have provided genetic and biochemical evidence that HSP 21 is activated by the GUN 5‐dependent retrograde signaling pathway, and stabilizes PSII by directly binding to its core subunits such as D1 and D2 proteins under heat stress. We further demonstrate that the constitutive expression of HSP 21 sufficiently rescues the thermosensitive stability of PSII and survival defects of the gun5 mutant with dramatically improving granal stacking under heat stress, indicating that HSP 21 is a key chaperone protein in maintaining the integrity of the thylakoid membrane system under heat stress. In line with our interpretation based on several lines of in vitro and in vivo protein‐interaction evidence that HSP 21 interacts with core subunits of PSII , the kinetics of HSP 21 binding to the D1 and D2 proteins was determined by performing an analysis of microscale thermophoresis. Considering the major role of HSP 21 in protecting the core subunits of PSII from thermal damage, its heat‐responsive activation via the heat‐shock transcription factor HsfA2 is critical for the survival of plants under heat stress. Our findings reveal an auto‐adaptation loop pathway that plant cells optimize particular needs of chloroplasts in stabilizing photosynthetic complexes by relaying the GUN 5‐dependent plastid signal(s) to activate the heat‐responsive expression of HSP 21 in the nucleus during adaptation to heat stress in plants. Heat shock proteins (HSPs) function as chaperones, assisting in protein folding and preventing irreversible protein aggregation under heat stress. Here we provide genetic and biochemical evidence that HSP21 protects photosystem II against heat stress by directly binding to its core subunits.