Orange protein (DbOR) from the salt-tolerant green alga Dunaliella bardawil mediates photosynthesis against heat stress via interacting with DbPsbP1

In terrestrial plants, Orange (OR) protein can modulate carotenoid homeostasis, photosynthesis, and improve tolerance to environmental stress. However, the related regulatory mechanisms of OR protein are rarely reported in green algae. Until now, research on photosynthetic regulation mechanism is largely unknown in the halophilic green alga Dunaliella bardawil. In this study, one Orange gene (DbOR, encoding a DnaJ-like protein) and two PsbP genes (DbPsbP1/2, encoding oxygen-evolving enhancer protein 2 of photosystem II) were first isolated and characterized from D. bardawil. DbOR shared a highly conserved DnaJ cysteine-rich domain and two transmembrane domains. DbOR and DbPsbP1 had multiple subcellular localizations including nucleus, cytoplasm, and cell membrane, and they could interact with each other widespread in the cell including in the chloroplast. After short time of heat treatment (32 °C, 37 °C, and 40 °C for 4 h), the maximum quantum efficiency, electron transport rate, photosynthetic efficiency of D. bardawil cells could be quickly recovered in 1 d, although the biomass was decreased in some way at high temperature. We believe that the up-regulation of DbOR and DbPsbP1/2 genes under 37 °C or/and 40 °C for 4 h and the interaction of DbOR and DbPsbP1 could enhance the stability of DbPsbP1, thus protecting the photosynthetic apparatus in response to heat stress and contribute to the increase of electron transport rate and photosynthetic efficiency of D. bardawil during the recovery from heat stress. Our study enhances the understanding of the function and mechanisms of algal OR protein in regulating photosynthesis and stress tolerance in D. bardawil and provides a gene source for the cultivation of algae or plants with high photosynthesis and stress resistance.