Fig. 1. HS1 regulates rice thermotolerance by stabilizing PsaC. A, Seedling and leaf phenotypes of wild type (WT) Wuyunjing 7 (WYJ7) and hs1 mutant before (CK) and after heat stress. Scale bars, 1 cm. B, Changes in total chlorophyll content in WT and hs1 mutant before (CK) and after heat stress. C, Changes in relative expression level of HS1 in 14-day-old seedlings of WT after heat stress. The UBQ5 gene was used as a reference gene. D, Ultrastructural changes in chloroplasts of WT and hs1 mutant before (CK) and after heat stress. Cp, Chloroplast; G, Grana; S, Starch grain; OG, Osmophilic globule. The red boxes correspond to the magnified areas shown on the left. E, DAB (3,3ʹ-diaminobenzidine) and NBT (nitro blue tetrazolium) staining results of leaves from WT and hs1 mutant before (CK) and after heat stress. Scale bars, 1 cm. F and G, Changes in peroxide content (F) and peroxidase activity (G) in WT and hs1 mutant before (CK) and after heat stress. H2O2, Hydrogen peroxide; MDA, Malondialdehyde; SOD, Superoxide dismutase; CAT, Catalase. H, TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay results of WT and hs1 mutant before (CK) and after heat stress. Scale bars, 50 μm. Red signals indicate propidium iodide staining and green signals indicate TUNEL-positive cells. I, Changes in the abundance of other photosynthetic proteins in WT and hs1 mutant before (CK) and after heat stress.1×, 1/2×, 1/4×, and 1/8× indicate the dilution factors of the protein added to the corresponding lanes. J,Results of SFLC (split firefly luciferase complementation) assay between HS1 and PsaC proteins. The SFLC assay demonstrates that HS1 interacts with PsaC in Nicotiana benthamiana. The combination of nLUC (N-terminal fragment of luciferase) and cLUC (C-terminal fragment of luciferase) were used as a negative control. K, Changes in relative expression level of PsaC in 14-day-old seedlings of WT and hs1 mutant before (CK) and after heat stress. The UBQ5 gene was used as a reference gene. L, Changes in PsaC protein abundance in WT and hs1 mutant before (CK) and after heat stress. M, In vitro stability of PsaC protein in total proteins from WT and hs1 mutant after heat incubation. PsaC-GST indicates in vitro-induced PsaC protein with a GST tag, and Ponceau staining indicates the total amount of protein loaded in each lane. GST, Glutathione S-transferase. N, Working model of HS1 sensing high temperatures and regulating PsaC protein stability. In the WT plant, HS1 responds to heat stress by stabilizing PsaC, thereby protecting chloroplast function and intracellular reactive oxygen species (ROS) homeostasis (left panel). In the hs1 mutant, HS1 fails to maintain PsaC stability, leading to the destruction of chloroplast structure (right panel). The excessive ROS production disrupts intracellular redox balance, resulting in DNA damage. Consequently, hs1 exhibits high-temperature sensitivity, manifested as severe leaf albinism after heat stress. Heat treatment was conducted at 35 ºC for 7 d. Data are mean ± SD (n = 3) in B, C, F, G, and K. ns indicates P > 0.05, and ** indicates P < 0.01 (Student’s t-test).