*Article* **BAG9 Confers Thermotolerance by Regulating Cellular Redox Homeostasis and the Stability of Heat Shock Proteins in** *Solanum lycopersicum*

**Huamin Huang 1,†, Chenxu Liu 1,†, Chen Yang 1, Mukesh Kumar Kanwar 1, Shujun Shao 1, Zhenyu Qi 2,3 and Jie Zhou 1,2,4,5,\***


**Abstract:** The Bcl-2-associated athanogene (BAG) family, a group of co-chaperones that share conservative domains in flora and fauna, is involved in plant growth, development, and stress tolerance. However, the function of tomato *BAG* genes on thermotolerance remains largely unknown. Herein, we found that the expression of *BAG9* was induced during heat stress in tomato plants. Knockout of the *BAG9* gene by CRISPR/Cas9 reduced, while its overexpression increased thermotolerance in tomato plants as reflected by the phenotype, photosynthesis rate, and membrane peroxidation. Heat-induced reactive oxygen species and oxidative/oxidized proteins were further increased in *bag9* mutants and were normalized in *BAG9* overexpressing plants. Furthermore, the activities of antioxidant enzymes, ascorbic acid (AsA)/dehydroascorbic acid (DHA), and reduced glutathione (GSH)/oxidized glutathione (GSSG) were reduced in *bag9* mutants and were increased in *BAG9* overexpressing plants under heat stress. Additionally, BAG9 interacted with Hsp20 proteins in vitro and in vivo. Accumulation of Hsp proteins induced by heat showed a reduction in *bag9* mutants; meanwhile, it was increased in *BAG9* overexpressing plants. Thus, BAG9 played a crucial role in response to heat stress by regulating cellular redox homeostasis and the stability of heat shock proteins.

**Keywords:** antioxidants; BAG9; Hsps; *Solanum lycopersicum*; thermotolerance
