*Article* **Genome-Wide Identification of the Bcl-2 Associated Athanogene (BAG) Gene Family in** *Solanum lycopersicum* **and the Functional Role of** *SlBAG9* **in Response to Osmotic Stress**

**Hailong Jiang, Yurong Ji, Jiarong Sheng, Yan Wang, Xiaoya Liu, Peixiang Xiao and Haidong Ding \***

Joint International Research Laboratory of Agriculture and Agri-Product Safety of Ministry of Education of China, College of Bioscience and Biotechnology, Yangzhou University, Yangzhou 225009, China; jhl19923574146@163.com (H.J.); jyr0806@163.com (Y.J.); myyjjfb@163.com (J.S.); wy1123406945@163.com (Y.W.); arielnxy@outlook.com (X.L.); xiaopeixiang\_1998@163.com (P.X.)

**\*** Correspondence: hdding@yzu.edu.cn

**Abstract:** The Bcl-2-associated athanogene (BAG) proteins are a family of multi-functional group of co-chaperones regulators, modulating diverse processes from plant growth and development to stress response. Here, 10 members of *SlBAG* gene family were identified based on the available tomato (*Solanum lycopersicum*) genomic information and named as *SlBAG1-10* according to their chromosomal location. All SlBAG proteins harbor a characteristic BAG domain, categorized into two groups, and SlBAG4, SlBAG7, and SlBAG9 of group I contain a plant-specific isoleucine glutamine (IQ) calmodulin-binding motif located in the N terminus. The quantitative real-time PCR expression analysis revealed that these *SlBAG* genes had organ-specific expression patterns and most *SlBAG* genes were differentially expressed in multiple abiotic stresses including drought, salt, high temperature, cold, and cadmium stress as well as abscisic acid and H2O2. In addition, heterologous overexpression of *SlBAG9* increased the sensitivity of Arabidopsis to drought, salt, and ABA during seed germination and seedling growth. The decreased tolerance may be due to the downregulation of stress-related genes expression and severe oxidative stress. The expression levels of some stress and ABA-related genes, such as *ABI3*, *RD29A*, *DREB2A*, and *P5CS1*, were significantly inhibited by *SlBAG9* overexpression under osmotic stress. Meanwhile, the overexpression of *SlBAG9* inhibited the expression of *FSD1* and *CAT1* under stress conditions and the decreased levels of superoxide dismutase and catalase enzyme activities were detected accompanying the trends in the expression of both genes, which resulted in H2O2 accumulation and lipid peroxidation. Taken together, these findings lay a foundation for the future study of the biological function of *SlBAG* genes in tomato.

**Keywords:** antioxidant defense; arabidopsis; bioinformatic analysis; *SlBAG* genes; *SlBAG9*; *Solanum lycopersicum*
