**1. Introduction**

B cell lymphoma 2 (Bcl-2)-associated athanogene (BAG) protein is a relatively conservative protein in animals and plants. BAG proteins share a common conserved BAG domain (BD), which interacts with the ATPase domain of heat-shock protein 70 (Hsc70/Hsp70 in the C-terminal region but is generally different in the N-terminal region, which gives specificity to specific proteins and pathways [1]. In animals, BAGs are widely involved in many biological processes, such as tumor regulation, apoptosis and stress response [1,2]. In plants, seven BAG family genes were first discovered in Arabidopsis [3,4]. The family of plant BAG protein can be divided into two categories. The first category contains Nterminal ubiquitin-like (UBL) domain and BAG domain, including AtBAG1–4 proteins. They may be direct homologues of animal BAG1, with high similarity in structure and function. The second type (BAG5-7 proteins) contains an isoleucine glutamine (IQ) motif binding to Ca2+-free calmodulin (CaM) near BAG domain, which is unique to plants [3].

**Citation:** Jiang, H.; Ji, Y.; Sheng, J.; Wang, Y.; Liu, X.; Xiao, P.; Ding, H. 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. *Antioxidants* **2022**, *11*, 598. https:// doi.org/10.3390/antiox11030598

Academic Editor: Nafees A. Khan

Received: 16 February 2022 Accepted: 17 March 2022 Published: 21 March 2022

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Plant BAG proteins are involved in biological processes such as plant programmed cell death (PCD) and autophagy, and play an important role in plant response to abiotic stresses such as salt, drought, temperature, and pathogen infection. Some BAG family genes have been explored in other plants such as rice, tomato, and wheat [5–9], but presently, as a model plant Arabidospis has gained the primary research attention, while relatively little is known about the functional research on other species.

The Arabidopsis seedlings overexpressing *AtBAG1* challenged with salinity stress and decreased seedling growth [10]. The *atbag4* mutant plants were more sensitive to salt stress, while the tobacco plants overexpressing *AtBAG4* gene showed stronger tolerance to ultraviolet (UV), low temperature stress, oxidative stress, drought, and salt stress than the wild type [3]. AtBAG1-AtBAG4 bound to Hsc70 through the BAG domain. The *atbag5* mutant showed the delayed aging, while the overexpression transgenic lines showed the premature aging. As a signaling hub, AtBAG5 linked the Ca2+ signaling network with the Hsc70 chaperone system to regulate plant senescence [11,12]. AtBAG6 cleavage triggered autophagy and plant defense and AtBAG6 [13]. AtBAG7 was an endoplasmic reticulum (ER) localization protein that played a central regulatory role in the heat-induced unfolded protein response (UPR) pathway [14]. The double-faced role of AtBAG7 in plant–phytophthora interaction has been found recently [15]. However, it is gratifying that, nowadays, studies on other plants species are becoming available. Transgenic rice plants overexpressing *OsBAG4* showed tolerance to NaCl stress [16]. OsBAG4 is an active regulator of disease resistance and a rice E3 ubiquitin ligase EBR1 targeted OsBAG4 for proteasome degradation [17]. Soybean *GmBAG6a* gene overexpressed in Arabidopsis had the ability to resist nematode infection [18]. Overexpressing BAG family gene *HSG1* from grapes in Arabidopsis plants showed obvious resistance to high temperature [19]. Overexpression of wheat *TaBAG2* increased Arabidopsis heat tolerance [6]. *SlBAG2* and *SlBAG5b* mediated tomato leaf tolerance to dark stress and senescence [8].

Based on the publication of plant genome sequence databases, the BAG gene family have been proved to exist in different species, such as *Arabidopsis thaliana* [3], *Oryza sativa* [5,20], and *Physcomitrium patens* [21]. We have previously tried to explore the key high-temperature-responsive genes in tomato using integrative analysis of transcriptome and proteome [22] and *SlBAG9* with a high expression level under high temperature stress was screened at the transcriptional and protein levels, which aroused our interest in the BAG gene family. In this study, ten *SlBAG* genes were identified, the related characteristics of *SlBAG* gene and protein were studied, and the gene expression pattern under different stress conditions were explored. In addition, further research on the function of *SlBAG9* was carried out. Heterologous overexpression of *SlBAG9* in Arabidopsis increased the sensitivity of Arabidopsis to drought, salt, and abscisic acid (ABA), which was reflected in the decrease in seed germination rate and seedling growth, low expression levels of stress/ABA-responsive genes and activities of superoxide dismutase (SOD) and catalase (CAT), and aggravated oxidative damage. Taken together, these findings lay a foundation for the future study of the biological function of *SlBAG* genes in tomato.

#### **2. Materials and Methods**
