**3. Results**

#### *3.1. Structure and Expression Analysis of Sl1*

The *Sl1* (Solyc09g089890) gene that was previously reported as one of the highly expressed genes in tomato roots under Cd stress was selected as the target gene in the current study [25]. We examined the time course of the relative expression level of *Sl1* in roots exposed to Cd stress. After 7 d treatment, the expression of *Sl1* reached the peak, which was 4.9-fold of that in control (Figure 1). Moreover, we examined the relative expression of the *Sl1* gene in different tissues including leaf, stem, root, flower, and fruit. The results showed that the relative expression level of *Sl1* in roots was 6-fold of that in leaves and 1.6-fold of that in stems (Figure 1).

The *Sl1* gene locates on chromosome 9 with only one exon on coding sequences (CDS) that encodes 349 amino acids. We used InterPro (https://www.ebi.ac.uk/interpro/ (accessed on 20 October 2021)) and Expasy (https://web.expasy.org/compute\_pi/ (accessed on 22 October 2021)) to analyze thC terminus structure of the Sl1 protein. The results show that the isoelectric point and molecular weight of Sl1 protein are 8.56 and 40.14 kDa, respectively, and Sl1 contains a RING domain of 127 to 170 amino acids (Figure S2A,B).

**Figure 1.** The relative expression level of *Sl1* in tomato plants. (**A**) The relative expression level of *Sl1* in different tissues of tomato. (**B**) Time course of the relative expression level of *Sl1* in tomato root with and without Cd stress. The data presented here are the average of three biological replicates (±SD). Different letters indicate a significant difference (*p* < 0.05, Tukey's test).
