Genome-Wide Analysis of Strictosidine Synthase-like Gene Family Revealed Their Response to Biotic/Abiotic Stress in Poplar

Abstract

The strictosidine synthase-like (SSL) gene family is a small plant immune-regulated gene family that plays a critical role in plant resistance to biotic/abiotic stresses. To date, very little has been reported on the SSL gene in plants. In this study, a total of thirteen SSLs genes were identified from poplar, and these were classified into four subgroups based on multiple sequence alignment and phylogenetic tree analysis, and members of the same subgroup were found to have similar gene structures and motifs. The results of the collinearity analysis showed that poplar SSLs had more collinear genes in the woody plants Salix purpurea and Eucalyptus grandis. The promoter analysis revealed that the promoter region of PtrSSLs contains a large number of biotic/abiotic stress response elements. Subsequently, we examined the expression patterns of PtrSSLs following drought, salt, and leaf blight stress, using RT-qPCR to validate the response of PtrSSLs to biotic/abiotic stresses. In addition, the prediction of transcription factor (TF) regulatory networks identified several TFs, such as ATMYB46, ATMYB15, AGL20, STOP1, ATWRKY65, and so on, that may be induced in the expression of PtrSSLs in response to adversity stress. In conclusion, this study provides a solid basis for a functional analysis of the SSL gene family in response to biotic/abiotic stresses in poplar.

Keywords: Populus trichocarpa; SSL; STR; Strictosidine synthase; family; stress.

Figure 1

Phylogenetic analysis of SSLs in Populus trichocarpa (Ptr), Arabidopsis thaliana (At), and Salix purpurea (Sup). Neighbor-joining (NJ) method with 10,000 bootstrap replicates was applied to draw a phylogenetic tree by MEGA7 software. The tree was divided into four groups; each color represents one group. Black stars indicate PtrSSLs.

Figure 2

Gene structure and protein motif of the SSL gene family in poplar. Colorful boxes delineate different motifs. The clustering was performed according to the phylogenetic analysis.

Figure 3

Chromosomal localization and collinearity analysis of the PtrSSL family in poplar. Heatmap in the outer circles indicate gene density on chromosomes, with red and gray lines indicating duplication gene pairs of PtrSSLs and collinear gene pairs in the poplar genome, respectively.

Figure 4

Collinearity analysis of the SSL genes from poplar and five other species. The SSL collinear genes are connected with a red line, while other collinear genes are connected with gray line.

Figure 5

Cis-elements analysis of poplar SSL genes promoters. Different colors represent different genes. The element counts are shown in Supplementary Table S3.

Figure 6

The expression patterns of PtrSSLs in roots, stems, and leaves. PtrActin was used as a reference gene. The expression of genes in root was set to 1. The data were processed using the 2^−ΔΔCt method.

Figure 7

Transcriptional regulatory networks involving PtrSSLs. Pink nodes represent PtrSSLs, and green nodes represent potential regulators of PtrSSLs upstream.

Figure 8

Expression patterns of PtrSSLs in response to NaCl. X-axis shows stress treatment time points, and Y-axis represents the relative expression level. The data were processed using the 2^−ΔΔCt method. Gene expression in 0 h was set to 1, and expression in the other time points was relative to it; t-test, * p < 0.05, and ** p < 0.01.

Figure 9

Expression patterns of PtrSSLs in response to drought. X-axis shows stress treatment time points, and Y-axis represents the relative expression level. The data were processed using the 2^−ΔΔCt method. Gene expression in 0 h was set to 1, and expression in the other time points was relative to it; t-test, * p < 0.05, and ** p < 0.01.

Figure 10

Expression patterns of PtrSSLs in response to leaf blight (Alternaria alternata). X-axis shows stress treatment time points, and Y-axis represents the relative expression level. The data were processed using the 2^−ΔΔCt method. Gene expression in 0 h was set to 1, and expression in the other time points was relative to it; t-test, * p < 0.05, and ** p < 0.01.