Molecular Evolution and Stress and Phytohormone Responsiveness of SUT Genes in Gossypium hirsutum

Abstract

Sucrose transporters (SUTs) play key roles in allocating the translocation of assimilates from source to sink tissues. Although the characteristics and biological roles of SUTs have been intensively investigated in higher plants, this gene family has not been functionally characterized in cotton. In this study, we performed a comprehensive analysis of SUT genes in the tetraploid cotton Gossypium hirsutum. A total of 18 G. hirsutum SUT genes were identified and classified into three groups based on their evolutionary relationships. Up to eight SUT genes in G. hirsutum were placed in the dicot-specific SUT1 group, while four and six SUT genes were, respectively, clustered into SUT4 and SUT2 groups together with members from both dicot and monocot species. The G. hirsutum SUT genes within the same group displayed similar exon/intron characteristics, and homologous genes in G. hirsutum At and Dt subgenomes, G. arboreum, and G. raimondii exhibited one-to-one relationships. Additionally, the duplicated genes in the diploid and polyploid cotton species have evolved through purifying selection, suggesting the strong conservation of SUT loci in these species. Expression analysis in different tissues indicated that SUT genes might play significant roles in cotton fiber elongation. Moreover, analyses of cis-acting regulatory elements in promoter regions and expression profiling under different abiotic stress and exogenous phytohormone treatments implied that SUT genes, especially GhSUT6A/D, might participate in plant responses to diverse abiotic stresses and phytohormones. Our findings provide valuable information for future studies on the evolution and function of SUT genes in cotton.

Keywords: abiotic stress; cotton; expression profile; phylogenetic relationship; phytohormone; sucrose transporter.

FIGURE 1

Neighbor-joining phylogenetic tree of SUT proteins from three Gossypium species and other plants SUT homologs. The SUT proteins were classified into five groups, which are shown in different colors. Loci and numbers of different plant SUT groups are listed in Supplementary Tables S3, S4, respectively.

FIGURE 2

Locations and syntenic relationships of SUT genes from G. hirsutum, G. arboreum and G. raimondii. Chromosomes of G. hirsutum, G. arboreum and G. raimondii are indicated by pink, blue, and yellow, respectively. Putative homologous SUT genes between homoeologous chromosomes of G. hirsutum At and Dt subgenomes, the At subgenome and G. arboreum, the Dt subgenome and G. raimondii, and G. arboreum and G. raimondii are connected by red, purple, green and blue, respectively. Gray line link homologous genes located on non-homoeologous chromosomes in the At and Dt subgenomes. Duplicated gene pairs in G. arboreum and G. raimondii are connected by yellow lines.

FIGURE 3

Phylogenetic relationships and structure of G. hirsutum SUT genes. (A) Neighbor-joining phylogenetic tree of G. hirsutum SUT proteins. The SUT genes in G. hirsutum were classified into three groups, SUT1, SUT4, and SUT2, which are represented by purple, blue, and orange, respectively. (B) Exon–intron structures of SUT genes. Green boxes and black lines represent exons and introns, respectively.

FIGURE 4

Tissues-specific expression profiles of G. hirsutum SUT genes. Expressions of nine paralogous SUT gene pairs were investigated in roots, stems, leaves, sepals, petals, stamens, carpels, ovules at 0 DPA, and fibers during different developmental stages by qRT-PCR. The GhHis3 gene was used as an internal control. Error bars indicate the standard deviations of three independent experiments.

FIGURE 5

Putative cis-acting regulatory elements related to stress and hormone response in promoters of G. hirsutum SUT genes.

FIGURE 6

Expression analysis of G. hirsutum SUT genes by qRT-PCR under different abiotic treatments: (A) cold; (B) heat; (C) drought; (D) salt. Transcript levels of each gene were calculated by the 2^-ΔΔCt method and expressed relative to non-stressed values (0 h) set to 1. GhHis3 was used as a housekeeping gene. Error bars represent the standard deviation of three independent experiments.

FIGURE 7

Relative transcriptional expression levels of G. hirsutum SUT genes based on qRT-PCR under different plant hormones treatments: (A) IAA; (B) GA; (C) SA. Transcript levels of each gene were calculated by the 2^-ΔΔCt method and expressed relative to the non-stressed values (0 h) set to 1. GhHis3 was used as a housekeeping gene. Error bars represent the standard deviation of three independent experiments.