Exogenous Application of Phytohormones Promotes Growth and Regulates Expression of Wood Formation-Related Genes in Populus simonii × P. nigra

Abstract

Although phytohormones are known to be important signal molecules involved in wood formation, their roles are still largely unclear. Here, Populus simonii × P. nigra seedlings were treated with different concentrations of exogenous phytohormones, indole-3-acetic acid (IAA), gibberellin (GA₃), and brassinosteroid (BR), and the effects of phytohormones on growth were investigated. Next, 27 genes with known roles in wood formation were selected for qPCR analysis to determine tissue-specificity and timing of responses to phytohormone treatments. Compared to the control, most IAA, GA₃, and BR concentrations significantly increased seedling height. Meanwhile, IAA induced significant seedling stem diameter and cellulose content increases that peaked at 3 and 30 mg·L^-1, respectively. Significant increase in cellulose content was also observed in seedlings treated with 100 mg·L^-1 GA₃. Neither stem diameter nor cellulose content of seedlings were affected by BR treatment significantly, although slight effects were observed. Anatomical measurements demonstrated improved xylem, but not phloem, development in IAA- and BR-treated seedlings. Most gene expression patterns induced by IAA, GA₃, and BR differed among tissues. Many IAA response genes were also regulated by GA₃, while BR-induced transcription was weaker and slower in Populus than for IAA and GA₃. These results reveal the roles played by phytohormones in plant growth and lay the foundation for exploring molecular regulatory mechanisms of wood formation in Populus.

Keywords: Populus simonii × P. nigra; expression profiles; phytohormone; qPCR; wood formation related gene.

Figure 1

Effects of exogenously applied indole-3-acetic acid (IAA), gibberellin (GA₃) and brassinosteroid (BR) on plant growth and cellulose synthesis: plant height (A–C), stem diameter (D–F) and cellulose content (G–I). * and ** indicate significant differences at p < 0.05 and p < 0.01.

Figure 2

Effects of phytohormones on xylem and phloem differentiation in 5-month-old Populus. (A) Cross sections of stems in phytohormone-treated and untreated plants. (B) Measurement of xylem thickness. (C) Measurement of phloem thickness. (D) The ratio of xylem thickness to phloem thickness. * and ** indicate significant differences in comparison with control at p < 0.05 and p < 0.01, respectively. Cambial zone (Ca), Phloem (Ph), Phloem fiber (Pf), Xylem (Xy), Bar = 100 µm.

Figure 3

Hierarchical clustering of gene expression by qPCR under IAA, GA₃, and BR treatments. Treatment times are indicated at the bottom of the figure. The samples were harvested at time points of 0.5 h, 1.5 h, 3 h, 24 h, 2 d, 3 d and 4 d. (A) Stem, (B) Root, (C) Leaf. * indicates significant differences in comparison with control (|log₂^f°^{ld change}| > 1 and p < 0.05).

Figure 3

Hierarchical clustering of gene expression by qPCR under IAA, GA₃, and BR treatments. Treatment times are indicated at the bottom of the figure. The samples were harvested at time points of 0.5 h, 1.5 h, 3 h, 24 h, 2 d, 3 d and 4 d. (A) Stem, (B) Root, (C) Leaf. * indicates significant differences in comparison with control (|log₂^f°^{ld change}| > 1 and p < 0.05).

Figure 3

Hierarchical clustering of gene expression by qPCR under IAA, GA₃, and BR treatments. Treatment times are indicated at the bottom of the figure. The samples were harvested at time points of 0.5 h, 1.5 h, 3 h, 24 h, 2 d, 3 d and 4 d. (A) Stem, (B) Root, (C) Leaf. * indicates significant differences in comparison with control (|log₂^f°^{ld change}| > 1 and p < 0.05).

Figure 4

Correlation network of genes in response to phytohormones. (A) IAA, (B) GA₃, (C) BR. Each node represents a cell wall biosynthesis-related gene (green), a transcription factor (red) or a signal transduction-related gene (yellow).