Transcriptomic and metabolomic analyses reveal how girdling promotes leaf color expression in Acer rubrum L

Abstract

Background: Acer rubrum L. (red maple) is a popular tree with attractive colored leaves, strong physiological adaptability, and a high ornamental value. Changes in leaf color can be an adaptive response to changes in environmental factors, and also a stress response to external disturbances. In this study, we evaluated the effect of girdling on the color expression of A. rubrum leaves. We studied the phenotypic characteristics, physiological and biochemical characteristics, and the transcriptomic and metabolomic profiles of leaves on girdled and non-girdled branches of A. rubrum.

Results: Phenotypic studies showed that girdling resulted in earlier formation of red leaves, and a more intense red color in the leaves. Compared with the control branches, the girdled branches produced leaves with significantly different color parameters a*. Physiological and biochemical studies showed that girdling of branches resulted in uneven accumulation of chlorophyll, carotenoids, anthocyanins, and other pigments in leaves above the band. In the transcriptomic and metabolomic analyses, 28,432 unigenes including 1095 up-regulated genes and 708 down-regulated genes were identified, and the differentially expressed genes were mapped to various KEGG (kyoto encyclopedia of genes and genomes) pathways. Six genes encoding key transcription factors related to anthocyanin metabolism were among differentially expressed genes between leaves on girdled and non-girdled branches.

Conclusions: Girdling significantly affected the growth and photosynthesis of red maple, and affected the metabolic pathways, biosynthesis of secondary metabolites, and carbon metabolisms in the leaves. This resulted in pigment accumulation in the leaves above the girdling site, leading to marked red color expression in those leaves. A transcriptome analysis revealed six genes encoding anthocyanin-related transcription factors that were up-regulated in the leaves above the girdling site. These transcription factors are known to be involved in the regulation of phenylpropanoid biosynthesis, anthocyanin biosynthesis, and flavonoid biosynthesis. These results suggest that leaf reddening is a complex environmental adaptation strategy to maintain normal metabolism in response to environmental changes. Overall, the results of these comprehensive phenotype, physiological, biochemical, transcriptomic, and metabolomic analyses provide a deeper and more reliable understanding of the coevolution of red maple leaves in response to environmental changes.

Keywords: Acer rubrum L.; Anthocyanin; Chlorophyll; Girdling; Leaf coloration; Metabolomics; Physiological; Transcriptome.

Fig. 1

A, B, C, D show the repeated experiments, which the change of foliar color between the treated bunches and control branches. Based on the selection, a branch from the tree was ligated tightly with plastic strip in middle part of the branch and tagged on each tree. E shows the variation of parameters L, a, b. F shows the difference in the leaf anthocyanin between the treated bunches and control branches. (US represents the upper stem; LS represents the lower stem; CB represents the control branch)

Fig. 2

Soluble sugar and flavonoid concentrations of leaves collected from upper stem, lower stem and control branch. Standard error bars represent ± 1 SE. Different letters (a and b) behind the data indicate significant (P < 0.05) differences between leaf positions. (US represents the upper stem; LS represents the lower stem; CB represents the control branch)

Fig. 3

Statistical analysis of all differentially expressed genes (DEGs) Volcano plot of DEGs

Fig. 4

KEGG pathway annotation of Acer rubrum L. transcripts. The vertical axis shows the annotations of the KEGG metabolic pathways. The horizontal axis represents the gene numbers annotated in each pathway

Fig. 5

Statistical analysis of KEGG pathway enrichment of up-regulated DEGs. Each circle represents a KEGG pathway, the ordinate represents the pathway name, and the abscissa is the enrichment factor. The larger the enrichment factor is, the greater the degree of enrichment is. The circle color represents q-value, the smaller the q-value is, the more reliable the enrichment significance is. The size of circle indicates the number of genes enriched in the pathway, and the larger the circle, the more abundant the genes is

Fig. 6

Go enrichment histogram of differential genes. The horizontal axis shows the ratio of the genes annotated to the total number of genes annotated. The ordinate represents the name of the Go entry. The label to the right of the figure represents the category to which the Go entry belongs

Fig. 7

KOG classification of Acer rubrum L. transcripts. The capital letters on the horizontal axis indicate the KOG categories,which are explained below the histogram, and those on the vertical axis indicate the number of genes

Fig. 8

OPLS-DA S-plot (The abscissa represents the covariance between principal components and metabolites, and the ordinate represents the correlation coefficient between principal components and metabolites. The metabolites closer to the upper right corner and lower left corner in the figure indicate more significant differences. Red dots indicate that the VIP of these metabolites is ≥ 1, while green dots indicate that the VIP of these metabolites is < 1.)