Physiological characteristics and gene regulation mechanism of juvenile leaves of Acer rubrum L. during leaf color transformation in spring
Acer rubrum L., a widely used ornamental colored-leaf tree species, has great utility in both residential and urban landscaping. However, unsuitable environmental conditions tend to reduce the intensity of color change, greatly reducing the ornamental value of this species. Here, we investigated the discoloration of A. rubrum leaves from red to green during maturation. We first quantified leaf-color change in the L*, a*, b* color space, and found the most noticeable difference in the a* value, which changed from positive (more red) to negative (more green). In green leaves, photosynthetic pigment content was four-fold greater than that in red leaves, and anthocyanin content was significantly lowed (a 78.33% decrease). Consistent with this, levels of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as non-photochemical quenching, were significantly higher in red leaves. The activity levels of phenylalanine ammonia lyase (PAL), an initial enzyme in the anthocyanin synthesis pathway, were significantly positively correlated with anthocyanin accumulation. In contrast, polyphenol oxidase (PPO) enzyme activity was not correlated with any with other indicators. Transcriptome sequencing identified 2,161 differentially expressed genes (DEGs) between the red leaves and the green leaves (1,253 upregulated). Some of these DEGs (e.g., 4-coumarate: coenzyme A ligase (4CL), anthocyanidin synthase (ANS), phenylalanine ammonia lyase (PAL), flavonol synthase (FLS), chalcone synthase (CHS), dihydroflavonol 4-reductase (DFR), and flavanone 3-hydroxylase (F3H)) encoded important enzymes in the anthocyanin metabolic pathway, while others (e.g., MYB111 (EZV62_000212), MYB12 (EZV62_010323), and bHLH3 (EZV62_023045)) regulated anthocyanin accumulation. Our results have led to a clearer understanding of the physiological and genetic mechanisms underlying leaf-color change in A. rubrum, and provide a basis from which to improve the ornamental properties of colored-leaf tree species.