Accumulation of Anthocyanidins Determines Leaf Color of Liquidambar Formosana as Revealed by Transcriptome Sequencing and Metabolism Analysis

Liquidambar formosana is important for its ornamental value in China; it is increasingly used for landscaping and gardening trees due to its diverse leaf colors and seasonal changes. Varieties including either a fixed leaf color, the purplish ‘Fuluzifeng’ (ZF), or seasonal changes in leaf color, the reddish ‘Nanlinhong’ (NLH) have been bred and registered as new plant varieties under the International Union for the Protection of New Plant Varieties (UPOV) system. To gain practical insights into the anthocyanin biosynthetic process, transcriptome sequencing (Illumina) was performed to clarify the metabolic pathways present in the three seasonal changes in leaf colors in NLH and in the springtime purple-red color of ZF. qRT-PCR was used to verify the speculation. Based on the differentially expressed genes and flavonoids analyses, the spring, summer, and autumn leaves of NLH were compared to study the seasonal differences. NLH and ZF were compared to study the formation mechanism of the two leaf colors in spring. Transcriptome sequencing produced a total of 121,216 unigenes from all samples, where 48 unigenes were differentially expressed and associated with the anthocyanidin pathway. The expression levels of LfDFR and LfANS genes corresponded to the accumulation of concentrations of cyanidins in spring (NLHC) and autumn leaves (NLHQ), respectively, with different shades of red. Moreover, the LfF3′5′H gene corresponded to the accumulation of flavonols and delphinidins in purple-red leaves (ZFC). Cyanidin and peonidin were the key pigments in red and dark-red leaves, and purple-red leaves were co-pigmented by cyanidins, pelargonidins, and delphinidins.