Transcriptome profiling provides insights into the fruit color development of wild Lycium ruthenicum Murr. from Qinghai-Tibet Plateau

Lycium ruthenicum Murr. is an important ecological and economic species in the Qaidam Basin of Qinghai-Tibet Plateau. Its black fruits (BF) are rich in anthocyanins, which have health-promoting properties for humans and thus provide nutritional benefits for this plant. Although the fruit quality of natural white fruit (WF) is affected by the disappearance of pigmentation in phenotypes, this phenomenon provides an opportunity to unravel the complex color metabolic networks. In this study, anthocyanin profiling confirmed that WF was formed due to anthocyanin loss. Transcriptome analysis of BF and WF revealed 101,466 unigenes, 261 of which were identified as the putative homologs of color-related genes in other species. Genes encoding the enzymes involved in flavonoid biosynthesis were also identified systematically. The structural gene expression levels of chalcone synthase (CHS), chalcone isomerase (CHI), flavonoid 3'5'-hydroxylase (F3'5'H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), and anthocyanidin 3-O-glucosyltransferase (UFGT) were highly similar and significantly positively correlated with anthocyanin accumulation rate in BF. In particular, F3'5'H, UFGT, ANS, and DFR expression levels in BF were 2391, 119, 96, and 85 times higher than those in WF at S3 (35 days after anthesis), respectively. This result strongly suggests that the low expression of these genes in WF is responsible for the anthocyanin loss. Meanwhile, the expression patterns of the anthocyanin regulatory genes were also investigated by qRT-PCR. Mass sequencing data were obtained and annotated by deep sequencing and provided a platform for future function and molecular biological research on L. ruthenicum Murr.