High-throughput RNA sequencing was performed for comprehensively analyzing the transcriptome of the purple sweet potato. A total of 58,800 unigenes were obtained and ranged from 200 nt to 10,380 nt with an average length of 476 nt. The average expression of one unigene was 34 reads per kb per million reads (RPKM) with a maximum expression of 1,935 RPKM. At least 40,280 (68.5%) unigenes were identified to be protein-coding genes, in which 11,978 and 5,184 genes were homologous to Arabidopsis and rice proteins, respectively. Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) analysis showed that 19,707 (33.5%) unigenes were classified to 1,807 terms of GO including molecular functions, biological processes, and cellular components and 9,970 (17.0%) unigenes were enriched to 11,119 KEGG pathways. We found that at least 3,553 genes may be involved in the biosynthesis pathways of starch, alkaloids, anthocyanin pigments, and vitamins. Additionally, 851 potential simple sequence repeats (SSRs) were identified in all unigenes. Transcriptome sequencing on tuberous roots of the sweet potato yielded substantial transcriptional sequences and potentially useful SSR markers which provide an important data source for sweet potato research. Comparison of two RNA-sequence datasets from the purple and the yellow sweet potato showed that UDP-glucose-flavonoid 3-O-glucosyltransferase was one of the key enzymes in the pathway of anthocyanin biosynthesis and that anthocyanin-3-glucoside might be one of the major components for anthocyanin pigments in the purple sweet potato. This study contributes to the molecular mechanisms of sweet potato development and metabolism and therefore that increases the potential utilization of the sweet potato in food nutrition and pharmacy.
BackgroundSiraitia grosvenorii (Luohanguo) is an herbaceous perennial plant native to southern China and most prevalent in Guilin city. Its fruit contains a sweet, fleshy, edible pulp that is widely used in traditional Chinese medicine. The major bioactive constituents in the fruit extract are the cucurbitane-type triterpene saponins known as mogrosides. Among them, mogroside V is nearly 300 times sweeter than sucrose. However, little is known about mogrosides biosynthesis in S. grosvenorii, especially the late steps of the pathway.ResultsIn this study, a cDNA library generated from of equal amount of RNA taken from S. grosvenorii fruit at 50 days after flowering (DAF) and 70 DAF were sequenced using Illumina/Solexa platform. More than 48,755,516 high-quality reads from a cDNA library were generated that was assembled into 43,891 unigenes. De novo assembly and gap-filling generated 43,891 unigenes with an average sequence length of 668 base pairs. A total of 26,308 (59.9%) unique sequences were annotated and 11,476 of the unique sequences were assigned to specific metabolic pathways by the Kyoto Encyclopedia of Genes and Genomes. cDNA sequences for all of the known enzymes involved in mogrosides backbone synthesis were identified from our library. Additionally, a total of eighty-five cytochrome P450 (CYP450) and ninety UDP-glucosyltransferase (UDPG) unigenes were identified, some of which appear to encode enzymes responsible for the conversion of the mogroside backbone into the various mogrosides. Digital gene expression profile (DGE) analysis using Solexa sequencing was performed on three important stages of fruit development, and based on their expression pattern, seven CYP450s and five UDPGs were selected as the candidates most likely to be involved in mogrosides biosynthesis.ConclusionA combination of RNA-seq and DGE analysis based on the next generation sequencing technology was shown to be a powerful method for identifying candidate genes encoding enzymes responsible for the biosynthesis of novel secondary metabolites in a non-model plant. Seven CYP450s and five UDPGs were selected as potential candidates involved in mogrosides biosynthesis. The transcriptome data from this study provides an important resource for understanding the formation of major bioactive constituents in the fruit extract from S. grosvenorii.
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