Comprehensive transcriptome analyses of different Crocus flower tissues uncover genes involved in crocin biosynthesis

Abstract

The stigma of Crocus sativus is used in traditional Chinese medicine and has drawn attention as a rich source of crocin, a compound with a reported activity that counters various cancers, depression, and cardiovascular diseases. However, our knowledge of crocin biosynthesis in Crocus is still limited. To identify the genes that encode key enzymes responsible for crocin production, transcriptome analyses of Crocus stigma, petal, and stamen were performed. There were 109 136 unigenes in the three Crocus flower tissues: 10 862 unigenes were expressed explicitly in stigmas. A total of 469 and 335 down-regulated differentially expressed genes in comparison to stigmas were detected in stamens and petals, respectively. There were 290 down-regulated genes in both tissues. The gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes analysis revealed that most genes in the two tissues served a similar function. The results of the differences of gene expression among the three tissues showed that most of the carotenoid pathway genes in stigmas had a high expression which was a benefit for producing zeaxanthin, a substrate of crocin. Fourteen candidate genes were identified in Crocus from the co-down-regulated genes, and the genes were expressed explicitly in stigmas and were predicted to be involved in crocin biosynthesis. Overall, the expressions of genes in the crocin biosynthesis pathway in the three Crocus flower tissues had a positive relationship with their crocin content, and some new candidate genes related to crocin biosynthesis were identified. The results can help elucidate the crocin biosynthesis pathway in Crocus.