Identification of critical genes associated with lignin biosynthesis in radish (Raphanus sativus L.) by de novo transcriptome sequencing

Radish is an important root vegetable crop with high nutritional, economic, and medicinal value. Lignin is an important secondary metabolite possessing a great effect on plant growth and product quality. To date, lignin biosynthesis-related genes have been identified in some important plant species. However, little information on characterization of critical genes involved in plant lignin biosynthesis is available in radish. In this study, a total of 71,148 transcripts sequences were obtained from radish root, of which 66 assembled unigenes and ten candidate genes were identified to be involved in lignin monolignol biosynthesis. Full-length cDNA sequences of seven randomly selected genes were isolated and sequenced from radish root, and the assembled unigenes covered more than 80% of their corresponding cDNA sequences. Moreover, the lignin content gradually accumulated in leaf during the developmental stages, and it increased from pre-cortex to cortex splitting stage, followed by a decrease at thickening stage and then increased at mature stage in root. RT-qPCR analysis revealed that all these genes except RsF5H exhibited relatively low expression level in root at thickening stage. The expression profiles of Rs4CL5, RsCCoAOMT1, and RsCOMT genes were consistent with the changes of root lignin content, implying that these candidate genes may play important roles in lignin formation in radish root. These findings would provide valuable information for identification of lignin biosynthesis-related genes and facilitate dissection of molecular mechanism underlying lignin biosynthesis in radish and other root vegetable crops.