Long read reference genome-free reconstruction of a full-length transcriptome from Astragalus membranaceus reveals transcript variants involved in bioactive compound biosynthesis

Astragalus membranaceus, also known as Huangqi in China, is one of the most widely used medicinal herbs in Traditional Chinese Medicine. Traditional Chinese Medicine formulations from Astragalus membranaceus have been used to treat a wide range of illnesses, such as cardiovascular disease, type 2 diabetes, nephritis and cancers. Pharmacological studies have shown that immunomodulating, anti-hyperglycemic, anti-inflammatory, antioxidant and antiviral activities exist in the extract of Astragalus membranaceus. Therefore, characterising the biosynthesis of bioactive compounds in Astragalus membranaceus, such as Astragalosides, Calycosin and Calycosin-7-O-β-d-glucoside, is of particular importance for further genetic studies of Astragalus membranaceus. In this study, we reconstructed the Astragalus membranaceus full-length transcriptomes from leaf and root tissues using PacBio Iso-Seq long reads. We identified 27 975 and 22 343 full-length unique transcript models in each tissue respectively. Compared with previous studies that used short read sequencing, our reconstructed transcripts are longer, and are more likely to be full-length and include numerous transcript variants. Moreover, we also re-characterised and identified potential transcript variants of genes involved in Astragalosides, Calycosin and Calycosin-7-O-β-d-glucoside biosynthesis. In conclusion, our study provides a practical pipeline to characterise the full-length transcriptome for species without a reference genome and a useful genomic resource for exploring the biosynthesis of active compounds in Astragalus membranaceus.