Revealing the full-length transcriptome of caucasian clover rhizome development

Background: Caucasian clover ( Trifolium ambiguum M. Bieb.) is a strongly rhizomatous, low-crowned perennial leguminous and ground-covering grass. The species may be used as an ornamental plant and is resistant to cold, arid temperatures and grazing due to a well-developed underground rhizome system and a strong clonal reproduction capacity. However, the posttranscriptional mechanism of the development of the rhizome system in caucasian clover has not been comprehensively studied. Additionally, a reference genome for this species has not yet been published, which limits further exploration of many important biological processes in this plant. Result: We adopted PacBio Sequencing and Illumina Sequencing to identify differentially expressed transcripts in five tissues taproot (T1), horizontal rhizome (T2), swelling of taproot (T3), rhizome bud (T4) and rhizome bud tip (T5) of the caucasian clover rhizome. In total, we obtained 19.82 GB clean data and 80,654 nonredundant transcripts were analysed. Additionally, we identified 78,209 open reading frames (ORFs), 65,227 coding sequences (CDSs), 58,276 simple sequence repeats (SSRs), 6,821 alternative splicing (AS) sites, 24,29 long noncoding RNAs (lncRNAs) and 4,501 putative transcription factors (TFs) from 64 different families. Compared with other tissues, T5 exhibited more differentially expressed genes, and co-upregulated genes in T5 are mainly annotated as involved in phenylpropanoid biosynthesis. We also identified betaine aldehyde dehydrogenase (BADH) as a highly expressed gene-specific to T5. A weighted gene co-expression network analysis (WGCNA) cluster analysis of transcription factors and physiological indicators were combined to reveal 11 candidate genes (MEgreen-GA3), three of which belong to the HB-KNOX family, that are up-regulated in T3. We analysed 276 differential transcripts involved in hormone signaling and transduction, and the largest number of transcripts are associated with the IAA signaling pathway, with significant up-regulation in T2 and T5. Conclusions: : Taken together, this study contributes to our understanding of gene expression across five different tissues and provides preliminary insight into rhizome growth and development in caucasian clover.