Uncovering the mechanism preliminarily of formation and development of taro corm in vitro by morphological physiology and transcriptomic analysis

•The parenchymal cells expand accompanied with starch increasing during corm expansion.•The content of IAA, ZR and ABA were positively correlated with taro corm expansion.•These genes involved in starch metabolism probably promote the expansion of corm. In vitro virus-free corms as propagules provide a feasible alternative to the convenient transport and the high survival rate of transplant for typical asexual crop of taro (Colocasia esculenta (L.) Schott). However, the mechanism of formation and expansion of in vitro corm remains unclear. In this study, in vitro corms were obtained from virus-free test-tube plantlet of taro in optimized induced medium (Murashige-Skoog medium + 2 mg L−1 6-Benzylaminopurine + 0.5 mg L−1 1-naphthalene acetic acid + 3% sucrose) (OIM), and then inoculated into the medium with Murashige-Skoog medium + 8% sucrose (MS8) for corm expansion. During this process, morphological observations by paraffin section, plant growth regulators (PGRs) content determination by enzyme-linked immunosorbent assay, and transcriptome assay by Illumina platform were carried out at days 0 (T0), 45 (T1), 90 (T2), and 135 (T3) under different corm expansion stages. The results showed that parenchymal cells enlarge continuously, accompanied with starch particles increase during corm expansion. Besides, three PGRs of indoleacetic acid (IAA), zeatin-riboside (ZR), and abscisic acid (ABA) were positively correlated with corm expansion, the contents of IAA and ABA remained at a high level. Moreover, the transcriptome results showed that plant hormone signal transduction and the starch and sucrose metabolism pathways are enriched significantly based on Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The genes (SBE2.2, GBSS, SUS4), related to starch biosynthesis were up-regulated, meanwhile, three genes of AMYs involved in starch metabolism were down-regulated, which probably promote starch accumulation and corms expansion. Eight genes from plant hormone signal transduction and the starch and sucrose metabolism pathway were selected randomly to validate the result of RNA-seq, and the RT-qPCR results were consistent with the RNA-seq data. These results will provide valuable information for uncovering the mechanism of in vitro corm expansion.