Cellular observation was completed via paraffin sectioning, analysis of the transcriptomic profile via next?generation sequencing technology, and visualization, as well as enrichment analysis of differentially expressed genes, via MapMan.

Through the morphological analysis of P. japonica’s rhizome elongation, we discovered that the elongation was directly promoted by the growth of -14 young internodes in the rhizome shoot. The anatomical analysis results further revealed that internodes with lengths lower than 0.4 cm had strong cell division ability, and distinctive cell elongation was found in a 1.0 cm long internode. Long and short parenchymal cells were apparent in the 1.0 cm internode and its vascular tissue such as vessel cells and fiber cells were much longer and more developed than in the 0.4 cm internode. It was also observed that pith tissue broke to form the pith cavity in the 1.0 cm internode. The RNA?Seq analysis between 0.4 and 1.0 cm internodes revealed a transition of transcriptomic activities. This included a transition from cell division and basic cellular metabolism in the 0.4 cm internode to active cell growth processes such as cell wall biosynthesis and secondary metabolism, and in the 1.0 cm internode to Programmed Cell Death processes such as ethylene signal pathways and (reactive oxygen species, ROS) burst genes.

There is a transition of cell growth processes from cell division to cell elongation during P. japonica’s rhizome internode elongation. Additionally, developmental events such as pith cavity formation exist in the 1.0 cm internode, which might be due to upregulation of genes related to cell growth and Programmed Cell Death.