A model system to study secondary growth and bast fibre development: the hemp hypocotyl peeks behind the wall

Cannabis sativa L. is an annual herbaceous crop grown for the production of long extraxylary fibers, the bast fibers, rich in cellulose and used both in the textile and biocomposite sectors. Despite being herbaceous, hemp undergoes secondary growth and this is well exemplified by the hypocotyl. The hypocotyl was already shown to be a suitable model to study secondary growth in other herbaceous species, namely Arabidopsis thaliana and it shows an important practical advantage, i.e. elongation and radial thickening are temporally separated. This study focuses on the mechanisms marking the transition from primary to secondary growth in the hemp hypocotyl by analysing the suite of events accompanying vascular tissue and bast fiber development. RNA-Seq transcriptomics, imaging and quantification of phytohormones were carried out on four representative developmental stages (i.e. 6-9-15-20 days after sowing) to provide a first comprehensive profiling of the events associated with primary and secondary growth in hemp. This multidisciplinary approach provides cell wall-related snapshots of the growing hemp hypocotyl and identifies marker genes associated with the young (expansins, β-galactosidases and transcription factors involved in light-related processes) and the older hypocotyl (secondary cell wall biosynthetic genes and transcription factors). Time-course analysis of hemp hypocotyl transcriptome (6, 9, 15 and 20 days after sowing) with 3 biological replicates for each time point.