Abstract
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. 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 comprehensive overview 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).
Highlights
Hemp (Cannabis sativa L.) is a herbaceous plant native to Central Asia known for its medicinal and textile applications for over 10,000 years
In order to study the development of the secondary tissues in hemp, hypocotyls aged between 6 (H6) and 20 days (H20) were studied
In H6 the cortical parenchyma constitutes the bulk of the section, with collenchyma acting as supporting tissue and medullar parenchyma occupying a limited area (Figure 1A)
Summary
Hemp (Cannabis sativa L.) is a herbaceous plant native to Central Asia known for its medicinal and textile applications for over 10,000 years. The woody fibers of industrial hemp are used to manufacture a concrete-like material known as “hempcrete,” which is very light in weight, while the cellulosic fibers find application in the biocomposite sector for the creation of bioplastics. The stem of C. sativa provides two types of fibers, i.e., the xylem and the phloem fibers. The latter are known as bast fibers. The cell walls of xylem fibers are impregnated with lignin to ensure strength and resistance to negative sap pressure (Wang et al, 2013), while those of the bast fibers are mainly made of crystalline cellulose (and can account for up to 75–80% of the dry mass; Guerriero et al, 2013). The other components of bast fiber cell walls are hemicelluloses (4%, including xyloglucan in the PCW and xylan in the SCW), pectins (4%), proteins (3%), lignin (2%) and traces of phenolic acids (
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