Studying Secondary Growth and Bast Fiber Development: The Hemp Hypocotyl Peeks behind the Wall

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).

Transcriptomic profiling of hemp bast fibres at different developmental stages

“…Although the LM5 antibody recognizing β-1,4-galactans does not label hemp bast fibres 22, 63 , it may be possible that pectic galactans in hemp fibres are masked and therefore do not react with the LM5 antibody.…”

Plant ‘muscles’: fibers with a tertiary cell wall

“…The presence of RG‐I with side chains of β‐(1→4)‐galactan was detected in the tertiary cell walls of nonflax plants by biochemical characterization (Gorshkova et al ., ; Guedes et al ., ) and by immunocytochemistry (Bowling & Vaughn, ; Gritsch et al ., ; Guedes et al ., ). Immunolabeling with an antibody recognizing β‐1,4‐galactans is not always successful in the tertiary cell wall, as reported for phloem fibers of hemp (Blake et al ., ; Behr et al ., ). However, antibodies specific for the backbone of RG‐I and for β‐(1→4)‐galactosidase heavily bind tertiary cell walls of primary and secondary hemp fibers (Chernova et al ., ).…”

Development of Hemp Fibers: The Key Components of Hemp Plastic Composites

“…This makes it quite difficult to obtain intrusively growing fibers for analysis by methods of biochemistry or molecular biology. The important step to identify molecular players involved in various stages of hemp fiber development was performed by the analysis of the transcriptome in hypocotyls of different age [33] and in different parts of young stems [34], both of which may contain intrusively growing fibers. However, the analyzed samples contained complex mixture of tissues and the early stages of fiber development were not fully identified.…”