Figure 5.

Schematic model showing the key processes occurring at each stem internode/tissue analysed. The cartoon on the right hand‐side refers to a longitudinal view of intrusively growing bast fibres. Young internodes in intrusive growth (TOP/MID) are characterized by cell wall expansion and jasmonic acid (JA) biosynthesis, probably due to the signal perceived by neighboring parenchyma cells (PC). An active auxin metabolism is also observed, with peroxisomal β‐oxidation processes linked to the conversion IAA to IBA. ROS are thus produced, whose effects are mitigated by flavonoids. Flavonoid biosynthetic genes are indeed highly expressed in young internodes and flavonoids are high in amount at the MID. The core tissues at the BOT have a predominance of ontologies related to secondary cell wall formation and xylan/cellulose/lignin deposition. A gene related to thermospermine biosynthesis is also highly expressed in the core, where it controls xylem differentiation. In the fibre‐rich tissues at the BOT, ontologies related to the C starvation syndrome are evident, because of the sucrose demand and UDP‐glucose need for bast fibre thickening. Senescence processes, known to be controlled by salicylic acid (SA), are also observed