Abstract text: Tertiary cell walls (TCWs) provide mechanical support and can generate tensile forces for plant movement. However, whether the regulatory networks governing constitutive and induced TCW formation are shared or distinct remains unclear. Using flax, where phloem fibers deposit TCWs constitutively while xylem fibers form TCWs only upon gravistimulation, we compared transcriptional programs underlying both modes. RNA-seq of isolated phloem fibers and xylem tissues from pulling and opposite stem sides at 1 and 8 hours post-gravistimulation revealed both fiber types deploy a shared TCW-related gene set (RRTs, RGGAT, GALS, FLAs, BGAL, COBL4). Mechanical stimuli trigger TCW deposition via rapid chloroplast metabolic shifts, branched-chain amino acid and jasmonate synthesis, implicating plastids as signaling hubs. Phloem fibers exhibit rapid asymmetric responses including ethylene pathway activation on the opposite side within 1 hour, while xylem tissues lag but show pronounced TCW gene upregulation on the pulling side by 8 hours. We identified common orthologous structural (listed above) and transcription factor genes (WRKY20, BLH2, LBD4) that are upregulated not only in both constitutive and induced TCW in flax but also during tension wood formation in poplar, suggesting a conserved regulatory module across species and tissue types.

With partial financial support: RSF #26-16-00294.