How It All Begins: Molecular Players of the Early Graviresponse in the Non-elongating Part of Flax Stem

Plants have developed two major approaches to adjust their position in accordance with the direction of the gravity vector: differential cell growth at the opposing sides of elongating regions and complex processes in non-elongating stem parts, like reaction wood development. Gravistimulation of flax plants induces the formation of gravitropic curvature in non-elongating stem parts, largely associated with modifications in phloem and xylem fibers. To gain knowledge about key “triggers” and “forward players” that induce negative gravitropic reactions, transcriptome profiling of the phloem fibers and xylem tissues from pulling and opposite stem sides 1 and 8 hours after gravistimulation was carried out. As the first reaction, activation of the processes associated with RNA synthesis and protein folding in both tissues and both stem sides was detected; later, activation of kinases and transferases occurred. Among the major induced changes, transcriptomic data indicate rapid and substantial shifts in chloroplast metabolism in all analyzed tissues, including temporal activation of the branched-chain amino-acid pathway, adjustment of light-harvesting complexes, and jasmonic acid biosynthesis. Auxin transporter genes were activated only in the xylem, whereas other auxin-related genes were barely upregulated 1 hour after stem inclination in any analyzed sample. The asymmetric changes between stem sides included the sharp activation of ethylene-related genes in the phloem fibers of the opposite stem side, as well as tertiary cell wall deposition in both the phloem and xylem fibers of the pulling stem side during later stages of graviresponse. The obtained results provide informative insights into the graviresponse mechanisms.