The involvement of a Wall Associated Kinase in the response to gravistimulation in poplar stems
Résumé
Plants are able to sense organ tilt and reorient their growth according to the gravity vector. In angiosperm woody species, the motor for the stem straightening in parts undergoing cambial growth is differential maturation between the two sides of the stem. The dynamics of the gravitropic response involved the sensing of the local inclination angle (gravisensing) as well as the sensing of the local curvature which progressively takes place in the straightening process (Bastien et al (2013). If one wants to specifically address the question of the gravisensing / graviresponse in plants undergoing secondary growth, the gravistimulation should not induce organ deformation. This is why we chose to tilt staked poplar trees in our experimental design. From this, one challenging research question is the identification of molecular actors specifically involved in gravisensing / graviresponse. Since it has been proposed that plant cells could sense gravity through the cytoskeleton-plasma membrane-cell wall continuum, wall associated kinases (WAK) with an extracellular part that can bind to pectins localized in the cell wall, appear as molecular candidates. Using in silico approaches, we showed that the WAK family in poplar is the largest characterized to date with 175 sequences. Gene expression was analyzed in various organs and tissues. Most WAKs were weakly expressed. Only 28 WAKs were expressed in the stem. One WAK is of particular interest showing differential expression after tilting of the stem. The protein was localized in young xylem and bark cells devoid of amyloplasts. These data suggest that a WAK could be involved in the sensing/response to gravistimulation in a manner independent of the displacement of amyloplats which is behind the starch-statolith hypothesis (Sack, 1997).