There are much discussion about the relation between gravity-induced the reorientation of MTs and elongation rate of epidermal cells in roots and shoots.However,we still dont know whether gravity is intimately related with cell expansion through their presumed role in regulating orientation of cortical MTs in the cell.In the present study,we examined reorientation of microtubules and phenotype of cell wall in the upper and lower part of Arabidopsis hypocotyls under normal gravitational and simulated weightlessness conditions.