Gravitropic curvature growth of Arabidopsis hypocotyls mainly occurred in the rapid growing Elongation Zone (EZⅠ),not in the slow-growing Elongation Zone (EZⅡ).By examining reorientation of Microtubules (MT) and phenotype of the cell wall in the EZⅠ and the EZⅡ of Arabidopsis hypocotyls under normal gravitational condition,it is found that MTs in the rapid growing epidermal cells were mainly in the transverse direction,while those in the non-growing epidermal cells were in the longitudinal directions.However,this difference in cortical MT arrays between the EZⅠ and EZⅡ cells disappeared when the seedlings were exposed to the simulated microgravity condition on a horizontal clinostat.Field emission scanning electron microscopy revealed that the surface texture of epidermal cells,like the direction of the MT,in the EZⅠ and the EZⅡ also became similar when exposed to the simulated microgravity condition.This result indicated that simulate microgravity could modify the potential differentiation between the EZⅠ and the EZⅡ by affecting the orientation of cortical MT in the epidermal cells.