Roots of land plants apply both gravitropism and hydrotropism in response to gravity and moisture gradients, respectively, in controlling their growth orientation. Both responses enable efficient water acquisition by plants. In the gravitropic response of seedling roots, columella cells in the root cap perceive gravity and re-localize the auxin-efflux carrier PIN3 so that the plant hormone auxin can move downward to the lateral root cap. Then, PIN2 plays a role in transporting the auxin from the lateral root cap to the elongation zone along the lower side of the re-oriented roots. Thus, auxin is redistributed with a greater accumulation on the bottom side, leading differential growth to bend downwards. In hydrotropism, however, the root cap is dispensable; as such, instead of auxin abscisic acid (ABA) and MIZ1 play positive roles in inducing a hydrotropic response of Arabidopsis roots. It was found that both MIZ1 and ABA function in the cortex of the elongation zone for the induction of hydrotropism. In addition, the overexpression of MIZ1 results in the enhancement of the hydrotropic response, which in turn results in a greater tolerance to drought stress. These results imply that a molecular mechanism unique to hydrotropism exists, and that the manipulation of hydrotropic ability is useful for improving plant production under various conditions.