In plants, the abscisic acid (ABA) pathway is activated by drought conditions; this signals the guard cells to close the stomatal pores, which conserves water. Lemichez et al. show that the Arabidopsis guanosine triphosphatase AtRac1 is involved in reorganization of the actin cytoskeleton in guard cells in response to ABA. Transgenic plants expressing a dominant negative form of AtRac1 or a constitutively active form of AtRac1 exhibited stomatal closure in the absence of ABA in the former and an inability to decrease stomatal aperture in response to ABA in the latter. Comparison of AtRac1 activity in wild-type plants and mutant plants (abi1-1), which are defective in stomatal closure and actin reorganization response in the presence of ABA, showed that ABA normally inactivates AtRac1 and that this is blocked in the abi1-1 mutant. Finally, the defect in ABA signaling with regard to stomatal pore closure in the aba1-1 plants could be overcome by transgenic expression of the dominant negative AtRac1 mutant. These results show that, like the vertebrate Rac proteins, plant Rac homologs have similar functions--to regulate the actin cytoskeleton in response to external stimuli.
E. Lemichez, Y. Wu, J.-P. Sanchez, A. Mettouchi, J. Mathur, N.-H. Chua, Inactivation of AtRac1 by abscisic acid is essential for stomatal closure. Genes Dev. 15, 1808-1816 (2001). [Abstract] [Full Text]