LOCALIZATION OF A RHO GTPASE IMPLIES A ROLE IN TIP GROWTH AND MOVEMENT OF THE GENERATIVE CELL IN POLLEN TUBES

The Rho family GTPases function as key molecular switches, controlling a variety of actin-dependent cellular processes, such as the establis hment of cell polarity, cell morphogenesis, and movement in diverse eu karyotic organisms. A novel subfamily of Rho GTPases, Rep, has been id entified in plants, Protein gel blot and RNA gel blot hybridization an alyses indicated that one of these plant Rho GTPases, Rop1, is express ed predominantly in the male gametophyte (pollen and pollen tubes), Ce ll fractionation analysis of pollen tubes showed that Rop is partition ed into soluble and particulate fractions, The particulate Rop could b e solubilized with detergents but not with salts, indicating that it i s tightly bound to membranes, The membrane association appears to resu lt from membrane anchoring via a geranylgeranyl group because an in vi tro isoprenylation assay demonstrated that Rop1Ps is geranylgeranylate d. Subcellular localization, using indirect immunofluorescence and con focal microscopy, showed that Rop is highly concentrated in the cortic al region of the tube apex and in the periphery of the generative cell . The cortical Rop protein at the apex forms a gradient with decreasin g concentration from tip to base and appears to be associated with the plasma membrane. These results suggest that the apical Rop GTPase may be involved in the signaling mechanism that controls the actin-depend ent tip growth of pollen tubes, Localization of the Rop GTPase to the periphery of the generative cell is analogous to that of myosin, sugge sting that the Rop GTPase plays an important role in the modulation of an actomyosin motor system involved in the movement of the generative cell.