The localization of Rac GTPase in Picea willsonii pollen tubes implies roles in tube growth and the movement of the tube nucleus and sperm cells

The Rac/Rop small GTPases are central regulators of important cellular processes in plants. But their roles in pollen and pollen tube development in plants are still not well understood, especially in gymnosperms. In the present paper, a 23-kDa polypeptide was detected in pollen tube plasma membrane and cytosolic fractions in Picea willsonii pollen tubes using the antisera against the NtRac1 protein and Arabidopsis Rops. The Rac1 protein appears to be associated with the plasma membrane based on detergent experiments. Subcellular localization using indirect immunofluorescence and confocal microscopy showed that the detected Rac1 is concentrated in the cortical region of the tube apex, especially in the growing fast tubes, and that little is present in arrested tubes. Interestingly, a Rac/Rop protein was observed in the tube nucleus and sperm cells of P. willsonii pollen tubes, and almost little exist while these cells inside pollen grains. These results suggest that Rac/Rop GTPases are involved in the signaling mechanism that controls pollen tube growth and might point to a potential function in signaling to the migration of the tube nucleus and sperm cells in pollen tubes in P. willsonii.