Fig. 3.

The tip growth system underlies the growth guidance of pollen tubes. a Illustration of the signaling part of the guidance model. The external guidance signal promotes ROP1 activation through the positive feedback. A signal gradient leads to an asymmetric distribution of active ROP1 and exocytosis. By regulating the cell wall mechanical properties, asymmetric exocytosis results in the change of growth direction. b Observing the growth guidance of Arabidopsis pollen tubes using the semi-in vitro assay with AtLURE1-containing gelatin beads. Arrowhead indicates the tip of the pollen tube being observed. Numbers are time after the beads were placed nearby the tip of the pollen tube. Scale bar: 50 μm. c, d Simulation and observation of active ROP1 redistribution prior to the turning of a pollen tube during growth guidance. c Simulation of a pollen tube when exposed to a gradient of guidance signal. Color of the cell surface and the background indicates the level of active ROP1 and the concentration of guidance signal, respectively. Scale bar: 5 μm. d Confocal microscopy images showing the turning of a pollen tube expressing CRIB4-GFP attracted by an AtLURE1-containing gelatin bead (upper left to the pollen tube). The images are representative of ten samples. Numbers show time (min:s). Scale bar: 5 μm