The initiation and organization of microtubules in germinating pear (Pyrus communis L.) pollen.

To study microtubule organization in germinating pear (Pyrus communis L., cv., Bartlett) pollen, we removed the pollen wall by freeze-fracturing before treating the resultant pollen protoplasts by conventional immunofluorescence procedures. Results reveal that axial bundles of microtubules are present in the generative cell of both inactivated and activated pollen grains. Microtubules are not present in the vegetative cells of inactivated pollen, but they are present in the vegetative cells of activated pollen grains. Microtubule nucleation occurs in the vegetative cell cortex. Subsequently, the microtubules grow as branching arrays through most of the vegetative cell cortex except at the apertures where they form localized converging or criss-cross patterns. Eventually, in a germinated pollen grain, the microtubules form network-like arrays through most of the pollen grain and a collar of short arrays at the base of the pollen tube. It is suggested that the role of vegetative cell microtubules in pollen germination is indirect through their mediation of the conformational changes in actin organization that are essential for pollen germination.