Microtubule reorganization during pollen development of rice (Oryza sativaL.)

Anthers of rice (Oryza sativa L.) at different stages of development were c ryofixed. freeze-substituted, and embedded in methacrylate. Sections were t hen cut and immuno-labeled with anti-tubulin to localize microspore microtu bules. Changes in microtubule distribution pattern were followed by confoca l fluorescence microscopy. To facilitate description, pollen development ha s been divided into four developmental stages (twenty-four phases). (i) The young-microspore stage (phases 1-5) is characterized by the formation of a vacuole. When the vacuole enlarges. the: nucleus moves to the periphery of the cell. Afterwards the nucleus migrates to a site opposite the germ pore . Between the germ pore and the nucleus there is a strand of cytoplasm whic h contains some microtubules that run in parallel to the pore-nucleus axis. (ii) At the first-mitosis stage (phases 6-12) a perinuclear band of tubule s appears which eventually girdles the nucleus. (iii) At the generative-cel l development stage (phases 13-20), after the first mitosis a generative ce ll forms at a site opposite the germ pore. It is initially lens-shaped and part of its wall is appressed closely to the plasma membrane of the microsp ore. The wall of the generative cell contains cellulose and callose. Later the generative cell detaches from the microspore wall and migrates into the cytoplasm of the vegetative cell. In the vegetative cytoplasm the generati ve cell becomes spherical. (iv) At the second-mitosis and sperm formation s tage (phases 21-24) the mitotic division is symmetrical. Before division, t he shape of the generative cell changes from spherical to spindle-shaped. A fter cell division two sperm cells form. although they do not detach from e ach other. Later, cytoplasmic processes containing microtubules develop at the two ends of the sperm cells. These long processes remain linked to each other to form a paired unit.