Microtubule reorganization during pollen development of rice (Oryza sativa L.)

Anthers of rice (Oryza sativa L.) at different stages of development were cryofixed, freeze-substituted, and embedded in methacrylate. Sections were then cut and immuno-labeled with anti-tubulin to localize microspore microtubules. Changes in microtubule distribution pattern were followed by confocal fluorescence microscopy. To facilitate description, pollen development has 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 which 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 tubules appears which eventually girdles the nucleus. (iii) At the generative-cell development stage (phases 13–20), after the first mitosis a generative cell 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 microspore. 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 generative cell becomes spherical. (iv) At the second-mitosis and sperm formation stage (phases 21–24) the mitotic division is symmetrical. Before division, the shape of the generative cell changes from spherical to spindle-shaped. After cell division two sperm cells form, although they do not detach from each 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.