The relationship between organelle distribution and the cytoskeleton was examined during microsporogenesis in Lilium longiflorum. The distribution pattern of plastid and mitochondrial nucleoids was followed by fluorescence microscopy after staining with 4′,6-diamidino-2-phenylindole (DAPI). Although the plastid nucleoids gradually enlarged during prophase I, by anaphase I of meiosis they were randomly distributed in the cytoplasm of each microsporocyte. At telophase I the plastid nucleoids were aggregated in the equatorial region of the cell. After entering prophase II the plastid nucleoids were randomly distributed in the cytoplasm, and at telophase II they had reaggregated to the equators of the two cells. After the completion of meiosis they were located at the two poles of each young microspore. This distinct cell polarity of plastid nucleoids was preserved in isolated protoplasts. In all cells where the distribution of plastid nucleoids was non-random, the nucleoids were invariably situated furthest away from the interphase and telophase nuclei. However, the distribution of mitochondrial nucleoids throughout meiotic division showed little cell polarity.
Analysis of the microtubule and actin cytoskeletons during microsporogenesis revealed that the microtubules radiated out from the cell nuclei only at the stages when the distribution of plastids showed polarity, whereas the actin filaments were usually randomly oriented throughout the cytoplasm, independent of the plastid arrangement and of the organization of microtubule cytoskeleton. The radiating microtubules seemed to exclude the plastids from around the cell nuclei. Treatment of cultured pollen tetrads with colchicine disrupted the plastid polarity, probably by depolymerizing the radiating microtubules, resulting in a random distribution of the plastid nucleoids. Treatment with cytochalasin B, however, had no effect on the arrangement of plastids.
These results demonstrate that microtubules function in the movement and distribution of plastids in male reproductive cells of higher plants. Further, it is assumed that the system of radiating microtubules that controls the distribution of plastids during male meiosis is also involved in the subsequent formation of male gametes, which are deficient in plastids in many angiosperm plants, including this lily.
