Non-random Nuclear Orientation during Dna Synthesis in Tradescantia Pollen Grains

A polarized division occurs normally in the development of microspores in plants. The result is a presumptive vegetative cell and generative cell within the young pollen grain. The generative nucleus in Tradescantia synthesizes DNA and considerably later, in the pollen tube, it divides to form two sperm nuclei. The vegetative nucleus probably degenerates in the pollen tube. In Tradescantia, it was long ago suggested by Sax (5) that the ability of one daughter cell to develop as the generative cell depended upon its placement in the pollen grain. According to Sax, normal development required the formation of vacuoles which positioned the division spindle (and consequently the daughter nuclei) perpendicular to the long axis of the microspore. Thus the fate of the two cells was determined actually before the division which produced them. This hypothesis has never been tested directly, but some evidence which will be given below argues against it. In any case, while the placement in the pollen grain may determine whether the two daughter cells develop normally or not, it is of interest to know if any interaction occurs between the two cells after the division. Woodard (8) noted a sudden change in nucleoprotein metabolism in both cells about 40 hours after the microspore division. This may be a consequence of the loss of the wall between the two cells, or its reduction in thickness. The original description of Tradescantia pollen grain development by Sax and Edmonds (6) noted that the wall was transitory; recent electron micrographs by Gay and Murayama (reference 2, and personal communication) show that the wall undergoes a progressive thinning, finally becoming about 300 A thick. The result of in vitro labeling of young Tradescantia pollen grains in short term incubations with radioactive thymidine (7) have revealed an unexpected correlation between the young generative cell and the vegetative cell. The vegetative cell appears to respond to DNA synthesis in the generative nucleus by an increased mobility of its nucleus in the cytoplasm. This is discussed in the present report.