DEREPRESSION OF THE CELL-CYCLE BY STARVATION IS INVOLVED IN THE INDUCTION OF TOBACCO POLLEN EMBRYOGENESIS

Microspectrophotometry following Feulgen staining and autoradiography following (H-3)-thymidine labelling were used to study cell-cycle events during pollen development in tobacco (Nicotiana tabacum L.). During normal gametophytic pollen development in the anther and in vitro the generative nucleus passes through the S phase to the G2 phase soon after microspore mitosis, while the vegetative nucleus remains arrested in G1 (=G0). During embryogenic induction by an in vitro starvation treatment of immature pollen ongoing DNA replication in the generative nucleus is completed and followed by DNA replication in the vegetative cell in a large fraction of the pollen grains. Addition of the DNA replication inhibitor hydroxyurea to the starvation medium postpones S phase entry until the pollen is transferred to a rich medium and does not affect embryo formation. These results demonstrate that one of the crucial events of embryogenic induction is the derepression of the G1 arrest in the cell cycle of the vegetative cell.