Plant Physiol, December 2002, Vol. 130, pp. 1776-1787

Cell Death Processes during Expression of Hybrid Lethality in Interspecific F₁ Hybrid between Nicotiana gossei Domin and Nicotiana tabacum

Faculty of Agriculture, Kyoto Prefectural University, 1-5 Hangi-cho, Shimogamo, Sakyo-ku, Kyoto 606-8255 Japan (M.M., K.M., H.Y., M.I.); and Research Institute for Biological Sciences Okayama, Kayo-cho, Okayama 716-1241 Japan (K.O.)

Hybrid lethality, a type of reproductive isolation, is a genetically controlled event appearing at the seedling stage in interspecific hybrids. We characterized the lethality of F₁ hybrid seedlings from Nicotiana gossei Domin and Nicotiana tabacum cv Bright-Yellow 4 using a number of traits including growth rate, microscopic features of tissues and cells, ion leakage, DNA degradation, reactive oxygen intermediates including superoxide radical (O₂) and hydrogen peroxide (H₂O₂), and expression of stress response marker genes. Lethal symptoms appeared at 4 d after germination in the basal hypocotyl and extended toward both the hypocotyl and root of the plants grown at 26°C. Microscopic analysis revealed a prompt lysis of cell components during cell death. Membrane disruption and DNA degradation were found in the advanced stage of the lethality. The death of mesophyll cells in the cotyledon was initiated by the vascular bundle, suggesting that a putative factor inducing cell death diffused into surrounding cells from the vascular tissue. In contrast, these symptoms were not observed in the plants grown at 37°C. Seedlings grown at 26°C generated larger amounts of reactive oxygen intermediate in the hypocotyl than those grown at 37°C. A number of stress response marker genes were expressed at 26°C but not at 37°C. We proposed that a putative death factor moving systemically through the vascular system induced a prompt and successive lysis of the cytoplasm of cells and that massive cell death eventually led to the loss of the hybrid plant.