Overproduced ethylene causes programmed cell death leading to temperature-sensitive lethality in hybrid seedlings from the cross Nicotiana suaveolens x N-tabacum

Reproductive isolation mechanisms (RIMs) often become obstacles in crossbreeding. Hybrid lethality is a subtype of RIM but its physiological mechanism remains poorly elucidated. Interspecific hybrids of Nicotiana suaveolens Lehm. x N. tabacum L. cv. Hicks-2 expressed temperature-sensitive lethality. This lethality was induced by programmed cell death (PCD) that was accompanied by the characteristic changes of animal apoptosis in hybrid seedlings at 28 degreesC but not at 36 degreesC. When hybrid seedlings were cultured at 28 degreesC, DNA fragmentation started in the cotyledon, and nuclear fragmentation subsequently progressed with lethal symptoms spreading throughout the seedlings. At 28 degreesC, ethylene production in hybrid seedlings was detectable at a high level compared with the level in parental seedlings. In contrast, the ethylene production rate in hybrid seedlings cultured at 36 degreesC was equal to that in parental seedlings. Treatment with ethylene biosynthetic inhibitors, amino-oxyacetic acid and amino-ethoxyvinyl glycine, suppressed lethal symptoms and apoptotic changes, and also prolonged survival of hybrid seedlings. Thus, the increase in the ethylene production rate correlated closely with expression of lethal symptoms and apoptotic changes in hybrid seedlings. From these observations, we conclude that overproduced ethylene acts as an essential factor mediating PCD and subsequent lethality in hybrid seedlings. Furthermore, the present study has provided the first evidence that ethylene is involved in the phenomenon of hybrid lethality.