Nuclear male sterility induced by pollen-specific expression of a ribonuclease

The promoter of a rice pollen-specific gene, PS1, has been fused to the Bacillus amyloliquefaciens Barnase gene which encodes a secreted ribonuclease. The PS1-Barnase chimeric gene has been introduced into tobacco. These transgenic tobacco plants show normal vegetative and floral development, but they display a range of reproductive properties from slightly reduced in fertility to completely sterile. Barnase mRNAs are detectable in the pollen from transgenic plants which do not show an obvious fertility-related phenotype, and in a few plants which have a mildly reduced-fertile phenotype. However, transgenic plants with a severely reduced-fertile or sterile phenotype do not accumulate detectable amounts of Barnase mRNA in their pollen, and the quality of their RNA is poor, presumably because of extensive RNA degradation. Reciprocal crosses between these transgenic plants and wild-type controls showed that the reduced-fertile phenotype is associated only with the transgenic pollen. When used as the female parent, these PS1-Barnase transgenic plants are fully fertile. Anthers in the severely sterile transgenic plants develop normally, but the majority of their pollen grains have abnormal morphology and they fail to germinate. These results indicate that expression of a pollen-specific cytotoxic gene induces lethality in pollen and may lead to severely reduced male fertility.