Nasrallah2 Section of Plant Biology, Division of Biological Sciences, Cornell University, Ithaca, New York 14853 A chimeric toxic gene consisting of the diphtheria toxin A chain gene fused to a promoter previously shown to direct pistil- and anther-specific expression was used to genetically target cell killing in transgenic Arabidopsis. Flowers of Arabidopsis transformants that carried the toxic gene fusion had distinct structural defects. The papillar cells at the stigma surface were stunted and were biosynthetically inactive. Anther development was also impaired by toxic gene expres- sion, leading to abnormalities in anther dehiscence, pollen morphology, and pollen germination. The combined defects of pistil and anther rendered transformants that carried the toxic gene fusion self-sterile. However, the transformants were cross-fertile with untransformed plants: the viable pollen of ablated plants was rescued by wild-type stigmas, and, strikingly, the ablated papillar cells allowed the growth of wild-type pollen. INTRODUCTION Genetic ablation methodology, in which a toxic gene is placed under the control of a tightly regulated promoter, is a powerful tool for the analysis of developmental processes and the role specific cell types play in complex tissues. The genetically tar- geted death of specific cells has been achieved by this approach in mammalian systems (Breitman et al., 1987; Palmiter et al., 1987; Behringer et al., 1988) and plants (Mariani et al., 1990, 1992; Thorsness et al., 1991). In one application of this method, we directed expression of the diphtheria toxin A chain (DT-A), a potent inhibitor of translation (reviewed in Colher, 1977), from the promoter of the S Locus Glycoprotein (SLG) gene, a gene derived from the self-incompatibility locus of Brassica. The SLG promoter is active exclusively in specific tissues of the pistil and anther (Nasrallah et