The S-ribonuclease gene of Petunia hybrida is expressed in nonstylar tissue, including immature anthers.

To determine the ability of isolated S-locus promoter sequences to direct organ-specific gene expression, we used microprojectile bombardment to introduce chimeric S-allele/beta-glucuronidase genes into different tissues of Petunia hybrida for transient expression. Histochemical staining showed that S-locus/beta-glucuronidase fusions were expressed in pistil, ovary, and petal tissue. No expression of the chimeric genes was detected in leaves or in mature pollen, either by histochemical staining or by fluorescence assays. RNA blot hybridization confirmed that low levels of S-locus mRNA accumulate in petals and ovaries in vivo. Analysis of the expression pattern of S-locus promoter deletions showed that sequences in the immediate vicinity of the TATA box were sufficient to confer qualitatively correct organ-specific expression of beta-glucuronidase. To further investigate the potential for S-ribonuclease expression in pollen, we used the polymerase chain reaction to amplify RNA accumulated in developing anthers. These assays demonstrated that mRNA for the S-ribonuclease accumulates to low levels in developing anthers several days prior to corolla opening and pollen anthesis. We discuss these results in light of current models of self-incompatibility.