Multiple elements of the S2-RNase promoter from potato (Solanum tuberosum L.) are required for cell type-specific expression in transgenic potato and tobacco

A functional analysis of the promoter of the S2-RNase gene from potato was performed in transgenic potato and tobacco plants, using a deletion series of S2-RNase promoter GUS fusions. A detailed histochemical and quantitative analysis of the transgenic tobacco plants revealed that S2 promoter fragments ranging in size from 5.6 kb in length down to 0.2 kb mediate a weak developmentally regulated expression in the pistil, and strong ectopic expression in pollen. In the pistil, different expression patterns were seen depending on the transformant, the predominant one being characterised by expression in the stigma and the transmitting tract of the style, whereas a few plants showed expression exclusively either in the stigma or in the stylar transmitting tissue. All transformants also showed GUS expression in the placental epidermis of the ovary. Two sequences that are conserved between the potato S1-RNase and S2-RNase promoters, termed motif I and motif III, are located in a fragment of the S2 promoter extending from position −200 to bp −100, and motif II, located between bp −498 and −480, was identified on the basis of sequence comparisons between pistil-specific promoters. Motif II was found to be dispensible for pistil-specific and for pollen-specific expression. Two submotifs, A and B, were identified within motif I. Both were essential for expression in the pistil but only B was necessary for expression in pollen. Although motif III has a similar bipartite structure and sequence to motif I, it was not sufficient to confer either pollen- or pistil-specific expression. However, deletion of motif III abolished pollen-specific expression in transient expression experiments, suggesting that an interaction between the two sequence motifs may be needed to specify cell type-specific expression. In transgenic potato the S2-RNase promoter also mediates expression in pollen and in the pistil; however, significantly fewer plants showed expression than in tobacco, with most plants also exhibiting GUS expression in other tissues.