TRANSGENE-INDUCED SILENCING OF S-LOCUS GENES AND RELATED GENES IN BRASSICA

The S-locus receptor kinase gene, SRK, and the S-locus glycoprotein ge ne, SLG, are required for the self-incompatibility response in Brassic a. A transgenic approach was used to investigate the function of these genes. Several chimeric genes predicted either to interfere with SRK function in self-incompatible hosts and possibly generate loss-of-func tion mutants, or to complement a defective SRK in a self-compatible ho st and possibly generate gain-of-function plants, were designed. The t ransgenes led to a dramatic reduction in the expression of endogenous S-locus and related genes, thus limiting the usefulness of transgenic approaches for the dissection of S-locus gene function in Brassica. Se veral features of this 'homology-dependent gene silencing' are describ ed, including its tissue-specific nature, the lack of a direct correla tion between the severity of silencing and the extent of sequence simi larity shared by the silenced gene and the transgene, its association with increased cytosine methylation of silenced endogenous genes in at least some cases, and its persistence in transgenic progenies that do not inherit the transgene. Most significantly, silencing of S genes i n self-incompatible host plants results in the breakdown of self-incom patibility in the stigma, whereas a 75% reduction in S gene transcript s in anthers does not affect pollen phenotype.