Transgene-induced silencing of S-locus genes and related genes in Brassica

The S-locus receptor kinase gene, SRK, and the S-locus glycoprotein gene, SLG, are required for the self-incompatibility response in Brassica. 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-function mutants, or to complement a defective SRK in a self-compatible host and possibly generate gain-of-function plants, were designed. The transgenes 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. Several features of this 'homology-dependent gene silencing' are described, including its tissue-specific nature, the lack of a direct correlation between the severity of silencing and the extent of sequence similarity 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 in self-incompatible host plants results in the breakdown of self-incompatibility in the stigma, whereas a 75% reduction in S gene transcripts in anthers does not affect pollen phenotype.