Role of inverted DNA repeats in transcriptional and post-transcriptional gene silencing.

Transgenes and endogenous genes are sensitive to silencing, in particular when the genes are tandemly repeated. Their expression can be transcriptionally or post-transcriptionally repressed, or both. It is remarkable that very often, two or more genes or parts of the genes are arranged as inverted repeats (IR). Many of such IRs are dominant silencing loci. They can repress the expression of homologous genes elsewhere in the genome in trans which is usually associated with an increase in the level of DNA methylation. Trans-silencing has been explained by DNA-DNA pairing between a repetitive silencing locus and a homologous target locus. However, there is accumulating evidence that the trans effect might be mediated by dsRNA transcribed from the IR (trans)genes. Besides dsRNA-directed DNA methylation, dsRNA in plants as well as in other systems also induces the degradation of homologous RNAs and silence genes post-transcriptionally. These findings indicate that several features associated with gene silencing can be attributed to the activities of dsRNA, which would explain why inverted transgene repeats are such efficient silencing loci.

Flavonols are not essential for fertilization in Arabidopsis thaliana.

Flavonols are plant metabolites suggested to serve a vital role in fertilization of higher plants. Petunia and maize plants mutated in their flavonol biosynthesis are not able to set seed after self-pollination. We have investigated the role of these compounds in Arabidopsis thaliana. Like in all other plant species, high levels of flavonols could be detected in pollen of wild-type A. thaliana. No flavonols were detected in reproductive organs of the A. thaliana tt4 mutant in which the chs gene is mutated. Surprisingly, this mutant did set seed after self-fertilization and no pollen tube growth aberrations were observed in vivo. The role of flavonols during fertilization of Arabidopsis is discussed.