In Arabidopsis thaliana , four different dicer-like (DCL) proteins have distinct but partially overlapping functions in the biogenesis of microRNAs (miRNAs) and siRNAs from longer, noncoding precursor RNAs. To analyze the impact of different components of the small RNA biogenesis machinery on the transcriptome, we subjected dcl and other mutants impaired in small RNA biogenesis to whole-genome tiling array analysis. We compared both protein-coding genes and noncoding transcripts, including most pri-miRNAs, in two tissues and several stress conditions. Our analysis revealed a surprising number of common targets in dcl1 and dcl2 dcl3 dcl4 triple mutants. Furthermore, our results suggest that the DCL1 is not only involved in miRNA action but also contributes to silencing of a subset of transposons, apparently through an effect on DNA methylation.