The exosome and 3'-5' RNA degradation in plants.

One of the most versatile RNA degradation machines in eukaryotes is the 3'-5' RNA exosome. It consists of nine conserved subunits forming the core complex, which associates with active ribonucleases, RNA binding proteins, helicases and additional co-factors. While yeast and human exosome core complexes are catalytically inactive, the plant core complex has probably retained a phosphorolytic activity. Intriguingly, the down-regulation of individual subunits of the plant core complex in Arabidopsis mutants led to distinct developmental defects, suggesting an unequal contribution of the core subunits to the in vivo activities of the plant exosome complex. In addition, some of the plant core subunits as well as some associated factors are encoded by duplicated genes, which may have both overlapping and specific functions. Together, these results suggest an unique and complex organisation of exosome-mediated RNA degradation processes in plants. This chapter reviews our current knowledge of plant exosomes and discusses the impact of 3'-5' RNA degradation on the posttranscriptional control of plant genome expression.