Deadenylation of mRNA is often the first and rate-limiting step in mRNA decay. 
PARN, a poly(A)-specific 3' --> 5' ribonuclease which is conserved in many 
eukaryotes, has been proposed to be primarily responsible for such a reaction, 
yet the importance of the PARN function at the whole-organism level has not been 
demonstrated in any species. Here, we show that mRNA deadenylation by PARN is 
essential for viability in higher plants (Arabidopsis thaliana). Yet, this 
essential requirement for the PARN function is not universal across the 
phylogenetic spectrum, because PARN is dispensable in Fungi (Schizosaccharomyces 
pombe), and can be at least severely downregulated without any obvious 
consequences in Metazoa (Caenorhabditis elegans). Development of the Arabidopsis 
embryos lacking PARN (AtPARN), as well as of those expressing an enzymatically 
inactive protein, was markedly retarded, and ultimately culminated in an arrest 
at the bent-cotyledon stage. Importantly, only some, rather than all, 
embryo-specific transcripts were hyperadenylated in the mutant embryos, 
suggesting that preferential deadenylation of a specific select subset of mRNAs, 
rather than a general deadenylation of the whole mRNA population, by AtPARN is 
indispensable for embryogenesis in Arabidopsis. These findings indicate a 
unique, nonredundant role of AtPARN among the multiple plant deadenylases.