Recently, we and others have reported that mRNAs may be polyadenylated in plant 
mitochondria, and that polyadenylation accelerates the degradation rate of 
mRNAs. To further characterize the molecular mechanisms involved in plant 
mitochondrial mRNA degradation, we have analyzed the polyadenylation and 
degradation processes of potato atp9 mRNAs. The overall majority of 
polyadenylation sites of potato atp9 mRNAs is located at or in the vicinity of 
their mature 3'-extremities. We show that a 3'- to 5'-exoribonuclease activity 
is responsible for the preferential degradation of polyadenylated mRNAs as 
compared with non-polyadenylated mRNAs, and that 20-30 adenosine residues 
constitute the optimal poly(A) tail size for inducing degradation of RNA 
substrates in vitro. The addition of as few as seven non-adenosine nucleotides 
3' to the poly(A) tail is sufficient to almost completely inhibit the in vitro 
degradation of the RNA substrate. Interestingly, the exoribonuclease activity 
proceeds unimpeded by stable secondary structures present in RNA substrates. 
From these results, we propose that in plant mitochondria, poly(A) tails added 
at the 3' ends of mRNAs promote an efficient 3'- to 5'- degradation process.