3'-5' exoribonuclease that catalyzes 3' maturation of chloroplast and mitochondrion ribosomal RNAs; degrades short nucleotidic extensions to generate the mature 3'-ends. Involved in the maturation of 23S, 16S and 5S rRNAs. Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Expressed in seedlings, roots, leaves and flowers. Shorter 3' nucleotide extensions of mitochondrial mRNAs and reduced accumulation of several chloroplast rRNA species. Germinates only on sucrose-containing media, with white cotyledons and pale green rosette leaves. Belongs to the RNR ribonuclease family.