C/D-box snoRNP assembly and trafficking in plants: organelle-derived insights into 2′- <i>O</i> -methylation

Two major classes of small nucleolar ribonucleoprotein (snoRNP) complexes have been identified in eukaryotic cells: C/D-box snoRNPs are responsible for 2'-O-methylation (2'-O-Me) of RNA, while H/ACA-box snoRNPs catalyse the conversion of uridine to pseudouridine (Ψ). In this review, we examine the current state of knowledge regarding C/D-box snoRNPs in plants. This knowledge is primarily derived from studies conducted on the model plant Arabidopsis thaliana. We provide a summary of reports concerning the organisation and expression of C/D-box snoRNAs, as well as the proteins that form the C/D-box snoRNP. In addition, we review the factors that are potentially involved in, or have been characterised as being involved in, the process of assembling and translocating C/D-box snoRNPs from the nucleoplasm to the nucleolus via Cajal bodies. Finally, the subject of 2'-O-methylation of ribosomal RNA (rRNA) in chloroplasts and mitochondria is presented, with a focus on the role of site-specific enzymes, as observed in bacterial systems, and in contrast to the involvement of nuclear C/D-box snoRNPs complexes.