Ribosome Biogenesis Factor HRR25 in Arabidopsis Growth and Development

Ribosomes drive translation of the genome into the proteome in all living cells and ribosome biogenesis is coupled to cellular homeostasis and growth rate. Improper ribosome assembly often results in nonfunctional proteins. Whereas ribosome biogenesis is well described in bacteria, yeast, and mammals, little is known in plants. Yeast ribosome biogenesis requires multiple rRNA processing events in coordination with 200 ribosome biogenesis factors (RBFs). In this process, HRR25 is an essential gene coding for a casein kinase1-like (CK1L) protein (Hrr25p) that through the Ltv1p-Enp1 complex regulates incorporation of the two small subunit ribosomal proteins, RPS3 and RPS10, that line the mRNA entry channel of the 40S small subunit; key ribosome maturation events. Furthermore, a Hrr25p-Nup53p (nuclear pore complex) interaction suggests that the mobility of Hrr25p during cell division may effect the cell cycle. In Arabidopsis thaliana 14 CK1L isoforms (1-13, 9α- β) have been reported as plasmodesmata-associated kinases. Phylogenetic analysis of these CK1L isoforms shows five of the 14 closely cluster with yeast Hrr25p. What are the role(s) of these five Hrr25 orthologs in Arabidopsis ribosome biogenesis and plant growth and development?