Ykä Helariutta: Recruitment of bifunctional regulator thermospermine to methylated ribosomes directs xylem fate

Polyamines are often associated with ribosomes and thought to stabilize their integrity. In Arabidopsis, the polyamine thermospermine induces translation of SUPPRESSOR-OF\u000210 ACAULIS51 (SAC51) and SAC51-LIKEs (SACLs) that inhibit heterodimerization of LONESOME-HIGHWAY (LHW) and TARGET-OF-MONOPTEROS5 for xylem fate, but its mode of function has remained elusive. Here, we report a methyltransferase, OVERACHIEVER, that writes m3U2952 in the peptidyl transferase center (PTC) of the 25S rRNA. Residue m3U2952 promotes functional thermospermine binding to a specific site connecting the P-site tRNA with 15 rRNA residues in the PTC. This interaction bifunctionally enhances translation of SACLs but inhibits that of LHW. Our study uncovers the dependency between a conserved rRNA base methylation and a polyamine in orchestrating cell fate decisions, highlighting a role for the ribosome chemical landscape in translational regulation