Polyamines are often associated with ribosomes and are thought to stabilise their integrity. However, no specific cellular functions have yet been attributed to ribosome-bound polyamines despite their roles in diverse biological processes, including the regulation of cell proliferation, differentiation, autophagy and ageing.
In Arabidopsis, the polyamine thermospermine (tSpm) affects xylem cell fate. tSpm induces translation of SUPPRESSOR-OF-ACAULIS51 (SAC51) and SAC51-LIKEs (SACLs), which inhibit heterodimerisation of the xylem development proteins LONESOME-HIGHWAY (LHW) and TARGET-OF-MONOPTEROS5. This study from the Helariutta, Kirpekar and Warren labs, featuring cryo-electron microscopy analysis by Alex Faile, reports a methyltransferase, OVERACHIEVER, that methylates the peptidyl transferase centre of the 25S ribosomal RNA (rRNA). The methylated residue m3U2952 promotes functional tSpm binding to a specific site that connects the P-site transfer RNA (tRNA) with rRNA residues in the peptidyl transferase centre. This interaction enhances the translation of SACLs but inhibits that of LHW.
The study uncovers the dependency between a conserved rRNA base methylation and a polyamine in orchestrating cell fate decisions by allowing the ribosome to act as a signalling sensor. Understanding how plants fine-tune their vascular development offers a promising path to optimise growth traits that are critical to agriculture and forestry, including the production of commercially valuable materials including wood, paper, edible roots and bioproducts.