Specific enhancement of the translation of thermospermine-responsive uORF-containing mRNAs by ribosomal mutations in Arabidopsis thaliana

Auxin-induced xylem formation in angiosperms is negatively regulated by thermospermine, whose biosynthesis is also induced by auxin. In Arabidopsis thaliana, loss-of-function mutants of ACL5, which encodes thermospermine synthase, exhibit a dwarf phenotype accompanied by excessive xylem formation. Studies of suppressor mutants that recover from the acl5 dwarf phenotype suggest that thermospermine alleviates the inhibitory effect of an upstream open-reading frame (uORF) on the main ORF translation of SAC51 mRNA. Many suppressor mutations for acl5 have been mapped to the uORF conserved in the SAC51 family or to ribosomal protein genes, such as RPL10A, RPL4A, and RACK1A. In this study, we identified newly isolated acl5 suppressors, sac501, sac504, and sac506, which are additional alleles of RPL10A and the uORFs of SAC51 family members, SACL1 and SACL3, respectively. To investigate whether acl5-suppressor alleles of ribosomal genes broadly affect translation of uORF-containing mRNAs, we examined GUS activity in several 5'-GUS fusion constructs. Our results showed that these alleles enhanced GUS activity in SAC51 and SACL3 5'-fusion constructs but had no effect on other 5'-fusion constructs unrelated to thermospermine response. This suggests that these ribosomal proteins are specifically involved in the thermospermine-mediated regulation of mRNA translation.