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

Auxin-induced xylem formation in angiosperms is negatively regulated by thermospermine, whose biosynthesis is also induced by auxin. In <i>Arabidopsis thaliana</i>, loss-of-function mutants of <i>ACL5</i>, which encodes thermospermine synthase, exhibit a dwarf phenotype accompanied by excessive xylem formation. Studies of suppressor mutants that recover from the <i>acl5</i> dwarf phenotype suggest that thermospermine alleviates the inhibitory effect of an upstream open-reading frame (uORF) on the main ORF translation of <i>SAC51</i> mRNA. Many suppressor mutations for <i>acl5</i> have been mapped to the uORF conserved in the <i>SAC51</i> family or to ribosomal protein genes, such as <i>RPL10A</i>, <i>RPL4A</i>, and <i>RACK1A</i>. In this study, we identified newly isolated <i>acl5</i> suppressors, <i>sac501</i>, <i>sac504</i>, and <i>sac506</i>, which are additional alleles of <i>RPL10A</i> and the uORFs of <i>SAC51</i> family members, <i>SACL1</i> and <i>SACL3</i>, respectively. To investigate whether <i>acl5</i>-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 <i>SAC51</i> and <i>SACL3</i> 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.