Mutations in Ribosomal Proteins, RPL4 and RACK1, Suppress the Phenotype of a Thermospermine-Deficient Mutant of <i>Arabidopsis thaliana</i>

<div><p>Thermospermine acts in negative regulation of xylem differentiation and its deficient mutant of <i>Arabidopsis thaliana</i>, <i>acaulis5</i> (<i>acl5</i>), shows excessive xylem formation and severe dwarfism. Studies of two dominant suppressors of <i>acl5</i>, <i>sac51-d</i> and <i>sac52-d</i>, have revealed that <i>SAC51</i> and <i>SAC52</i> encode a transcription factor and a ribosomal protein L10 (RPL10), respectively, and these mutations enhance translation of the <i>SAC51</i> mRNA, which contains conserved upstream open reading frames in the 5’ leader. Here we report identification of <i>SAC53</i> and <i>SAC56</i> responsible for additional suppressors of <i>acl5. sac53-d</i> is a semi-dominant allele of the gene encoding a receptor for activated C kinase 1 (RACK1) homolog, a component of the 40S ribosomal subunit. <i>sac56-d</i> represents a semi-dominant allele of the gene for RPL4. We show that the GUS reporter activity driven by the CaMV 35S promoter plus the <i>SAC51</i> 5’ leader is reduced in <i>acl5</i> and restored by <i>sac52-d</i>, <i>sac53-d</i>, and <i>sac56-d</i> as well as thermospermine. Furthermore, the <i>SAC51</i> mRNA, which may be a target of nonsense-mediated mRNA decay, was found to be stabilized in these ribosomal mutants and by thermospermine. These ribosomal proteins are suggested to act in the control of uORF-mediated translation repression of <i>SAC51</i>, which is derepressed by thermospermine.</p>