The SAC51 Family Plays a Central Role in Thermospermine Responses in Arabidopsis

Plant Cell Physiol. 2016 Aug;57(8):1583-92. doi: 10.1093/pcp/pcw113. Epub 2016 Jul 7.


The acaulis5 (acl5) mutant of Arabidopsis thaliana is defective in the biosynthesis of thermospermine and shows a dwarf phenotype associated with excess xylem differentiation. SAC51 was identified from a dominant suppressor of acl5, sac51-d, and encodes a basic helix-loop-helix protein. The sac51-d mutant has a premature termination codon in an upstream open reading frame (uORF) that is conserved among all four members of the SAC51 family, SAC51 and SACL1-SACL3 This suggests that thermospermine cancels the inhibitory effect of the uORF in main ORF translation. Another suppressor, sac57-d, has a mutation in the conserved uORF of SACL3 To define further the function of the SAC51 family in the thermospermine response, we analyzed T-DNA insertion mutants of each gene. Although sacl1-1 may not be a null allele, the quadruple mutant showed a semi-dwarf phenotype but with an increased level of thermospermine and decreased sensitivity to exogenous thermospermine that normally represses xylem differentiation. The sac51-1 sacl3-1 double mutant was also insensitive to thermospermine. These results suggest that SAC51 and SACL3 play a key role in thermospermine-dependent negative control of thermospermine biosynthesis and xylem differentiation. Using 5' leader-GUS (β-glucuronidase) fusion constructs, however, we detected a significant enhancement of the GUS activity by thermospermine only in SAC51 and SACL1 constructs. Furthermore, while acl5-1 sac51-1 showed the acl5 dwarf phenotype, acl5-1 sacl3-1 exhibited an extremely tiny-plant phenotype. These results suggest a complex regulatory network for the thermospermine response in which SAC51 and SACL3 function in parallel pathways.

Keywords: Arabidopsis; Thermospermine; Translation; Xylem differentiation; uORF.

MeSH terms

  • Amino Acid Sequence
  • Arabidopsis / cytology
  • Arabidopsis / drug effects
  • Arabidopsis / genetics*
  • Arabidopsis / physiology
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Basic Helix-Loop-Helix Transcription Factors / genetics
  • Basic Helix-Loop-Helix Transcription Factors / metabolism*
  • Gene Expression Regulation, Plant
  • Genes, Reporter
  • Models, Biological
  • Mutagenesis, Insertional
  • Open Reading Frames / genetics
  • Phenotype
  • Seedlings / cytology
  • Seedlings / drug effects
  • Seedlings / genetics
  • Seedlings / physiology
  • Sequence Alignment
  • Spermine / analogs & derivatives*
  • Spermine / metabolism
  • Spermine / pharmacology
  • Xylem / cytology
  • Xylem / drug effects
  • Xylem / genetics
  • Xylem / physiology


  • Arabidopsis Proteins
  • Basic Helix-Loop-Helix Transcription Factors
  • SAC51 protein, Arabidopsis
  • Spermine
  • thermospermine