title: The SAC51 Family Plays a Central Role in Thermospermine Responses in Arabidopsis.
abstract: 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.
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Qingqing
authors.initials: Q
authors.lastname: Cai
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Fukushima
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Mai
authors.initials: M
authors.lastname: Yamamoto
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Nami
authors.initials: N
authors.lastname: Ishii
authors.affiliation: Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Sakamoto
authors.affiliation: Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Kurata
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan.
authors.forename: Hiroyasu
authors.initials: H
authors.lastname: Motose
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan perfect@cc.okayama-u.ac.jp.
authors.forename: Taku
authors.initials: T
authors.lastname: Takahashi
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000166:cytology / Q000187:drug effects / Q000235:genetics* / Q000502:physiology; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D017930:Genes, Reporter; D008954:Models, Biological; D016254:Mutagenesis, Insertional; D016366:Open Reading Frames / Q000235:genetics; D010641:Phenotype; D036226:Seedlings / Q000166:cytology / Q000187:drug effects / Q000235:genetics / Q000502:physiology; D016415:Sequence Alignment; D013096:Spermine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D052584:Xylem / Q000166:cytology / Q000187:drug effects / Q000235:genetics / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D013096:Spermine; C515850:SAC51 protein, Arabidopsis; C021097:thermospermine
keywords: Arabidopsis; Thermospermine; Translation; Xylem differentiation; uORF