title: Thermospermine levels are controlled by an auxin-dependent feedback loop mechanism in Populus xylem.
abstract: Polyamines are small polycationic amines that are widespread in living organisms. Thermospermine, synthesized by thermospermine synthase ACAULIS5 (ACL5), was recently shown to be an endogenous plant polyamine. Thermospermine is critical for proper vascular development and xylem cell specification, but it is not known how thermospermine homeostasis is controlled in the xylem. We present data in the Populus model system supporting the existence of a negative feedback control of thermospermine levels in stem xylem tissues, the main site of thermospermine biosynthesis. While over-expression of the ACL5 homologue in Populus, POPACAULIS5, resulted in strong up-regulation of ACL5 expression and thermospermine accumulation in leaves, the corresponding levels in the secondary xylem tissues of the stem were similar or lower than those in the wild-type. POPACAULIS5 over-expression had a negative effect on accumulation of indole-3-acetic acid, while exogenous auxin had a positive effect on POPACAULIS5 expression, thus promoting thermospermine accumulation. Further, over-expression of POPACAULIS5 negatively affected expression of the class III homeodomain leucine zipper (HD-Zip III) transcription factor gene PttHB8, a homologue of AtHB8, while up-regulation of PttHB8 positively affected POPACAULIS5 expression. These results indicate that excessive accumulation of thermospermine is prevented by a negative feedback control of POPACAULIS5 transcript levels through suppression of indole-3-acetic acid levels, and that PttHB8 is involved in the control of POPACAULIS5 expression. We propose that this negative feedback loop functions to maintain steady-state levels of thermospermine, which is required for proper xylem development, and that it is dependent on the presence of high concentrations of endogenous indole-3-acetic acid, such as those present in the secondary xylem tissues.
authors: Instituto de Biologia Experimental e Tecnológica, Apartado 12, 2781-901, Oeiras, Portugal.
authors: Ana
authors: A
authors: Milhinhos
authors: Jakob
authors: J
authors: Prestele
authors: Benjamin
authors: B
authors: Bollhöner
authors: Andreia
authors: A
authors: Matos
authors: Francisco
authors: F
authors: Vera-Sirera
authors: José L
authors: JL
authors: Rambla
authors: Karin
authors: K
authors: Ljung
authors: Juan
authors: J
authors: Carbonell
authors: Miguel A
authors: MA
authors: Blázquez
authors: Hannele
authors: H
authors: Tuominen
authors: Célia M
authors: CM
authors: Miguel
mesh_terms: D000595:Amino Acid Sequence; D029681:Arabidopsis Proteins / Q000235:genetics; D001483:Base Sequence; D018506:Gene Expression Regulation, Plant; D007210:Indoleacetic Acids / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D016350:Leucine Zippers / Q000235:genetics; D008954:Models, Biological; D008969:Molecular Sequence Data; D010802:Phylogeny; D010937:Plant Growth Regulators / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D018515:Plant Leaves / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D018547:Plant Stems / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D030821:Plants, Genetically Modified; D032107:Populus / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D013096:Spermine / Q000031:analogs & derivatives* / Q000378:metabolism; D014197:Trees; D015854:Up-Regulation; D014934:Wood / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D052584:Xylem / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism
chemical_list: C414439:ACAULIS5 protein, Arabidopsis; D029681:Arabidopsis Proteins; D007210:Indoleacetic Acids; D010937:Plant Growth Regulators; D010940:Plant Proteins; D013096:Spermine; C030737:indoleacetic acid; C021097:thermospermine
keywords: ACAULIS5 (ACL5); POPACAULIS5; Populus tremula × Populus tremuloides; Populus trichocarpa; class III homeodomain leucine zipper transcription factors (HD-Zip III); polyamine; wood development